Salman Khan Speaks at GEL (Good Experience Live) Conference

WEBVTT
Kind: captions
Language: en

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(music)

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(jumpy music)

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(applause)

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Salman: It's exciting to be here.

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Well as Mark said, I am the founder
and the faculty of the Khan Academy.

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(laughter)

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Very bureaucratic organization.

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(laughter)

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And as he said, I started off,
just a little background on me,

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I kind of have a techie
background, a math background.

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And then I sold out, and I became
an analyst at a hedge fund.

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But while I was an
analyst at a hedge fund,

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I always had this little
bit of an itch to teach.

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And no one believes me, but my
motivation for working at a hedge fund,

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I didn't even know,
someone told me it's the

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best job to learn about everything.

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You get to see what happens to
crop prices when an oil tanker

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in this part of the world
disappears, and then that happens,

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and gold prices ...

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That's a great job, and then someone
else said and it pays pretty well.

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I said, "Oh."

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But I was at a, I sold out, I was
working as an analyst at a hedge fund,

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and I started tutoring my
cousins in New Orleans.

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And we would do this kind of
remote tutoring type of thing,

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and we'll talk more about the form factor,

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because that's what I really
want to make this talk about.

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And I wanted to scale up, so I
just started making YouTube videos.

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And fast forward about four or five years.

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and as Mark said, it's now
fourteen hundred videos.

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I started just doing Algebra.

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And after about seventy videos,
I said, "Gee, that's Algebra."

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And this was kind of a
part time thing for me.

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And then I said, "Maybe I
can do Pre-Algebra too."

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And then another seventy videos
later, I said, "That's Pre-Algebra.

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"I'll do Trigonometry. And maybe Calculus.

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"And maybe I'll do Arithmetic.
And I'll do Physics."

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And before we know it, fast forward,
there's fourteen hundred videos,

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like Mark had mentioned.

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And the YouTube people told me that
it's now the most used open course

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video library on, definitely on YouTube.

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More views than MIT, and
Stanford, and Berkeley.

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And as far as I know the Internet.

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And this is all from-

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(laughter and applause)

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And this is all from one dude who
is a guy working at hedge fund

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in his part time with
no budget whatsoever,

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really just starting to tutor his cousins.

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And now it's sixty thousand video
views, and fourteen million views,

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two hundred thousand
unique students a month.

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And you can kind of view it as it's
the largest school in the world.

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But before I go into the actual why,
and I want to make this kind of a

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conversation about why I think
it's working, because I think it

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has ... it can be moved to other
domains in all of our lives, I think.

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And I think it will tell us something
about how we learn and all of that.

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I want to show you a quick
video, just so you understand

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the form factor, and then we
can talk a little bit about

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why, maybe we collectively
think it might be working.

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So if you start up the video.

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Four x squared, squared, plus two
times the product of both terms

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two times four x squared, times y
squared, plus y squared, this term

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In October of eighteen o five,
in October of eighteen o five,

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the French Navy gets destroyed by the
British in the Battle of Trafalgar.

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This is Trafalgar.

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It's actually written here.

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Tra-falgar.

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And they get destroyed by Napoleon's
good old friend Admiral Horatio Nelson.

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Horatio Nelson.

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So this guys was definitely
a thorn in Napoleon's side.

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Horatio ...

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D eight positive T cell.

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It has a C D eight proteins on it.

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And this would be called
a C D four positive.

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C D four positive T cells.

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P of x y i plus q of x y j.

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Right this is just a
vector field, over ...

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Problem thirty four.

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A landscaper estimates that
landscaping a new park will take

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one person forty-eight hours.

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If four people work on the job,
and they each work six hour days,

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they each work six hour days ...

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(applause)

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And I'm curious, and don't, feel free,
I'm very open to the negative feedback.

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What are the things, if you were to
just compare that to your experience

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when you might have been exposed
to some of those same concepts

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either when you learned it
at high school or in college,

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or even if you've learned it
watch some of the other online
video libraries out there,

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what are the things that stood out
to you, that were a little different?

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Any?

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Audience member: You can fast forward.

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Salman: Yeah, no that's actually a huge.

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And I think that's
online video in general.

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The comment was that you can fast forward.

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I was going to talk about this later,
but there's this whole notion that

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anything online is kind
of just nice to have.

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It's not as good as a live interaction.

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And I think that assumption is something,

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this bias that we always have in life,

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that something for free, that has
zero incremental delivery cost

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has got to be worse than
something that's really

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expensive and hard to do.

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And if you think about it, there is
value of being in a room together.

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It's something like jail.

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It's a shared experience.

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We can get inspired together.

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But if we're, if it's a broadcast lecture,

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and it's kind of mundane,
and no one's really ...

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there's not a lot of interaction.

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it's an interesting question to
say, "Gee, is video in any way

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"less good than an
actual live interaction?"

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And because you can pause.

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You can repeat it.

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You don't have to take notes.

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It is actually something as far as
I can tell, better in every way.

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You watch it on your own terms,
whenever you want to watch it.

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If you don't understand a concept,
you can watch a previous video.

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You can look on Wikipedia
and see what that word meant.

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So, yeah that is one of the
... and I would say that's

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general of all online video,
not just the Khan Academy.

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Any other things that just kind of
popped out at, when you saw those videos?

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Yeah?

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Audience Member: You don't have to
worry about pleasing the teacher.

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In other words, you weren't
distracted by the social interaction.

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You're just listening
for your own benefit.

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Salman: Right. Right.

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So the comment was that you're not
worried about pleasing the teacher.

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You're just doing it for your own benefit.

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And that's actually ... when I
started, like my teaching career,

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and it's full time now, I
quit my job in September,

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I was tutoring my cousins
and I was doing it remotely.

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They were in New Orleans.

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I was in Boston.

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Now I'm in Northern California.

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And they were doing really well.

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But then I started doing
these YouTube videos.

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And I assumed it was not going to
be as good as the live interaction.

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I assumed, "Oh this is
just nice to have for them.

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"Maybe other people will watch it."

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I kind of said, "Oh, it's
a little bit of a legacy.

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"Maybe my great grandchildren
might be able to watch,

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"learn Calculus from these videos."

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I just thought it would be a
nice thing to put out there.

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But the first point of
feedback I got from my cousins,

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was it was a little bit of
a back-handed compliment,

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is that you're better on
YouTube than you are in person.

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(laughter)

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And after a little bit of getting,
not being defensive about that,

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I think they were saying exactly this,

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when we had the live interaction
and the one-on-one session,

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that's stressful for the student.

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That's stressful ... "Oh do you get this?"

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And they might kind of get it.

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It might be like an eighty
percent understanding of it.

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But they don't want to waste my time,

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and I'm their cousin,
and I'm volunteering.

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They don't want me to judge
them in any negative way.

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So, "Yeah, yeah, yeah. I get it."

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But that's bad information for me,
I'll move on to the next concept.

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They're stressed out the whole time.

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They want this whole
interaction to be over.

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But with this, you're not
afraid to ask the question.

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The question literally really is.

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You to to that, as Mark said there's
fourteen hundred videos, you're like,

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"You know what, I'm a little
shaky on negative numbers,

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"even though I'm in Calculus."

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There's no way in a traditional
classroom, that you'll have the guts

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to raise your hand and say,
"Wait, a negative times ...
why is that a positive?"

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(laughter)

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You can't do that.

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(laughter)

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But YouTube you can kind
of pause a video and say,

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"Let me go watch a little video, just to
make sure I understand that properly."

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Any other things popped out?

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Right here.

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Yeah, so the comment was,
colors, I repeat a lot

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and I cater to different learning styles.

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And I'll throw out another one
that I things that's related is

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that you don't see me.

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You don't see the teacher.

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And I'll tell you, huh?

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But you can hear me, exactly.

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Exactly.

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And I'll tell you why I did it.

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I don't want to make it sound
like I'm this grand desiger

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and I had all of this
interesting ideas in mind.

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I did it initially because I
did not have a video camera.

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(laughter)

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And I thought, well, when I did
it with my cousins in New Orleans

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they didn't see me either.

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But it seemed to be working with them.

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All they heard was my
voice on the speaker phone,

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and we had like a shared notepad.

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We were using Yahoo Doodle at the time.

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And if I wrote something,
they would see it.

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If they wrote something, I would see it.

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And we made the ... I just liked ...

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I don't know if there's a movement
against or for whiteboards,

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but I hate whiteboards.

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I think black is just like,
there's this emptiness.

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It's just like the universe.

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Then you can start kind of
putting in little bits of ...

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And it seems like such a
subtle thing, but for me,

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like that one video where
you see the white background,

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it's kind of like a struggle for me.

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I need that clean,
literally a clean slate.

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But I did it because it
worked with my cousins,

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and I didn't have a video camera.

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But as soon as I put it out there
like that, the feedback I immediately

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got from people on YouTube
is, it feels intimate.

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Even though they know that it's a
recording and I'm not really there,

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they said it actually feels like
you're sitting next to me tutoring me.

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It doesn't feel like ... think
about the other experience where

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I'm all the way over here, at a
chalkboard, "and the next step,"

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and I'm projecting over
to you and you're distant.

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You're not in any way
connected to the the teacher.

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When you have kind of this voice
in your head, it feels intimate.

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Actually some people have
said it feels even more.

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It feels like, not only are
we sitting next to each other,

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it feels like you're actually in my brain.

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(laughter)

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That I'm ... and even more, I've
actually gotten several people

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say that, "When I take the exam, as
soon as it ... when I think in other

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"domains I have a normal voice-

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(laughter)

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"but as soon as I see a calculus
problem, it's like, Well and the next.'"

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And you hear-

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(laughter)

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you-

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(laughter)

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So it's a ... any other notions?

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Yeah, yeah.

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Audience member: The audio is
consistent, unlike in a classroom,

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where the teacher turns
around and you can't hear.

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Salman: Oh yeah, no that's another
thing, that is actually one of

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motivations for doing it, is
I've looked, before I started

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I looked at a lot of this stuff.

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I'm not the first person
putting up videos.

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I looked at the MIT stuff,
which I think is great.

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And I was like, "Gee."

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The video camera was at that same seat.

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I went to MIT, and it's like
the same seat I used to sit in,

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and like fall asleep in.

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And sometimes ... I think
it's tremendous what they did.

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I don't know if anyone realizes the
history of this open course thing.

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In like two thousand or two
thousand one, it was all happening.

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Every other university was thinking of
a for-profit model distance learning.

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MIT said no, everything's for free.

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So I think it's like this
hugely awesome thing they did,

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but it's hard to see what's
going on in the classroom.

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And the other feedback I get is that,
the lecture is actually distracting.

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We naturally want to engage with
the human eyes and the hands,

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and that's what you want to look at.

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You don't want to look at the math.

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And when the professor's
there, that's what you look at.

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And you get disengaged and all.

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Any other?

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Any other?

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Here.

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Audience member: Go at your own speed.

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Salman: Go at your own speed.

00:11:28.714 --> 00:11:29.849
Exactly.

00:11:29.849 --> 00:11:32.265
You can go exactly ... some people
have actually they've written

00:11:32.265 --> 00:11:34.104
little apps that actually
will speed up or slow down

00:11:34.104 --> 00:11:36.425
the actual without changing the pitch.

