Conjugate acids and bases

WEBVTT
Kind: captions
Language: en

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We've seen a reasonable
number of acid

00:00:03.890 --> 00:00:05.390
reactions and base reactions.

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So let's just write down a few
of them for review and let's

00:00:08.160 --> 00:00:10.450
see if we can see a general
pattern here.

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And a lot of this might not
be any news to you.

00:00:12.630 --> 00:00:16.990
So if we have hydrogen flouride,
or if it's in an

00:00:16.990 --> 00:00:21.110
aqueous solution it's
hydrofluoric acid.

00:00:21.110 --> 00:00:24.320
We know that this is a weak
acid-- it doesn't disassociate

00:00:24.320 --> 00:00:24.850
completely.

00:00:24.850 --> 00:00:26.930
So it's in equilibrium.

00:00:26.930 --> 00:00:28.090
Some type of equilibrium.

00:00:28.090 --> 00:00:30.860
That doesn't mean the
concentrations are equal.

00:00:30.860 --> 00:00:32.110
This hydrogen disassociates.

00:00:35.610 --> 00:00:38.080
Actually, we know in reality,
it tags a ride along with

00:00:38.080 --> 00:00:40.700
another water molecule
and forms hydronium.

00:00:40.700 --> 00:00:44.760
And then you have-- and, of
course, this is still aqueous.

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Everything is going
on inside water.

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And then you have left over
your fluoride anion, or

00:00:53.090 --> 00:00:53.940
negative ion.

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And that's also in an
aqueous solution.

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And we could have rewritten--
actually, let me write another

00:01:00.000 --> 00:01:02.210
reaction here, just so you can
see the general pattern.

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Let me write another
acidic reaction.

00:01:05.500 --> 00:01:08.720
Let me write ammonium.

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So that's NH4 plus-- it's
ammonia with an extra hydrogen

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in an-- let me just write that--
in an aqueous solution.

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That can disassociate to one of
those hydrogens popping off

00:01:29.400 --> 00:01:31.790
in an aqueous solution.

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And then you have
ammonia, NH3.

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That's also in an aqueous
solution.

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Now both of these describe an
equilibrium reaction, but it

00:01:42.480 --> 00:01:45.110
kind of implies that we're
dealing with weak acids.

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You take an acid, and they're
producing hydrogen, which is

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at least the Arrhenius
definition of an acid if you

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looked at the Bronsted-Lowry
definition where they're

00:01:53.560 --> 00:01:55.920
donating protons to
the solution.

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They're creating hyrdonium,
they're donating protons to

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the water around it.

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But it kind of describes
it as an acid.

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But we know it's a weak acid,
so this reaction goes in two

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directions.

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So we can write the same
reaction essentially as a

00:02:08.310 --> 00:02:10.050
basic reaction.

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So we can-- instead of saying
hydrofluoric acid is our

00:02:13.550 --> 00:02:17.790
acid-- we could say hey, if I
just have a fluoride anion, if

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I just have a negative fluoride
here, I could say a

00:02:22.140 --> 00:02:27.430
negative fluoride, if I put
that in-- actually, I keep

00:02:27.430 --> 00:02:30.370
making that same mistake.

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The fluorine does not
have an l in it.

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I do that because
chloride does.

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Let me erase this.

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Hydrogen fluoride is HF.

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So it's just F there.

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Let me go to the
periodic table.

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See, I always confused fluorine
with chlorine because

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F is just for flourine.

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But you get the point.

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OK.

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So I could rewrite the same
weak acid equilibrium as a

00:02:55.260 --> 00:02:56.730
weak base equilibrium.

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Or I could say a negative
fluorine anion in an aqueous

00:03:01.150 --> 00:03:06.610
solution is in equilibrium
with-- and now we're saying

00:03:06.610 --> 00:03:09.810
I'm considering this a base,
which means that it's going to

00:03:09.810 --> 00:03:11.800
increase the concentration
of [UNINTELLIGIBLE].

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So what this might want to do is
it might want to grab some

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hydrogen from some of
the water that's

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in the aqueous solution.

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So it grabs some hydrogen and
becomes hydrogen flouride, or

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hydrofluoric acid.

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Let me do that in that
magenta color.

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It's aqueous.

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And where did it get
this hydrogen from?

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Well, it got it from one of
the surrounding water

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molecules, which was H2O.

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Since it gave away one
of the hydrogens now

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it's just OH minus.

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So the surrounding water
molecule is OH minus aqueous.

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Now these might look
different.

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This is donating a hydrogen to
the surrounding medium, and

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then you're left with just
the fluorine molecule.

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Well, this is essentially
creating a hydroxide molecule

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out of the surrounding medium so
it looks basic, but if you

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think about it, these reactions
are the same.

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I mean, you could have just
gone in reverse direction.

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You could say, hey, this is
going to react with some

00:04:17.190 --> 00:04:21.540
random hydronium molecule or
some random, free-standing

00:04:21.540 --> 00:04:22.520
proton out there.

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And then it could form hydrogen
fluoride, but we know

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that hydronium isn't just
sitting everywhere, that

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whenever you take the reverse
reaction, whenever you're

00:04:30.570 --> 00:04:33.500
going in this direction this
doesn't have to grab this

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hydrogen from an H3O.

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It could grab it from an H2O.

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And then you would have
this reaction.

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These are equivalent.

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And we could do the same thing
here for ammonium and amonia.