00:11:36.425 --> 00:11:38.262
Audience member: Well I
mean (unintelligible).

00:11:38.262 --> 00:11:39.570
Oh yeah, exactly.

00:11:39.570 --> 00:11:40.673
Audience Member: You can go
slow of you can go quickly.

00:11:40.673 --> 00:11:41.946
Salman: And that's the
thing that shocked me,

00:11:41.946 --> 00:11:43.697
I thought this is for my cousins,
they can fill in their blanks.

00:11:43.697 --> 00:11:46.446
But over and over again, I got
letters from gifted students

00:11:46.446 --> 00:11:48.820
and parents, saying that this is
allowed my students ... and people with

00:11:48.820 --> 00:11:52.068
learning disabilities that say that ...
and people with attention deficit disorder

00:11:52.068 --> 00:11:55.151
saying that ... and so it's all of these
things that just kind of fell into place.

00:11:55.158 --> 00:11:58.067
And I'll tell you kind of the
other thing I think makes it

00:11:58.067 --> 00:12:01.317
very different is, it's
just conversational.

00:12:01.317 --> 00:12:06.293
Some of it is because the form factor
is it feels like I'm next to you.

00:12:06.293 --> 00:12:09.646
I get to kind of chat about
it more with you as opposed to

00:12:09.646 --> 00:12:11.907
being pedantic and sitting at
a board, "and the next step."

00:12:11.907 --> 00:12:14.221
I can be very natural.

00:12:14.221 --> 00:12:18.984
A lot of people say how do
you prepare for these videos.

00:12:18.984 --> 00:12:19.898
Can I have a transcript?

00:12:19.898 --> 00:12:23.776
Can I have ... and what
I say is there is none.

00:12:23.776 --> 00:12:27.444
When I turn on the video, it is
literally a chain of thought process.

00:12:27.444 --> 00:12:32.194
It isn't scripted in any
way, and I started doing that

00:12:32.194 --> 00:12:35.153
actually out of laziness.

00:12:35.153 --> 00:12:36.447
You know, it's for my
cousins what do I care.

00:12:36.447 --> 00:12:39.239
I actually have a few of the videos
I actually answer telemarketer calls

00:12:39.239 --> 00:12:40.247
during-

00:12:40.247 --> 00:12:41.246
(laughter)

00:12:41.246 --> 00:12:43.709
the video.

00:12:43.709 --> 00:12:48.746
But the takeaway, all of these
things I have done out of laziness,

00:12:48.746 --> 00:12:50.296
I now can say oh, this
is brilliant and all,

00:12:50.296 --> 00:12:55.049
but the takeaway that people
love to see, and you never get to

00:12:55.049 --> 00:12:56.216
see it anywhere else.

00:12:56.216 --> 00:12:59.408
I mean, just like reflect back
on your entire education career.

00:12:59.408 --> 00:13:01.406
Everything you saw was
the finished product.

00:13:01.406 --> 00:13:03.606
The professor when they
teach that concept to you,

00:13:03.606 --> 00:13:05.324
then had done that problem
twenty times before.

00:13:05.324 --> 00:13:06.696
They reviewed the solution manual,

00:13:06.696 --> 00:13:08.195
and they just give you
the finished product.

00:13:08.195 --> 00:13:10.073
They're just like, "Oh, you're an
idiot, if you just don't immediately

00:13:10.073 --> 00:13:12.109
"do step A, B, C, and D."

00:13:12.109 --> 00:13:13.743
But when you watch these videos,
and I guarantee you, like

00:13:13.743 --> 00:13:16.074
ninety five percent of the videos,
when I'm doing it on screen

00:13:16.074 --> 00:13:18.238
I have never seen the problem before.

00:13:18.238 --> 00:13:22.576
And every now and then, every
now and then I make mistakes.

00:13:22.576 --> 00:13:27.598
I forget a negative sign, or I go down
a path that was not the right path.

00:13:27.598 --> 00:13:31.892
But ... this is probably the single
issue I get the most emails about.

00:13:31.892 --> 00:13:34.675
People say it's makes me
comfortable with mathematics.

00:13:34.675 --> 00:13:35.910
It allows me to see the art.

00:13:35.910 --> 00:13:39.673
It allows me to see someone who looks
like the know what they're doing,

00:13:39.673 --> 00:13:42.071
but they too stumble every now and then.

00:13:42.071 --> 00:13:44.365
They sometimes go down the wrong
path, and you can kind of see

00:13:44.365 --> 00:13:47.821
out loud the thought process
of going through this.

00:13:47.821 --> 00:13:52.278
The one thing ... Mark's saying,
tell us about what you really think

00:13:52.278 --> 00:13:56.454
makes Khan Academy a standout,
and why it seems to have

00:13:56.454 --> 00:13:58.409
struck a chord with people.

00:13:58.409 --> 00:14:02.320
And I think it's kind of an
artifact of how my own brain works.

00:14:02.320 --> 00:14:05.073
I'm this ... I don't know whether
I was exposed to something,

00:14:05.073 --> 00:14:09.077
or this is just how I learned to
learn, but I'm almost, I cannot

00:14:09.077 --> 00:14:13.656
process anything unless it's distilled
down into really, really, really

00:14:13.656 --> 00:14:17.600
simple nuggets, and it's almost ...
it's almost obvious by the time ...

00:14:17.600 --> 00:14:21.015
and it was almost difficult
when I was in school.

00:14:21.015 --> 00:14:23.004
If you just told me, gave
me a formula and said,

00:14:23.004 --> 00:14:24.993
"Just apply that formula,
steps A, B, and C,"

00:14:24.993 --> 00:14:26.984
I like, my brain just
starts, it can't do it.

00:14:26.984 --> 00:14:30.395
So I would fall behind in
class because I wanted to play

00:14:30.395 --> 00:14:32.937
with the equations and make
connections and understand

00:14:32.937 --> 00:14:36.902
why it was interesting and what's
it really saying about the universe.

00:14:36.902 --> 00:14:40.351
But later on obviously that
became a huge asset because

00:14:40.351 --> 00:14:42.741
in Eleventh grade when you get
exposed to a concept, you're like,

00:14:42.741 --> 00:14:46.445
"Oh, well I spent, I invested
all that time in Eighth grade,

00:14:46.445 --> 00:14:47.830
"and this concept that they're
saying is a new concept, that's just

00:14:47.830 --> 00:14:50.621
that Eighth grade concept, and they're
just re-branding it a little bit,

00:14:50.621 --> 00:14:53.120
"now it just looks like this
Eleventh grade concept."

00:14:53.120 --> 00:14:56.613
And while my peers are getting,
who are smarter than me,

00:14:56.613 --> 00:15:00.221
are getting stuck memorizing things,
and not knowing what they're doing,

00:15:00.221 --> 00:15:01.571
and then forgetting things the week after.

00:15:01.571 --> 00:15:03.225
I'm like this is frustrating.

00:15:03.225 --> 00:15:10.185
So, the single biggest goal in the
Khan Academy is to try to deliver

00:15:10.185 --> 00:15:12.323
things the way I wish
it was delivered to me.

00:15:12.323 --> 00:15:14.527
You get the intuition from the beginning.

00:15:14.527 --> 00:15:19.291
I will not make a video unless I
have a intuition on the concept.

00:15:19.306 --> 00:15:21.572
The best example is on entropy.

00:15:21.572 --> 00:15:23.696
I'm not like, you know ... when
I went through my education

00:15:23.696 --> 00:15:26.191
I wasn't this thermodynamics
jock, so I didn't really ...

00:15:26.191 --> 00:15:29.442
I could do the problems, and I
could get the question right,

00:15:29.442 --> 00:15:31.779
but I said I'm going to
make a video on entropy.

00:15:31.779 --> 00:15:36.122
I said I know the steps but none of
these textbooks are really doing the job,

00:15:36.122 --> 00:15:39.370
doing the trick, and really letting
me know what entropy really is.

00:15:39.370 --> 00:15:41.765
And I have just the suspicion that
it's not, they're not saying ...

00:15:41.765 --> 00:15:44.696
so I literally, I took two weeks
off, and I just pondered it,

00:15:44.696 --> 00:15:48.228
and I called every professor, every
buddy, everyone who I could talk to,

00:15:48.228 --> 00:15:50.445
and I'd say let's go have a
glass of wine about entropy.

00:15:50.445 --> 00:15:52.121
(laughter)

00:15:52.121 --> 00:15:55.074
And after about two weeks,
it clicked in my brain,

00:15:55.074 --> 00:15:58.528
I was like, "Now, I'm ready
to make a video on entropy."

00:15:58.528 --> 00:16:00.198
So if there's one takeaway
from the Khan Academy, is that

00:16:00.198 --> 00:16:03.777
it's just focused on,
narrowly focused on intuition.

00:16:03.777 --> 00:16:07.526
And just to show you, and to
really reinforce the point

00:16:07.526 --> 00:16:11.622
that I think, and it wasn't
obvious from the get go

00:16:11.622 --> 00:16:15.049
that this is not just a nice to have.

00:16:15.049 --> 00:16:19.219
This isn't just something that might make
an A minus student, into an A student

00:16:19.219 --> 00:16:20.221
or an A plus student.

00:16:20.221 --> 00:16:24.362
I want to read some of the letters.

00:16:24.362 --> 00:16:26.279
And I get on the order of now,
I get like a couple hundred

00:16:26.279 --> 00:16:30.096
letters a day from students,
and teachers, and home schoolers

00:16:30.096 --> 00:16:32.024
around the world now.

00:16:32.024 --> 00:16:35.033
The content's actually being distributed
in offline services into Ethopia

00:16:35.033 --> 00:16:37.215
and places where they
can't even find teachers.

00:16:37.215 --> 00:16:41.278
But these comments, some of them
are nice and funny and simple,

00:16:41.278 --> 00:16:45.571
but some of them really speak to there's
a deep hunger for this type of thing,

00:16:45.571 --> 00:16:47.033
and it's just not being addressed.

00:16:47.033 --> 00:16:49.193
This is a comment on a calculus video.

00:16:49.193 --> 00:16:52.031
"First time I smiled doing a derivative."

00:16:52.031 --> 00:16:54.165
(laughter)

00:16:54.165 --> 00:16:57.868
This person did a derivative,
and then they smiled.

00:16:57.868 --> 00:16:58.929
(laughter)

00:16:58.929 --> 00:16:59.684
They smiled.

00:16:59.684 --> 00:17:02.117
I remember EP calculus,
no one was smiling.

00:17:02.117 --> 00:17:03.545
(laughter)

00:17:03.545 --> 00:17:06.138
And then the next, the next comment,
this is in reply to that response,

00:17:06.138 --> 00:17:08.095
Same thing here.

00:17:08.095 --> 00:17:10.640
"I actually got a natural
high and a good mood

00:17:10.640 --> 00:17:12.893
"for the entire day, since I
remember seeing all of this matrix

00:17:12.893 --> 00:17:17.370
"text in class, and here I'm
all like I know Kung Fo."

00:17:17.370 --> 00:17:20.644
(loud laughter and applause)

00:17:20.644 --> 00:17:23.373
And the underlying theme here,
the underlying theme here,

00:17:23.373 --> 00:17:26.581
I used to think when I went though
my career, that only like math geeks,

00:17:26.581 --> 00:17:29.204
like me and Mark, we're
the only type of people

00:17:29.204 --> 00:17:31.257
that like to like understand things.

00:17:31.257 --> 00:17:34.451
And everyone else just wants to ...
but the takeaway here is that everyone,

00:17:34.451 --> 00:17:36.867
I don't care if whether you're
the math geek or whether you are

00:17:36.867 --> 00:17:39.189
the kid who's about to drop out,

00:17:39.189 --> 00:17:41.281
or you're the kid who thought
they hated mathematics.

00:17:41.281 --> 00:17:43.867
When you actually learn
something, really learn something,

00:17:43.867 --> 00:17:46.845
not just learn the steps
and jump through the hoops.

00:17:46.845 --> 00:17:50.523
When you really learn something,
it's arguably the highest high

00:17:50.523 --> 00:17:52.982
any human being can have.

00:17:52.982 --> 00:17:54.731
And you see it time ...
this is another one.

00:17:54.731 --> 00:17:56.389
This is an except.

00:17:56.389 --> 00:18:00.032
"My eldest kid is dancing around in
my room here because she is so excited

00:18:00.032 --> 00:18:03.206
"that she finally found someone
that teaches like this."

00:18:03.206 --> 00:18:06.173
She did a math problem,
and then she danced.

00:18:06.173 --> 00:18:12.371
(laughter)

00:18:12.371 --> 00:18:16.945
Now, this you know, I was working
as a analyst at a hedge fund,

00:18:16.945 --> 00:18:19.115
and the site traffic started to take off

00:18:19.115 --> 00:18:21.449
and I said, "Gee," you know I was
telling my wife "This is so exciting,

00:18:21.449 --> 00:18:24.469
"and I want to do this, and people are
writing me letters, but there's no,

00:18:24.469 --> 00:18:27.313
"it wasn't making ..."but you know I
said, "I have some savings, I could quit."

00:18:27.339 --> 00:18:29.623
I was really on the edge about this,
I said "I really want to do this,

00:18:29.623 --> 00:18:30.351
"but I can't figure out ... "

00:18:30.351 --> 00:18:32.626
And then I got this letter.

00:18:32.626 --> 00:18:35.469
And this letter, and I'll
read an excerpt of it.

00:18:35.469 --> 00:18:39.316
It to me, I mean, I was like if I only
got this letter this would be worth it,

00:18:39.316 --> 00:18:41.317
but this is, there could be
a million people like this,

00:18:41.317 --> 00:18:42.567
and this person's the only
person, I mean it's ...

00:18:42.567 --> 00:18:47.373
"Mr. Khan, no teacher has ever done
me any good. This may sound harsh,

00:18:47.373 --> 00:18:50.196
"but I mean it quite literally. I
was forced on medication to keep

00:18:50.196 --> 00:18:54.110
"from talking and chastised for
not speaking when called on.

00:18:54.110 --> 00:18:56.624
"About five years ago my family
collected enough money to move from

00:18:56.624 --> 00:18:58.950
"where I was born, so I could
have chance at having an education

00:18:58.950 --> 00:19:00.081
"and living a real life.

00:19:00.081 --> 00:19:03.464
"But without a real mastery of
elementary math I was slow to progress

00:19:03.464 --> 00:19:07.035
"I am now in college, and learning
more than I ever have in my life.

00:19:07.035 --> 00:19:09.724
"But an inadequate math background
has been holding me back,

00:19:09.724 --> 00:19:11.649
"I found the Khan
Academy in June of 2009."

00:19:11.649 --> 00:19:13.308
So it was like a year ago.

00:19:13.308 --> 00:19:16.356
"Right after I completed Math
141, a college Algebra course,

00:19:16.356 --> 00:19:18.873
"I spent the entire summer
on your YouTube page,

00:19:18.873 --> 00:19:21.282
"and I just wanted to thank you
for everything you're doing.

00:19:21.282 --> 00:19:22.622
"This is a God-send.

00:19:22.622 --> 00:19:25.227
"Last week I tested for a math
placement exam, and I'm now in

00:19:25.227 --> 00:19:26.574
Honors Math 200."

00:19:26.574 --> 00:19:28.480
Now this is the next sentence
is the one that really got me.

00:19:28.480 --> 00:19:29.569
This is on a placement exam.

00:19:29.569 --> 00:19:31.999
This is not on like the regular
exam you have in class where

00:19:31.999 --> 00:19:35.284
you know what you're studying for,
this is like all of mathematics.

00:19:35.284 --> 00:19:37.537
This is on the placement exam,
the entrance placement exam.

00:19:37.537 --> 00:19:41.285
"No question was answered
incorrectly on the placement exam."

00:19:41.285 --> 00:19:44.453
This was a kid who had basic
deficiencies in mathematics,

00:19:44.453 --> 00:19:46.705
and they watched YouTube
videos for a summer,

00:19:46.705 --> 00:19:49.286
and they did not get one question wrong.

00:19:49.286 --> 00:19:51.037
"I can say without any doubt,
you've changed my life,

00:19:51.037 --> 00:19:52.537
and the lives of everyone in my family."

00:19:52.537 --> 00:19:54.914
I mean you read that, and you're
like screw the hedge fund world.

00:19:54.914 --> 00:19:57.346
(applause)

00:19:57.346 --> 00:19:57.723
Good (unintelligible).

00:19:57.723 --> 00:19:58.558
Thank all of you all.

00:19:58.558 --> 00:20:02.558
(applause)


Khan Academy on PBS NewsHour (Edited)

WEBVTT
Kind: captions
Language: en

00:00:00.600 --> 00:00:03.010
JUDY WOODRUFF: Now the story
of a young man teaching

00:00:03.010 --> 00:00:04.610
math to the world.

00:00:04.610 --> 00:00:08.540
NewsHour correspondent
Spencer Michels reports.

00:00:08.540 --> 00:00:10.631
SALMAN KHAN: So this is
a quadratic equation,

00:00:10.631 --> 00:00:11.130
essentially.

00:00:11.130 --> 00:00:13.250
We're trying to
find the zeros of--

00:00:13.250 --> 00:00:15.010
SPENCER MICHELS: This
disembodied voice

00:00:15.010 --> 00:00:18.610
is heard every day by tens
of thousands of students

00:00:18.610 --> 00:00:23.300
struggling with math, both in
the US and around the world.

00:00:23.300 --> 00:00:26.550
33-year-old Salman Khan
recently quit his job

00:00:26.550 --> 00:00:29.160
as a hedge fund analyst
to devote himself

00:00:29.160 --> 00:00:33.400
to an unpaid job teaching
math on the internet.

00:00:33.400 --> 00:00:37.910
SALMAN KHAN: Welcome to the
presentation on basic addition.

00:00:37.910 --> 00:00:40.794
These are cancers,
or cancer cells.

00:00:40.794 --> 00:00:43.210
There's the investors and the
mortgage-backed securities--

00:00:43.210 --> 00:00:45.570
SPENCER MICHELS: He has
posted 1,200 lessons

00:00:45.570 --> 00:00:49.640
on YouTube, which appear
on an electronic blackboard

00:00:49.640 --> 00:00:51.870
and range in subject
from basic addition

00:00:51.870 --> 00:00:55.310
and advanced calculus
to science and finance.

00:00:55.310 --> 00:00:56.452
And they are free.

00:00:56.452 --> 00:00:58.160
SALMAN KHAN: Introduction
to trigonometry

00:00:58.160 --> 00:01:01.110
is where I teach people
about the trig functions.

00:01:01.110 --> 00:01:03.250
This is one of the
more popular videos.

00:01:03.250 --> 00:01:06.490
This has 179,000 views.

00:01:06.490 --> 00:01:10.630
And this is just what sine,
cosine, and tangent are.

00:01:10.630 --> 00:01:13.130
SPENCER MICHELS: Kahn lives
in California's Silicon Valley

00:01:13.130 --> 00:01:16.520
with his wife, a rheumatologist
in training at Stanford,

00:01:16.520 --> 00:01:17.880
and their new baby.

00:01:17.880 --> 00:01:21.230
He got the idea for Khan
Academy four years ago

00:01:21.230 --> 00:01:22.820
when he taught a
young cousin how

00:01:22.820 --> 00:01:25.500
to convert kilograms to grams.

00:01:25.500 --> 00:01:28.040
Many American students
have trouble with math,

00:01:28.040 --> 00:01:31.690
and studies show they lag behind
their counterparts in Asia

00:01:31.690 --> 00:01:34.330
and Europe in both
math and science.

00:01:34.330 --> 00:01:37.610
With Khan's help, his
cousin got good at math,

00:01:37.610 --> 00:01:39.880
and he eventually
had a new career

00:01:39.880 --> 00:01:43.340
tapping into anxieties
around the world.

00:01:43.340 --> 00:01:46.420
Now he records his lessons
from a converted closet

00:01:46.420 --> 00:01:48.260
in the back of his bedroom.

00:01:48.260 --> 00:01:49.662
He never goes on camera.

00:01:49.662 --> 00:01:52.120
SALMAN KHAN: It kind of feels
like the voice in their head.

00:01:52.120 --> 00:01:52.865
You're looking at it.

00:01:52.865 --> 00:01:54.910
It kind of feels like someone's
over your shoulder talking

00:01:54.910 --> 00:01:56.250
in your ear, as
opposed to someone

00:01:56.250 --> 00:01:57.910
at a blackboard that's
distant from you.

00:01:57.910 --> 00:02:00.076
SPENCER MICHELS: Although
he is not the first person

00:02:00.076 --> 00:02:02.690
to teach on the internet,
his simple operation

00:02:02.690 --> 00:02:06.550
has attracted more eyeballs
than most universities' sites.

00:02:06.550 --> 00:02:09.520
For Khan, teaching math
and science and finance

00:02:09.520 --> 00:02:11.050
is just the beginning.

00:02:11.050 --> 00:02:13.430
He's ready to expand
his YouTube site

00:02:13.430 --> 00:02:15.659
to include whatever
strikes his fancy.

00:02:15.659 --> 00:02:17.200
SALMAN KHAN: I want
to do everything.

00:02:17.200 --> 00:02:17.950
I want to do history.

00:02:17.950 --> 00:02:18.825
I want to do grammar.

00:02:18.825 --> 00:02:20.677
I want to do literally
every subject.

00:02:20.677 --> 00:02:23.010
SPENCER MICHELS: So far,
except for a little advertising

00:02:23.010 --> 00:02:26.430
revenue, he is donating
his time and equipment.

00:02:26.430 --> 00:02:28.570
It's a one-man
operation, but it's

00:02:28.570 --> 00:02:31.420
gotten so big he expects
to start soliciting

00:02:31.420 --> 00:02:35.760
outside support to keep
his academy growing.


Another Stoichiometry Example in a Solution

WEBVTT
Kind: captions
Language: en

00:00:00.500 --> 00:00:03.530
Let's do another stoichiometry
problem where the reaction is

00:00:03.530 --> 00:00:06.570
occurring inside
of a solution.

00:00:06.570 --> 00:00:09.550
So we have a metallic zinc and
it reacts with aqueous

00:00:09.550 --> 00:00:10.660
hydrochloric acid.

00:00:10.660 --> 00:00:13.270
We see that right there, the
metallic zinc. And then the

00:00:13.270 --> 00:00:16.980
hydrochloric acid is
dissolved in water,

00:00:16.980 --> 00:00:18.230
it's an aqueous solution.

00:00:18.230 --> 00:00:20.140
The solvent is water.

00:00:20.140 --> 00:00:22.310
Makes these products
right there.

00:00:22.310 --> 00:00:26.380
And they say, what volume of
0.25 molar-- so they're giving

00:00:26.380 --> 00:00:28.880
us the concentration
of the solution.

00:00:28.880 --> 00:00:32.640
What volume of 0.25 molar
hydrochloric acid in

00:00:32.640 --> 00:00:35.190
milliliters-- so we've got to
remember that, they want their

00:00:35.190 --> 00:00:40.850
answer in milliliters-- is
required to convert 11.8 grams

00:00:40.850 --> 00:00:44.910
of zinc completely
to products?

00:00:44.910 --> 00:00:46.880
So the way we can do this
we'll figure out.

00:00:46.880 --> 00:00:47.880
We know that we're
starting off with

00:00:47.880 --> 00:00:51.150
11.8 grams right there.

00:00:51.150 --> 00:00:53.880
And we just have to figure out
well, if we have 11.8 grams of

00:00:53.880 --> 00:00:56.950
zinc, how many moles
of zinc that is.

00:00:56.950 --> 00:01:00.000
And then, assuming this is a
balanced equation-- and I'm

00:01:00.000 --> 00:01:02.670
going to verify that right after
I finish this sentence--

00:01:02.670 --> 00:01:05.920
but assuming that this is a
balanced equation, for every

00:01:05.920 --> 00:01:08.170
mole of zinc we're going
to need two moles of

00:01:08.170 --> 00:01:09.350
hydrochloric acid.

00:01:09.350 --> 00:01:12.480
And then we can figure out
given how many moles of

00:01:12.480 --> 00:01:16.680
hydrochloric acid we need, then
how much of the solution

00:01:16.680 --> 00:01:18.870
do we need based on
its concentration?

00:01:18.870 --> 00:01:19.690
So let's do that.

00:01:19.690 --> 00:01:21.440
It's going to, I think,
be a lot simpler when

00:01:21.440 --> 00:01:22.350
we actually do it.

00:01:22.350 --> 00:01:25.540
But I just want to give you
the action plan before we

00:01:25.540 --> 00:01:26.280
break into it.

00:01:26.280 --> 00:01:28.520
But before we do anything,
let's verify that this is

00:01:28.520 --> 00:01:30.510
really a balanced equation.

00:01:30.510 --> 00:01:33.270
So you have 1 zinc on the
left-hand side, 1 zinc on the

00:01:33.270 --> 00:01:37.630
right-hand side, 2 hydrogens
on the left, 2 hydrogens on

00:01:37.630 --> 00:01:40.810
the right, 2 chlorines
on the left, 2

00:01:40.810 --> 00:01:42.140
chlorines on the right.

00:01:42.140 --> 00:01:43.200
So it looks balanced.

00:01:43.200 --> 00:01:44.490
Let's do the problem.

00:01:44.490 --> 00:01:47.220
So the first thing we want to
do-- we're starting off with

00:01:47.220 --> 00:01:56.865
11.8 grams of zinc, they give
us that information.

00:01:56.865 --> 00:01:58.650
Now we're going to figure
out how many moles

00:01:58.650 --> 00:02:01.790
of zinc that is.

00:02:01.790 --> 00:02:03.215
We're going to want to convert
this to moles.

00:02:03.215 --> 00:02:10.479
So it's going to be moles
of zinc over grams of

00:02:10.479 --> 00:02:12.880
zinc moles per gram.

00:02:12.880 --> 00:02:15.390
And if you look it up on a
periodic table, zinc has an

00:02:15.390 --> 00:02:21.700
atomic weight of 65.38, so
its molar mass is 65.38.

00:02:21.700 --> 00:02:29.100
So it's 1 mole for every 65.38
grams, or its 1/65, roughly,

00:02:29.100 --> 00:02:31.260
of a mole per gram.

00:02:34.700 --> 00:02:35.180
Actually, you know what?

00:02:35.180 --> 00:02:38.520
I'm just going to save our
calculator for the end.

00:02:38.520 --> 00:02:42.970
So the grams of zinc cancels
that with the grams of zinc,

00:02:42.970 --> 00:02:52.600
and we're left with 11.8 divided
by 65.38 moles of

00:02:52.600 --> 00:02:56.360
zinc.

00:02:56.360 --> 00:02:59.880
Now we're trying to figure out
how much of this solution of

00:02:59.880 --> 00:03:01.200
hydrochloric acid is required.

00:03:01.200 --> 00:03:03.260
So the next thing we want to
figure out is how many moles

00:03:03.260 --> 00:03:06.000
of hydrochloric acid
are required.

00:03:06.000 --> 00:03:09.450
And we see that for every mole
of zinc we need 2 moles of the

00:03:09.450 --> 00:03:11.830
hydrochloric acid.

00:03:11.830 --> 00:03:12.820
Let me write it this way.

00:03:12.820 --> 00:03:18.670
For every 1 mole of zinc--
we'll put that in the

00:03:18.670 --> 00:03:20.300
denominator.

00:03:20.300 --> 00:03:23.690
I'll switch colors just
arbitrarily here-- we need 2

00:03:23.690 --> 00:03:27.510
moles of hydrochloric acid.

00:03:27.510 --> 00:03:29.920
That is required.

00:03:29.920 --> 00:03:34.720
And so then we can multiply
the moles of zinc. We'll

00:03:34.720 --> 00:03:36.760
cancel out the moles of zinc.
That's why I wrote

00:03:36.760 --> 00:03:38.530
2/1 instead of 1/2.

00:03:38.530 --> 00:03:43.410
Two moles of hydrochloric acid
for every mole of zinc. And

00:03:43.410 --> 00:03:44.240
what does this give us?

00:03:44.240 --> 00:03:46.340
This gives us-- I'm not even
going to do the math.

00:03:46.340 --> 00:03:55.620
It's 11.8 times 2 over 65.38.

00:03:55.620 --> 00:03:58.480
And then we're left with moles
of hydrochloric acid

00:03:58.480 --> 00:04:03.060
required-- moles
of hydrochloric

00:04:03.060 --> 00:04:06.230
acid-- HCl-- required.

00:04:06.230 --> 00:04:07.820
So we haven't solved
the problem yet.

00:04:07.820 --> 00:04:10.530
We want to figure out
what volume of

00:04:10.530 --> 00:04:11.720
the solution we need.

00:04:11.720 --> 00:04:14.550
We know the actual number of
molecules we need, but we need

00:04:14.550 --> 00:04:17.170
to know how much solution
will contain

00:04:17.170 --> 00:04:19.519
that number of molecules.

00:04:19.519 --> 00:04:21.995
So we want to express
this in liters.

00:04:21.995 --> 00:04:23.570
Well, eventually we're going
to get to milliliters, but

00:04:23.570 --> 00:04:27.310
let's just go straight to liters
first because molarity

00:04:27.310 --> 00:04:29.580
is always expressed in liters.

00:04:29.580 --> 00:04:31.070
So let's multiply this.

00:04:31.070 --> 00:04:35.155
We want the moles of
HCl to cancel out.

00:04:35.155 --> 00:04:40.070
So let's put that in the
denominator, moles of

00:04:40.070 --> 00:04:42.290
hydrochloric acid.

00:04:42.290 --> 00:04:48.895
And we want liters of
hydrochloric acid, let me put

00:04:48.895 --> 00:04:53.900
it this way, liters
of 2.50 molar

00:04:53.900 --> 00:04:55.950
hydrochloric acid solution.

00:04:55.950 --> 00:04:59.430
When anyone puts this big M
there, that's molarity.

00:04:59.430 --> 00:05:01.720
You assume they're talking
about a solution because

00:05:01.720 --> 00:05:05.740
they're giving you the
concentration of the solute in

00:05:05.740 --> 00:05:07.700
the total solution.

00:05:07.700 --> 00:05:10.740
So how many moles are
there per liter?

00:05:10.740 --> 00:05:12.260
Well this concentration
is telling us.

00:05:12.260 --> 00:05:14.610
Molarity is moles per liter.

00:05:14.610 --> 00:05:22.810
So there are 2.50 moles for
every 1 liter of our solution.

00:05:22.810 --> 00:05:23.720
That's what that tells us.

00:05:23.720 --> 00:05:26.050
This is a 2.5 molar solution.

00:05:26.050 --> 00:05:28.400
Its molarity is 2.5.

00:05:28.400 --> 00:05:31.870
There are 2.52 moles
for every liter.

00:05:31.870 --> 00:05:34.030
And I wrote the moles in the
denominator because I wanted

00:05:34.030 --> 00:05:34.900
to cancel out here.

00:05:34.900 --> 00:05:37.020
So it's 1/2.5.

00:05:37.020 --> 00:05:39.280
So let's cancel those
two guys out.

00:05:39.280 --> 00:05:43.120
That cancels out with
that right there.

00:05:43.120 --> 00:05:47.470
And then we are left with 11.8
times 2 in the numerator.

00:05:51.910 --> 00:05:59.890
And in the denominator, we have
65.38 times 2-- let me

00:05:59.890 --> 00:06:02.120
write the times not as
a-- I'll write it

00:06:02.120 --> 00:06:04.690
that way-- times 2.50.

00:06:04.690 --> 00:06:13.870
And all we're left with in our
units is liters of 2.50 molar

00:06:13.870 --> 00:06:20.010
HCl solution-- and then in
that green-- required.

00:06:20.010 --> 00:06:22.060
We're almost done.

00:06:22.060 --> 00:06:23.620
Then we can get break out
the calculator and

00:06:23.620 --> 00:06:25.320
get the exact answer.

00:06:25.320 --> 00:06:27.280
They want the answer
in milliliters.

00:06:27.280 --> 00:06:29.180
So far we have the
answer in liters.

00:06:29.180 --> 00:06:30.810
I mean, there's going to be some
number here, I don't know

00:06:30.810 --> 00:06:31.480
what it is.

00:06:31.480 --> 00:06:35.960
But it's that number of liters
of the 2.5 molar hydrochloric

00:06:35.960 --> 00:06:40.370
acid solution is required.

00:06:40.370 --> 00:06:42.275
Let me just copy
and paste this.

00:06:42.275 --> 00:06:46.280
Let's convert this to
milliliters because that's

00:06:46.280 --> 00:06:47.530
what they want.

00:06:50.690 --> 00:06:52.930
So that's what we had from
the previous line.

00:06:52.930 --> 00:06:55.110
And to convert this to
milliliters-- we've done this

00:06:55.110 --> 00:07:01.440
drill before-- we want the
liters to cancel out.

00:07:01.440 --> 00:07:02.690
I'll just do it like this.

00:07:05.030 --> 00:07:07.870
And we want our answer
in milliliters.

00:07:07.870 --> 00:07:10.050
And how many milliliters
are there for a liter?

00:07:10.050 --> 00:07:14.850
Well there are 1,000 milliliters
per 1 liter.

00:07:14.850 --> 00:07:16.910
And all of these steps-- the way
I'm showing it to you, I'm

00:07:16.910 --> 00:07:18.270
making the dimensions
cancel out.

00:07:18.270 --> 00:07:23.080
But hopefully, they're making
some intuitive sense as well,

00:07:23.080 --> 00:07:29.360
that we divide by 2.5 at this
stage because we have more

00:07:29.360 --> 00:07:31.010
than 1 mole per liter.

00:07:31.010 --> 00:07:34.090
So however many moles we
require, we're going to have

00:07:34.090 --> 00:07:35.700
fewer liters required.

00:07:35.700 --> 00:07:37.930
So we're going to be dividing
by a number that's

00:07:37.930 --> 00:07:38.890
greater than 1.

00:07:38.890 --> 00:07:41.560
If that doesn't make sense to
you, just stick to the units.

00:07:41.560 --> 00:07:43.350
But at every one of these steps
I really want you to

00:07:43.350 --> 00:07:45.770
actually conceptualize
the numbers as well.

00:07:45.770 --> 00:07:47.560
But let's go back
to the problem.

00:07:47.560 --> 00:07:49.330
So we're converting
it to milliliters.

00:07:49.330 --> 00:07:53.860
So our final solution we're left
with, we have the liters.

00:07:53.860 --> 00:07:56.420
Cancelling out the liters, and
so we're left with, in the

00:07:56.420 --> 00:08:11.460
numerator, 11.8 times 2 times
1,000 over 65.38 times 2.50.

00:08:11.460 --> 00:08:14.880
And in the numerator we're left
with milliliters of 2.5

00:08:14.880 --> 00:08:16.920
molar hydrochloric acid.

00:08:20.310 --> 00:08:24.760
I'll just write it milliliters
of 2.50 molar

00:08:24.760 --> 00:08:25.980
hydrochloric acid.

00:08:25.980 --> 00:08:29.380
I don't want this 2.50 molar
HCl to confuse you.

00:08:29.380 --> 00:08:31.960
These aren't numbers in the
computation, this is a label

00:08:31.960 --> 00:08:33.760
to say what type of solution.

00:08:33.760 --> 00:08:38.840
We could have just written
over here liters of the

00:08:38.840 --> 00:08:40.370
solution in question.

00:08:40.370 --> 00:08:42.250
In case these numbers confuse
you, we could have written

00:08:42.250 --> 00:08:44.893
liters of the solution.

00:08:47.960 --> 00:08:52.060
And then this would be
milliliters of the solution.

00:08:52.060 --> 00:08:53.900
I don't want that 2.5
to confuse you.

00:08:53.900 --> 00:08:56.090
That's just the name of
the solution, or its

00:08:56.090 --> 00:08:56.800
concentration.

00:08:56.800 --> 00:08:58.390
That's how we're
identifying it.

00:08:58.390 --> 00:08:59.620
But now we've solved
the problem.

00:08:59.620 --> 00:09:04.020
We just need to calculate this
number, so let's break out the

00:09:04.020 --> 00:09:05.270
calculator.

00:09:07.630 --> 00:09:17.520
And you get 11.8 times 2 times
1,000 divided by 65.38.

00:09:17.520 --> 00:09:20.050
And we're also going
to divide by 2.5.

00:09:20.050 --> 00:09:23.210
It's in the denominator,
it's 144.

00:09:23.210 --> 00:09:26.080
And actually our lowest-- let's
see, a couple of numbers

00:09:26.080 --> 00:09:27.525
here only have three significant
digits, so we

00:09:27.525 --> 00:09:29.210
should really only stick to
three significant digits, so

00:09:29.210 --> 00:09:32.450
it's 144 milliliters.

00:09:32.450 --> 00:09:37.570
So this right here is 144
milliliters of, we could say

00:09:37.570 --> 00:09:40.490
the solution, or of
the 2.5 molar

00:09:40.490 --> 00:09:43.070
hydrochloric acid solution.

00:09:43.070 --> 00:09:43.930
And we're done.

00:09:43.930 --> 00:09:46.060
Hopefully you found that fun.


Stoichiometry of a Reaction in Solution

WEBVTT
Kind: captions
Language: en

00:00:00.380 --> 00:00:04.390
What we want to do in this video
is figure out the mass

00:00:04.390 --> 00:00:10.480
of calcium carbonate we need
to react completely with 25

00:00:10.480 --> 00:00:17.200
milliliters of 0.75 molar
hydrochloric acid solution.

00:00:17.200 --> 00:00:18.530
And we have the reaction here.

00:00:18.530 --> 00:00:21.520
And just to remind ourselves
what we're talking about we

00:00:21.520 --> 00:00:25.480
have this hydrochloric acid, and
it is dissolved in water.

00:00:25.480 --> 00:00:27.040
It is an aqueous solution.

00:00:27.040 --> 00:00:32.600
The solvent is water, and this
0.75 molar tells us the

00:00:32.600 --> 00:00:34.560
molarity of that solution.

00:00:34.560 --> 00:00:38.920
It tells us the concentration
or the moles of hydrochloric

00:00:38.920 --> 00:00:41.685
acid per liter of our
entire solution.

00:00:41.685 --> 00:00:45.880
And we need to figure out the
mass of this calcium carbonate

00:00:45.880 --> 00:00:48.900
that we need to react
completely with the

00:00:48.900 --> 00:00:50.590
hydrochloric acid solution.

00:00:50.590 --> 00:00:52.280
This amount of it.

00:00:52.280 --> 00:00:56.790
Now before we do any
stoichiometric problems, we

00:00:56.790 --> 00:01:00.150
have to make sure that our
actual equation is balanced.

00:01:00.150 --> 00:01:03.960
So let's do that first. So on
the left-hand side right here

00:01:03.960 --> 00:01:06.010
we've got a calcium.

00:01:06.010 --> 00:01:07.720
That's the only calcium.

00:01:07.720 --> 00:01:10.960
And on the right-hand side
we only have 1 calcium.

00:01:10.960 --> 00:01:13.660
So everything looks good from
the calcium point of view.

00:01:13.660 --> 00:01:15.620
On the left-hand side we
have a carbon here.

00:01:15.620 --> 00:01:16.390
I'll do it in pink.

00:01:16.390 --> 00:01:19.880
We have 1 carbon here, and then
we have no carbons here.

00:01:19.880 --> 00:01:22.570
And on the right-hand side
we only have 1 carbon.

00:01:22.570 --> 00:01:25.090
Everything looks good so far.

00:01:25.090 --> 00:01:28.000
On the left-hand side we
have 3 oxygens, no

00:01:28.000 --> 00:01:29.140
oxygens over here.

00:01:29.140 --> 00:01:32.060
And over here we have 1 oxygen
and then 2 more.

00:01:32.060 --> 00:01:33.460
We have 3 on the right-hand
side.

00:01:33.460 --> 00:01:36.490
Everything looks good so far.

00:01:36.490 --> 00:01:39.380
On the left-hand side we have
1 hydrogen right there.

00:01:39.380 --> 00:01:43.130
And then on the right-hand
side we have 2 hydrogens.

00:01:43.130 --> 00:01:44.660
We have 2 hydrogens here.

00:01:44.660 --> 00:01:48.020
So to balance it let me put 2--
let me put a 2 in front of

00:01:48.020 --> 00:01:50.310
the hydrochloric acid, the
aqueous solution of

00:01:50.310 --> 00:01:51.780
hydrochloric acid.

00:01:51.780 --> 00:01:55.250
Now the hydrogens balance out,
and we haven't done the

00:01:55.250 --> 00:01:58.570
chlorines yet, which is good,
because that would have messed

00:01:58.570 --> 00:02:00.430
up everything if they were
already balanced.

00:02:00.430 --> 00:02:00.810
But let's see.

00:02:00.810 --> 00:02:04.550
Now we have 2 chlorines
on the left-hand side.

00:02:04.550 --> 00:02:06.210
How many do we have on
the right-hand side?

00:02:06.210 --> 00:02:07.590
We have 2.

00:02:07.590 --> 00:02:11.580
So now this equation is balanced
after we threw that 2

00:02:11.580 --> 00:02:14.170
coefficient on the hydrochloric
acid.

00:02:14.170 --> 00:02:17.940
So now let's figure out what
mass of calcium carbonate do

00:02:17.940 --> 00:02:22.550
we need to react completely
with this much of 25

00:02:22.550 --> 00:02:27.350
milliliters of a 0.75 molar
hydrochloric acid solution.

00:02:27.350 --> 00:02:29.990
Now the whole way to think about
it is I've given-- this

00:02:29.990 --> 00:02:32.080
is how this problem is different
than everything else

00:02:32.080 --> 00:02:35.670
we've seen so far is that I
haven't given you the mass of

00:02:35.670 --> 00:02:36.890
hydrochloric acid.

00:02:36.890 --> 00:02:40.070
I haven't told you the moles
of hydrochloric acid.

00:02:40.070 --> 00:02:42.650
I've told you the amount
of solution containing

00:02:42.650 --> 00:02:45.350
hydrochloric acid and
the concentration.

00:02:45.350 --> 00:02:48.210
So what we're going to do is
take this information to

00:02:48.210 --> 00:02:51.820
figure out the moles of
hydrochloric acid in this

00:02:51.820 --> 00:02:55.420
reaction, how many moles do
we have in this solution.

00:02:55.420 --> 00:02:58.105
And then for every 2 moles of
hydrochloric acid we're going

00:02:58.105 --> 00:03:00.560
to need 1 mole of calcium
carbonate.

00:03:00.560 --> 00:03:01.650
You're going to see that.

00:03:01.650 --> 00:03:05.260
And then we just have to figure
out well, given that

00:03:05.260 --> 00:03:07.570
we're going to need half as many
moles here as there are

00:03:07.570 --> 00:03:09.690
here, how much mass is that.

00:03:09.690 --> 00:03:11.020
So let's do that.

00:03:11.020 --> 00:03:25.810
So we are starting off with the
25 milliliters of-- let me

00:03:25.810 --> 00:03:33.420
write this-- 0.750 molar HCl
solution, and let's convert

00:03:33.420 --> 00:03:37.770
this to moles of hydrochloric
acid.

00:03:37.770 --> 00:03:41.570
So the first thing we need to
think about is that molarity

00:03:41.570 --> 00:03:44.640
is always expressed in
moles per liter.

00:03:44.640 --> 00:03:46.540
This is expressed in milliliters
right now.

00:03:46.540 --> 00:03:48.440
So I want to get rid
of the milliliters.

00:03:48.440 --> 00:03:50.040
I want it to be in liters.

00:03:50.040 --> 00:03:54.560
So I want to multiply it by
liters-- I'll just write it as

00:03:54.560 --> 00:03:58.670
a capital L-- as liters
per milliliter.

00:03:58.670 --> 00:04:02.150
And we know we have 1 liter
per 1,000 milliliters, or

00:04:02.150 --> 00:04:05.330
there's 1/1000 of a liter per
milliliters, however you want

00:04:05.330 --> 00:04:06.080
to view it.

00:04:06.080 --> 00:04:08.820
That's going to cancel
out that milliliters.

00:04:08.820 --> 00:04:11.160
And then we want to convert--
we want to use this

00:04:11.160 --> 00:04:17.800
concentration information to
figure out the actual number

00:04:17.800 --> 00:04:21.700
of moles of hydrochloric acid
we're dealing with.

00:04:21.700 --> 00:04:37.440
So to do that we're going to
multiply it times 0.750 moles

00:04:37.440 --> 00:04:45.930
of hydrochloric acid per liter
of-- or I'll write this--

00:04:45.930 --> 00:04:52.815
0.750 molar hydrochloric
acid solution.

00:04:52.815 --> 00:04:55.310
So let me make sure
you understand

00:04:55.310 --> 00:04:56.010
what I'm doing here.

00:04:56.010 --> 00:04:59.005
I first want to get the 25
milliliters into liters, so

00:04:59.005 --> 00:05:01.450
I'm going to essentially divide
by 1,000 and the units

00:05:01.450 --> 00:05:02.090
will work out.

00:05:02.090 --> 00:05:04.790
We have a milliliters in the
numerator, milliliters in the

00:05:04.790 --> 00:05:07.680
denominator, we're left
with a liter on top.

00:05:07.680 --> 00:05:10.920
And then I want to convert
from the solution to the

00:05:10.920 --> 00:05:12.600
actual number of moles.

00:05:12.600 --> 00:05:15.080
And if something is a-- I could
put a 0 right here--

00:05:15.080 --> 00:05:22.220
0.75 molar, that means that
there is 0.75 moles of

00:05:22.220 --> 00:05:26.250
hydrochloric acid per liter
of the solution.

00:05:26.250 --> 00:05:27.770
This is just-- I'm just
saying you can

00:05:27.770 --> 00:05:29.340
call that the solution.

00:05:29.340 --> 00:05:30.950
And so this'll cancel out.

00:05:30.950 --> 00:05:34.550
We have the liters canceling
out with the liters.

00:05:34.550 --> 00:05:38.050
And then you can even say the
of 0.75 molar hydrochloric

00:05:38.050 --> 00:05:43.670
acid solution cancels out with
the of .750 molar hydrochloric

00:05:43.670 --> 00:05:44.870
acid solution.

00:05:44.870 --> 00:05:48.930
And then you're left with--
let me stay in the same

00:05:48.930 --> 00:06:04.740
color-- 25 times 1 times 0.750
divided by 1,000 moles of

00:06:04.740 --> 00:06:08.160
hydrochloric acid.

00:06:08.160 --> 00:06:10.170
And let's get our calculator
out to solve

00:06:10.170 --> 00:06:12.180
for this right here.

00:06:15.100 --> 00:06:23.250
So that is, let's see, 25 times
0.75 divided by 1,000 is

00:06:23.250 --> 00:06:28.140
equal to 0.01875.

00:06:28.140 --> 00:06:30.120
Let me write that down.

00:06:30.120 --> 00:06:38.040
So this is equal to 0.018--
if I want to stick with

00:06:38.040 --> 00:06:40.250
significant digits I really
don't have any more than three

00:06:40.250 --> 00:06:42.680
significant digits here, so I
really shouldn't have any more

00:06:42.680 --> 00:06:43.780
than three here.

00:06:43.780 --> 00:06:50.600
So I could say 0.018-- instead
of 75 I'll just round it-- 8

00:06:50.600 --> 00:06:55.470
moles of hydrochloric acid.

00:06:55.470 --> 00:06:59.100
And now we know that for every
2 moles of hydrochloric acid

00:06:59.100 --> 00:07:01.330
we need 1 mole of calcium
carbonate.

00:07:01.330 --> 00:07:05.520
So we want to know the moles of
calcium carbonate required.

00:07:05.520 --> 00:07:06.550
So let's do that.

00:07:06.550 --> 00:07:10.720
So let's multiply this times--
we want this thing

00:07:10.720 --> 00:07:11.740
to cancel out here.

00:07:11.740 --> 00:07:13.200
We want that to cancel out.

00:07:13.200 --> 00:07:17.980
So we want to know, so for every
mole of hydrochloric

00:07:17.980 --> 00:07:21.960
acid, or for every 2 moles of
hydrochloric acid, we see that

00:07:21.960 --> 00:07:25.460
there, for every 2 moles of
hydrochloric acid we need 1

00:07:25.460 --> 00:07:30.980
mole-- I'll do it in that same
orange color-- we need 1 mole

00:07:30.980 --> 00:07:36.510
of calcium carbonate
is required.

00:07:36.510 --> 00:07:42.030
And so these moles of
hydrochloric acid is going to

00:07:42.030 --> 00:07:44.200
cancel out with that moles
in the denominator.

00:07:44.200 --> 00:07:46.400
And we're essentially just going
to take this number and

00:07:46.400 --> 00:07:49.440
multiply it by 1 and divide
by 2, or just essentially

00:07:49.440 --> 00:07:51.510
divide it by 2.

00:07:51.510 --> 00:07:53.810
So let's do that.

00:07:53.810 --> 00:07:56.680
I could just take the original
number right here, divided by

00:07:56.680 --> 00:08:01.200
2-- I should only do it three
significant digits-- so it's

00:08:01.200 --> 00:08:07.920
really-- well I'll just stay
with this-- so 0, 0, 9, 3, 8

00:08:07.920 --> 00:08:10.320
just to round it.

00:08:10.320 --> 00:08:25.460
So this is equal to, we're left
with 0.00938 moles of

00:08:25.460 --> 00:08:29.740
calcium carbonate required.

00:08:29.740 --> 00:08:30.630
And we're not done yet.

00:08:30.630 --> 00:08:33.460
Remember, the question that we
asked is what's the mass of

00:08:33.460 --> 00:08:34.940
calcium carbonate required.

00:08:34.940 --> 00:08:37.240
So we need to figure out, we
need to essentially multiply

00:08:37.240 --> 00:08:40.159
this times the molar mass.

00:08:40.159 --> 00:08:43.850
So we want to convert this into
grams. Let's convert this

00:08:43.850 --> 00:08:47.040
into grams. I'll switch colors
just arbitrarily.

00:08:47.040 --> 00:08:49.770
And to put it into grams you
have to figure out the molar

00:08:49.770 --> 00:08:51.470
mass of calcium carbonate.

00:08:51.470 --> 00:08:53.690
And the molar mass of calcium--
you could look up

00:08:53.690 --> 00:08:55.390
the atomic weight of calcium
on a periodic

00:08:55.390 --> 00:08:57.520
table-- it is 40.

00:08:57.520 --> 00:09:00.240
And then you're going to have a
carbon in there, that's 12,

00:09:00.240 --> 00:09:01.370
we've seen that a lot.

00:09:01.370 --> 00:09:03.470
Then you have an oxygen,
which is 16, but we

00:09:03.470 --> 00:09:04.650
have three of them.

00:09:04.650 --> 00:09:09.350
So 16 times 3 is 48, so it's
going to be 40 plus 12 plus

00:09:09.350 --> 00:09:13.750
48, which is equal to 100.

00:09:13.750 --> 00:09:16.130
So it has a molar mass of 100.

00:09:16.130 --> 00:09:28.740
So times 100 grams of calcium
carbonate for every-- let me

00:09:28.740 --> 00:09:32.960
just write that-- grams of
calcium carbonate for every

00:09:32.960 --> 00:09:36.950
mole of calcium carbonate.

00:09:36.950 --> 00:09:38.970
And we know we're doing the
right thing by multiplying by

00:09:38.970 --> 00:09:40.430
100 because these units
cancel out.

00:09:40.430 --> 00:09:43.440
Moles of calcium carbonate in
the numerator, moles of

00:09:43.440 --> 00:09:45.120
calcium carbonate in
the denominator.

00:09:45.120 --> 00:09:47.120
So those cancel out.

00:09:47.120 --> 00:09:50.780
So we essentially just have to
multiply this thing times 100.

00:09:50.780 --> 00:09:51.960
So let's do that.

00:09:51.960 --> 00:09:54.050
So that thing times 100.

00:09:54.050 --> 00:09:59.000
So times 100 is equal to--
and we had rounded

00:09:59.000 --> 00:10:02.800
it-- so it's 0.938.

00:10:02.800 --> 00:10:03.670
So let me write that.

00:10:03.670 --> 00:10:08.070
0.938.

00:10:08.070 --> 00:10:16.660
So it equals 0.938 grams
of calcium carbonate.


Example of Finding Reactant Empirical Formula

WEBVTT
Kind: captions
Language: en

00:00:00.520 --> 00:00:04.370
This problem asks us, suppose
you isolate an acid from

00:00:04.370 --> 00:00:07.850
clover leaves and know that it
contains only the elements

00:00:07.850 --> 00:00:10.810
carbon, hydrogen, and oxygen.

00:00:10.810 --> 00:00:17.780
Heating 0.513 grams of the
acid in oxygen produces

00:00:17.780 --> 00:00:20.860
points-- let me write these
down or let me underline

00:00:20.860 --> 00:00:27.020
them-- so heating 0.513 grams of
the acid in oxygen produces

00:00:27.020 --> 00:00:34.960
0.501 grams of carbon dioxide
and 0.103 grams of water.

00:00:34.960 --> 00:00:38.130
What is the empirical formula
for the acid?

00:00:38.130 --> 00:00:39.960
So let's just write
the equation here.

00:00:39.960 --> 00:00:42.280
And once again, you can't
balance this equation because

00:00:42.280 --> 00:00:44.860
we don't know how many carbons
or hydrogens or oxygens we

00:00:44.860 --> 00:00:45.700
have in the acid.

00:00:45.700 --> 00:00:47.890
But let's just write it
down in general terms.

00:00:47.890 --> 00:00:50.610
So we have the acid.

00:00:50.610 --> 00:00:57.210
It has x carbons, y hydrogens,
and z oxygens, and we're going

00:00:57.210 --> 00:01:01.760
to heat it in the presence of
an abundance of oxygen.

00:01:01.760 --> 00:01:05.610
And when we do that the problem
tells us that this

00:01:05.610 --> 00:01:08.710
produces-- let me do it in
another color-- this produces

00:01:08.710 --> 00:01:14.450
some carbon dioxide
and some water.

00:01:17.020 --> 00:01:21.260
And they tell us the amounts
of carbon dioxide and water

00:01:21.260 --> 00:01:23.690
measured in grams, and they
tell us the amount of our

00:01:23.690 --> 00:01:24.630
original substance.

00:01:24.630 --> 00:01:25.520
Let me write that here.

00:01:25.520 --> 00:01:31.630
So we're heating 0.513 grams
of our original substance.

00:01:31.630 --> 00:01:41.880
And then that produces 0.501
grams of carbon dioxide, and

00:01:41.880 --> 00:01:48.080
0.103 grams of water.

00:01:48.080 --> 00:01:50.620
Now we're going to do this
problem very similar to the

00:01:50.620 --> 00:01:51.490
way that we've done
the last one.

00:01:51.490 --> 00:01:53.590
Remember, we just have figure
out the empirical formula.

00:01:53.590 --> 00:01:56.120
We just have to figure out the
ratio, the simplest whole

00:01:56.120 --> 00:02:00.310
number ratios of carbon to
hydrogens to oxygens.

00:02:00.310 --> 00:02:03.020
The only thing that makes this
slightly more difficult than

00:02:03.020 --> 00:02:06.360
the last one is we have oxygens
here, and the oxygens

00:02:06.360 --> 00:02:09.220
are kind of mixed in with
the two products.

00:02:09.220 --> 00:02:12.260
So we can't just look at the
moles of our two products and

00:02:12.260 --> 00:02:14.685
say oh, we must have this
many moles of oxygen.

00:02:14.685 --> 00:02:16.730
What we're going to do here is
we're going to figure out the

00:02:16.730 --> 00:02:22.780
moles of carbon dioxide using
this information there.

00:02:22.780 --> 00:02:26.120
And that'll tell us the moles of
carbon that must have come

00:02:26.120 --> 00:02:28.970
out of our mystery acid, because
this was the only

00:02:28.970 --> 00:02:30.230
source of carbon
that might have

00:02:30.230 --> 00:02:31.990
generated the carbon dioxide.

00:02:31.990 --> 00:02:34.370
Then we're going to use the
water to figure out the moles

00:02:34.370 --> 00:02:37.200
of hydrogen in the product,
which has to be the same as

00:02:37.200 --> 00:02:39.720
the moles of hydrogen in
our mystery substance.

00:02:39.720 --> 00:02:44.000
And then we can figure out the
mass of carbon and hydrogen

00:02:44.000 --> 00:02:47.610
out of this 0.513 grams, we'll
be able to figure out because

00:02:47.610 --> 00:02:50.000
we'll know how many moles of
carbon and how many moles of

00:02:50.000 --> 00:02:51.450
hydrogen we started off with.

00:02:51.450 --> 00:02:53.420
We'll be able to know their
mass, and then we'll be able

00:02:53.420 --> 00:02:56.880
figure out how many grams of
oxygen we must have had in our

00:02:56.880 --> 00:02:57.790
original substance.

00:02:57.790 --> 00:02:59.880
And then we can figure out
the moles from there.

00:02:59.880 --> 00:03:01.430
And then figure out
the ratios.

00:03:01.430 --> 00:03:04.100
If that confused you, hopefully
just solving through

00:03:04.100 --> 00:03:06.190
it will make a lot more sense.

00:03:06.190 --> 00:03:09.810
So first let's just figure out
the moles of carbon in our

00:03:09.810 --> 00:03:12.150
product, and the only product
here that had carbon was the

00:03:12.150 --> 00:03:13.200
carbon dioxide.

00:03:13.200 --> 00:03:16.380
And that'll tell us how many
moles of carbon we had in our

00:03:16.380 --> 00:03:18.940
original mystery
acid over here.

00:03:18.940 --> 00:03:26.310
So we are end up with 0.501
grams of carbon dioxide-- that

00:03:26.310 --> 00:03:27.430
tells us right there.

00:03:27.430 --> 00:03:30.500
And we want to convert this into
moles of carbon dioxide.

00:03:30.500 --> 00:03:36.550
So we want to multiply this
times moles of carbon dioxide

00:03:36.550 --> 00:03:39.940
per grams of carbon dioxide.

00:03:39.940 --> 00:03:42.750
And if you care about
dimensional analysis you know

00:03:42.750 --> 00:03:46.160
that because these grams of
carbon dioxide will cancel

00:03:46.160 --> 00:03:48.580
with these grams of
carbon dioxide.

00:03:48.580 --> 00:03:53.000
So how many grams is one
mole of carbon dioxide?

00:03:53.000 --> 00:03:55.000
And we've done this
multiple times.

00:03:55.000 --> 00:03:58.760
The molar mass of carbon
dioxide-- you have 1 carbon

00:03:58.760 --> 00:04:02.430
plus 2 oxygens, so 16 times 2.

00:04:02.430 --> 00:04:06.230
That is 12 plus 32 or 44.

00:04:06.230 --> 00:04:11.310
So one mole of carbon dioxide
has 44 grams of carbon

00:04:11.310 --> 00:04:14.610
dioxide, or 44 grams has 1 mole.
but we wrote it moles

00:04:14.610 --> 00:04:17.269
per gram because we want to
cancel out with the grams

00:04:17.269 --> 00:04:18.579
right there.

00:04:18.579 --> 00:04:25.300
So let's cancel that out, and
you're going to get 0.501

00:04:25.300 --> 00:04:28.760
divided by 44 moles
of carbon dioxide.

00:04:28.760 --> 00:04:30.540
Let's get the calculator out.

00:04:30.540 --> 00:04:39.970
So we've got 0.501 divided by
44 is equal to 0.-- we have

00:04:39.970 --> 00:04:42.265
three significant digits here,
and this is just a pure number

00:04:42.265 --> 00:04:46.190
or pretty close to a pure
number-- so 0.11-- I'll round

00:04:46.190 --> 00:04:51.370
it up to 4-- let me put it on
the side so I don't forget

00:04:51.370 --> 00:05:04.160
this-- so this tells us that we
have 0.0114 moles of carbon

00:05:04.160 --> 00:05:07.340
dioxide in our product.

00:05:07.340 --> 00:05:10.280
Now that also tells us for
every molecule of carbon

00:05:10.280 --> 00:05:13.560
dioxide we also have
one atom of carbon.

00:05:13.560 --> 00:05:16.730
So however many carbon dioxide
molecules we have, we have

00:05:16.730 --> 00:05:18.390
that same number of carbon.

00:05:18.390 --> 00:05:20.970
Remember, a mole is
just a number.

00:05:20.970 --> 00:05:25.570
When I say 0.0114 moles I'm
just telling you a number.

00:05:25.570 --> 00:05:30.850
So this tells me right there--
I'll stay in blue-- this tells

00:05:30.850 --> 00:05:37.660
me that we also have 0.0114
moles of carbon.

00:05:37.660 --> 00:05:39.270
That's what that tells me,
because for every carbon

00:05:39.270 --> 00:05:40.600
dioxide I have a carbon.

00:05:40.600 --> 00:05:43.460
And the carbons in the product
all had to come from my

00:05:43.460 --> 00:05:44.950
mystery substance.

00:05:44.950 --> 00:05:48.460
So how many grams of this
mystery substance must have

00:05:48.460 --> 00:05:49.410
been carbon?

00:05:49.410 --> 00:05:51.830
Well I had this many
moles of carbon in

00:05:51.830 --> 00:05:54.100
our original reactant.

00:05:54.100 --> 00:05:55.800
They all end up over here.

00:05:55.800 --> 00:05:59.200
Let's figure out their grams. So
let's multiply-- so times--

00:05:59.200 --> 00:06:01.660
now here we want to do grams per
mole, because we want to

00:06:01.660 --> 00:06:04.160
end up in grams. We want to
cancel out the moles.

00:06:04.160 --> 00:06:09.630
We want grams of carbon
per mole of carbon.

00:06:12.880 --> 00:06:16.630
And the molar mass of
carbon is 12-- it's

00:06:16.630 --> 00:06:17.330
a good one to memorize.

00:06:17.330 --> 00:06:21.170
So we have 12 grams of carbon
per mole of carbon-- we've got

00:06:21.170 --> 00:06:22.880
12 over 1 if you like.

00:06:22.880 --> 00:06:24.300
So what does this equal?

00:06:24.300 --> 00:06:27.280
How many grams of carbon are
we dealing with in this

00:06:27.280 --> 00:06:29.150
mystery substance right there?

00:06:29.150 --> 00:06:31.450
Get the calculator back.

00:06:31.450 --> 00:06:34.160
So if we multiply-- it's
actually this exact same

00:06:34.160 --> 00:06:45.720
number, let's just multiply it
by 12-- so times 12 is 0.137.

00:06:45.720 --> 00:06:47.070
So let me write that down.

00:06:47.070 --> 00:06:59.560
So we have 0.137 grams
of carbon in

00:06:59.560 --> 00:07:01.500
our original reactant.

00:07:01.500 --> 00:07:06.010
So of this 0.513 grams
of this mystery acid,

00:07:06.010 --> 00:07:09.570
0.137 grams are carbon.

00:07:09.570 --> 00:07:12.010
So that's what we got so far,
so we have two good

00:07:12.010 --> 00:07:12.530
take-aways.

00:07:12.530 --> 00:07:15.590
How many moles of carbon we had
over here, and how many

00:07:15.590 --> 00:07:18.180
grams of carbon that
translates to.

00:07:18.180 --> 00:07:21.880
Now let's do the same thing
for the hydrogen.

00:07:21.880 --> 00:07:23.140
Same drill.

00:07:23.140 --> 00:07:24.680
Let's do the hydrogen--
we're using that

00:07:24.680 --> 00:07:26.260
information right there.

00:07:26.260 --> 00:07:36.020
So we're starting off with 0.103
grams of water, and we

00:07:36.020 --> 00:07:38.060
want to turn this into
moles of water.

00:07:38.060 --> 00:07:44.270
So let's multiply it times
moles of water

00:07:44.270 --> 00:07:47.180
per grams of water.

00:07:47.180 --> 00:07:50.470
Once again, to cancel out the
grams of water, numerator and

00:07:50.470 --> 00:07:51.790
denominator.

00:07:51.790 --> 00:07:54.660
And what is the molar
mass of water?

00:07:54.660 --> 00:07:58.340
You have two hydrogens, so it's
2 times 1 plus the molar

00:07:58.340 --> 00:08:02.890
mass of oxygen, which is
16, so it equals 18.

00:08:02.890 --> 00:08:10.040
So 1 mole of water has 18 grams,
or 18 grams per mole.

00:08:10.040 --> 00:08:12.170
But we wrote 18 in the
denominator, because we want

00:08:12.170 --> 00:08:14.070
the grams into the
denominator.

00:08:14.070 --> 00:08:19.800
So these cancel out and
we have 0.103 divided

00:08:19.800 --> 00:08:22.090
by 18 moles of water.

00:08:22.090 --> 00:08:24.170
Let me get the calculator
back.

00:08:24.170 --> 00:08:31.640
So we have 0.103 divided
by 18 moles of water.

00:08:31.640 --> 00:08:33.460
Let me put this on the side
so I can still see

00:08:33.460 --> 00:08:36.659
it, very bad memory.

00:08:36.659 --> 00:08:44.900
So this tells us that
we have 0.00572

00:08:44.900 --> 00:08:50.600
moles of H2O, of water.

00:08:50.600 --> 00:08:55.340
We just converted these grams
into the number of molecules

00:08:55.340 --> 00:08:56.700
that we actually have.

00:08:56.700 --> 00:09:00.890
Now if we have this many moles
of H2O, how many moles of

00:09:00.890 --> 00:09:02.360
hydrogen do we have?

00:09:02.360 --> 00:09:04.850
For every molecule of
water we have two

00:09:04.850 --> 00:09:06.400
molecules of hydrogen.

00:09:06.400 --> 00:09:11.000
So you multiply that times 2--
so let me do that right here--

00:09:11.000 --> 00:09:23.380
so 0.00572 moles of H2O times,
we have 2 moles of hydrogen

00:09:23.380 --> 00:09:29.320
for every one mole, I
should say, of H2O.

00:09:29.320 --> 00:09:31.740
So we're just going to multiply
this by 2 to get

00:09:31.740 --> 00:09:32.795
moles of hydrogen.

00:09:32.795 --> 00:09:36.420
So let me get back the--
let's multiply it by 2.

00:09:36.420 --> 00:09:43.600
So times 2 is equal to 0.114.

00:09:43.600 --> 00:09:47.880
So this is equal to-- let
me get that back.

00:09:47.880 --> 00:09:50.270
I always forget how many
zeros there are, so

00:09:50.270 --> 00:09:51.740
I remember it now.

00:09:51.740 --> 00:10:02.470
OK, so it is equal to 0.0114
moles of hydrogen.

00:10:02.470 --> 00:10:05.965
So so far we figured out the
number of moles of carbon in

00:10:05.965 --> 00:10:09.120
our product and its mass that
must have been in the original

00:10:09.120 --> 00:10:10.570
reactant, the mystery
reactant.

00:10:10.570 --> 00:10:12.550
And we've done the moles
for the hydrogen.

00:10:12.550 --> 00:10:16.550
We have this many moles
of hydrogen.

00:10:16.550 --> 00:10:17.870
So we must have started
off with that

00:10:17.870 --> 00:10:19.110
many moles over here.

00:10:19.110 --> 00:10:21.470
Let's figure out the
mass of that.

00:10:21.470 --> 00:10:24.230
So what is the molar-- we want
to figure out this in grams--

00:10:24.230 --> 00:10:27.590
so times-- the molar mass
of hydrogen, it

00:10:27.590 --> 00:10:29.650
is 1 gram per mole.

00:10:29.650 --> 00:10:33.520
So hydrogen's easy--
times 1 gram of

00:10:33.520 --> 00:10:37.000
hydrogen per mole of hydrogen.

00:10:37.000 --> 00:10:39.350
I wanted the moles in the
denominator so it cancels out.

00:10:39.350 --> 00:10:41.110
Moles cancel with moles.

00:10:41.110 --> 00:10:44.710
So we also have-- I'll do it in
that same orange color-- we

00:10:44.710 --> 00:10:51.160
also have 0.0114 grams
of hydrogen.

00:10:51.160 --> 00:10:54.240
So that's another important
take-away.

00:10:54.240 --> 00:10:57.950
Now we have moles of carbon,
moles of hydrogen.

00:10:57.950 --> 00:11:01.900
If we can figure out moles of
oxygen in our original acid

00:11:01.900 --> 00:11:04.720
then we can figure out the
smallest whole number ratio

00:11:04.720 --> 00:11:08.580
between them, then we would
be able to find

00:11:08.580 --> 00:11:10.300
our empirical formula.

00:11:10.300 --> 00:11:13.580
And to figure out moles of
oxygen we need to first figure

00:11:13.580 --> 00:11:15.850
out how many grams of
oxygen we have here.

00:11:15.850 --> 00:11:19.740
And to do that we have a total
of 0.513 grams of this thing.

00:11:19.740 --> 00:11:22.840
We subtract out how much of it
is carbon, how much of it is

00:11:22.840 --> 00:11:26.510
hydrogen, and that'll tell
us how much is oxygen.

00:11:26.510 --> 00:11:28.690
Let me scroll down
a little bit.

00:11:28.690 --> 00:11:33.350
So if we want to figure out
how much is oxygen, we

00:11:33.350 --> 00:11:40.440
literally just take 0.5-- let me
write 0.513 grams-- let me

00:11:40.440 --> 00:11:45.490
just write this-- minus
the grams of carbon.

00:11:45.490 --> 00:11:49.900
So minus 0.137-- let me do in
that same color so you know

00:11:49.900 --> 00:11:53.450
where I'm getting these numbers
from-- minus our grams

00:11:53.450 --> 00:11:59.580
of carbon, 0.137 grams of carbon
minus-- I'll do it in

00:11:59.580 --> 00:12:03.130
the same orange color--
minus the-- where's

00:12:03.130 --> 00:12:04.690
our grams of hydrogen?

00:12:04.690 --> 00:12:12.280
Minus 0.011-- let me, just so
you get the whole idea.

00:12:12.280 --> 00:12:18.640
We started off with 513 grams
of acid, whatever it is,

00:12:18.640 --> 00:12:28.240
minus-- let me write it this way
even better-- CxHyOz minus

00:12:28.240 --> 00:12:38.600
our 0.137 grams of carbon,
minus our 0.0114 grams of

00:12:38.600 --> 00:12:42.750
hydrogen, will give us our grams
of oxygen that we're

00:12:42.750 --> 00:12:43.850
dealing with.

00:12:43.850 --> 00:12:45.980
So let's figure that out.

00:12:45.980 --> 00:12:47.990
Because everything in that
substance, everything in this

00:12:47.990 --> 00:12:49.810
mystery acid right here has
to be either carbon,

00:12:49.810 --> 00:12:50.980
hydrogen, or oxygen.

00:12:50.980 --> 00:12:52.820
We know how much is carbon
and hydrogen, the

00:12:52.820 --> 00:12:55.020
leftover has to be oxygen.

00:12:55.020 --> 00:13:07.220
So we have 0.513 minus 0.137
minus 0.0114 is equal to

00:13:07.220 --> 00:13:11.840
0.36-- I'll just round
it-- 0.365.

00:13:11.840 --> 00:13:21.570
So this is equal to 0.365
grams of oxygen.

00:13:21.570 --> 00:13:24.780
Let me make sure that
I got that right.

00:13:24.780 --> 00:13:28.320
Yeah, 0.365 grams of oxygen.

00:13:28.320 --> 00:13:29.910
So let's figure out how
many moles this is.

00:13:29.910 --> 00:13:37.990
So we're starting off, or this
mystery acid has 0.365 grams

00:13:37.990 --> 00:13:40.960
of oxygen, and we want to
convert that to moles.

00:13:40.960 --> 00:13:45.480
So times-- we want moles of
oxygen in the numerator, grams

00:13:45.480 --> 00:13:47.800
of oxygen in the denominator,
so they cancel.

00:13:47.800 --> 00:13:50.040
And we know the molar mass
for oxygen is 16.

00:13:50.040 --> 00:13:54.390
16 grams per mole-- right, the
16 has to match up with the

00:13:54.390 --> 00:14:00.420
grams. So this tells us that
we have 0.365 divided by 16

00:14:00.420 --> 00:14:02.310
grams of oxygen.

00:14:02.310 --> 00:14:03.780
So that's the number
right there.

00:14:03.780 --> 00:14:06.670
I'll just divide it by 16.

00:14:06.670 --> 00:14:12.640
So 0.22, I'll say 8
just to round it.

00:14:12.640 --> 00:14:17.260
Or 0.0228-- let me put it aside,
I want to make sure I

00:14:17.260 --> 00:14:18.960
don't lose any zeros here.

00:14:18.960 --> 00:14:26.440
So this tells us that we have
0.0228-- these guys cancel

00:14:26.440 --> 00:14:30.430
out-- moles of oxygen.

00:14:30.430 --> 00:14:31.790
We're almost done.

00:14:31.790 --> 00:14:37.400
We now know, we've now been
able to solve the moles of

00:14:37.400 --> 00:14:42.150
carbon in our original reactant
because all of this

00:14:42.150 --> 00:14:43.840
carbon had to come from here.

00:14:43.840 --> 00:14:46.195
We have our moles of hydrogen.

00:14:49.280 --> 00:14:51.260
We were able to figure out from
this information because

00:14:51.260 --> 00:14:53.040
all the hydrogen had
to come from here.

00:14:53.040 --> 00:14:55.050
And now we know the
moles of oxygen in

00:14:55.050 --> 00:14:56.700
our original reactant.

00:14:56.700 --> 00:14:59.760
Our moles, because the leftover
out of this 0.513

00:14:59.760 --> 00:15:02.580
grams had to be oxygen, so we
were able to figure out the

00:15:02.580 --> 00:15:04.920
moles of oxygen.

00:15:04.920 --> 00:15:07.100
So let's see if we can figure
out the smallest whole number

00:15:07.100 --> 00:15:09.230
ratio between the three.

00:15:09.230 --> 00:15:13.200
So the ratio-- and I'll write
the ratio like this-- we

00:15:13.200 --> 00:15:18.870
have-- let me do it in the same
colors-- we have 0.114

00:15:18.870 --> 00:15:24.690
moles of carbon, to--
and then we have

00:15:24.690 --> 00:15:30.380
0.0114 moles of hydrogen.

00:15:30.380 --> 00:15:32.750
These look nice, they're
actually the same numbers.

00:15:32.750 --> 00:15:38.280
So we're going to have a 1:1
ratio, to-- then we have the

00:15:38.280 --> 00:15:45.840
oxygen, 0.0228 moles
of oxygen.

00:15:45.840 --> 00:15:48.180
And this looks like exactly
twice times that.

00:15:48.180 --> 00:15:51.160
So if we divide everything, if
we divide-- this is a ratio,

00:15:51.160 --> 00:15:54.190
this is another way of writing
the ratio-- if we divide

00:15:54.190 --> 00:16:02.260
everything by 0.114-- let
me be careful here.

00:16:02.260 --> 00:16:05.710
This up here was 0.0114
moles of carbon.

00:16:05.710 --> 00:16:08.310
So this was a 0 over here.

00:16:08.310 --> 00:16:10.170
Let me clear it out.

00:16:10.170 --> 00:16:13.942
I want to be nice
and neat here.

00:16:13.942 --> 00:16:17.260
So let me clear this.

00:16:17.260 --> 00:16:19.530
I'm always afraid that I'm going
to lose a zero here.

00:16:19.530 --> 00:16:24.590
Our moles of carbon
was 0.0114.

00:16:24.590 --> 00:16:27.530
So let's divide everything by
this amount because that is

00:16:27.530 --> 00:16:30.000
kind of the smallest number of
moles, and that's exactly that

00:16:30.000 --> 00:16:30.910
number there.

00:16:30.910 --> 00:16:32.190
Do that in white.

00:16:32.190 --> 00:16:43.640
So you divide by 0.0114,
0.0114, 0.0114,

00:16:43.640 --> 00:16:45.140
and what do you get?

00:16:45.140 --> 00:16:47.030
What does this turn into?

00:16:47.030 --> 00:16:51.810
This turns into-- this is just
a 1-- so this is 1 mole of

00:16:51.810 --> 00:16:55.350
carbon for every-- this is
just a 1 right here-- for

00:16:55.350 --> 00:16:59.110
every-- I'll do it in the orange
color-- for every 1

00:16:59.110 --> 00:17:05.849
mole of hydrogen-- for every--
this is exactly 2, right?

00:17:05.849 --> 00:17:07.410
228 divided by 140, that's 2.

00:17:07.410 --> 00:17:13.480
This is exactly 2-- for every
2 moles of oxygen.

00:17:13.480 --> 00:17:16.400
So that tells us our empirical
formula for our mystery acid.

00:17:16.400 --> 00:17:19.920
It must be-- we don't know the
molecular formula, but we know

00:17:19.920 --> 00:17:24.619
the simplest ratios, whole
number ratios-- it must be 1

00:17:24.619 --> 00:17:29.110
carbon for every 1 hydrogen, and
for every 1 carbon and 1

00:17:29.110 --> 00:17:31.800
hydrogen we must
have 2 oxygens.

00:17:31.800 --> 00:17:34.540
So it's CHO2.

00:17:34.540 --> 00:17:38.660
That is the empirical formula
for our mystery acid.

00:17:38.660 --> 00:17:40.830
Hopefully you found that fun.