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We could write ammonia
as a base.

00:04:50.160 --> 00:04:56.000
NH3 is in equilibrium as a weak
base with-- it can grab a

00:04:56.000 --> 00:05:02.460
hydrogen from its surrounding
medium and become NH4 plus in

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an aqueous solution.

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And then it would have grabbed
that hydrogen, probably from a

00:05:07.010 --> 00:05:10.430
water molecule because that's
what's around it.

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And so that water molecule
will become an OH minus.

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And so now this looks ammonia
is a weak base.

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Ammonium is a weak acid.

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But these are equivalent
reactions.

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Now, you're probably already
seeing a relationship here.

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Ammonium is a weak acid.

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Ammonia is a weak base.

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And what's the difference
between the two?

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Just an H.

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Hydrofluoric acid is a weak
acid, just a fluorine anion.

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A negative fluorine
is a weak base.

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And what's the difference
between the two?

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They're just difference
of a hydrogen.

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Let me write that down.

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So let me write weak acid.

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And then you have
your weak base.

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So your weak base-- let me write
our weak acid is first.

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We had hydrofluoric acid, and
then the weak base is when you

00:06:08.750 --> 00:06:10.930
essentially just dump
the hydrogen, just

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the hydrogen proton.

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You kept the electron, so that's
why it's a negative

00:06:14.480 --> 00:06:15.880
charge right there.

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Hydrogen without its one
electron is just a proton

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because it has no neutrons.

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The other one was NH4 plus.

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You dump one of the hydrogens
and you get NH3.

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So what's the difference
going on?

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These are all minus
a hydrogen.

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Or if you go this way, you're
plus a hydrogen.

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So you have these kind
of conjugates.

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And this has all been a
long-winded way of introducing

00:06:45.800 --> 00:06:50.020
you to this idea that you have
these conjugate pairs.

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Like hydrofluoric acid, or
hydrogen fluoride and just the

00:06:53.230 --> 00:06:54.960
fluorine anions.

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So these are conjugate pairs.

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Which are essentially two
molecules that are identical

00:07:08.960 --> 00:07:12.070
except for a difference
in one hydrogen.

00:07:12.070 --> 00:07:13.860
No more than one hydrogen.

00:07:13.860 --> 00:07:15.720
One day there might be a test
where someone shows you two

00:07:15.720 --> 00:07:19.070
molecules that are separated by
two hydrogens-- those would

00:07:19.070 --> 00:07:21.940
not be conjugate pairs.

00:07:21.940 --> 00:07:28.980
For example, if I show you H2O
and OH minus, these are

00:07:28.980 --> 00:07:30.220
conjugate pairs.

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Because this over here is
exactly this minus a proton.

00:07:34.660 --> 00:07:37.500
And, in fact, let me be clear
that it's not just minus the

00:07:37.500 --> 00:07:39.850
hydrogen-- minus the proton.

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One of them is keeping
the electron.

00:07:41.630 --> 00:07:46.150
So this is minus a hydrogen
proton, this is plus a

00:07:46.150 --> 00:07:47.160
hydrogen proton.

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So the difference between
these two are

00:07:48.420 --> 00:07:50.320
just a hydrogen proton.

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So these are conjugate pairs.

00:07:51.730 --> 00:07:58.410
Now, if I were to say that H3O
and OH minus, you might be

00:07:58.410 --> 00:08:01.580
tempted to say, hey, this is
very acidic, this is a base,

00:08:01.580 --> 00:08:02.720
this is a conjugate pair.

00:08:02.720 --> 00:08:07.630
But no, there's a 2 H,
2 proton difference.

00:08:07.630 --> 00:08:09.160
This is H3O plus.

00:08:09.160 --> 00:08:10.980
There's a 2 proton difference,
so these are

00:08:10.980 --> 00:08:12.815
not conjugate pairs.

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So let me just cross that out.

00:08:20.410 --> 00:08:21.510
But these are.

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Now, you could say, if you have
H3O, you might say, hey,

00:08:25.200 --> 00:08:28.650
what's the conjugate base-- and
that's a new word I just

00:08:28.650 --> 00:08:32.350
introduced you to-- what's the
conjugate base for H3O if H3O

00:08:32.350 --> 00:08:33.179
is an acid?

00:08:33.179 --> 00:08:38.080
Well, you take one H from
it and you get H2O.

00:08:38.080 --> 00:08:41.510
So this is a conjugate pair.

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And I just said a word without
defining it, so now

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let me define it.

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Within every conjugate pair you
have an acid and a base.

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And if you say, oh, what is
the conjugate base for

00:08:51.740 --> 00:08:55.580
hydrofluoric acid, you get rid
of a hydrogen and you say, oh,

00:08:55.580 --> 00:08:59.350
it's just this fluorine anion.

00:08:59.350 --> 00:09:03.760
If you said, I have some of
ammonia, as a base, what is

00:09:03.760 --> 00:09:05.310
its conjugate acid?

00:09:05.310 --> 00:09:08.350
So if someone asks you, what is
a conjugate acid, you add a

00:09:08.350 --> 00:09:12.980
hydrogen proton to it and
you get ammonium.

00:09:12.980 --> 00:09:17.280
So I could call these
the conjugate acid.

00:09:17.280 --> 00:09:19.590
Let me just change terminology--
conjugate.

00:09:19.590 --> 00:09:22.130
And we'll see that actually, you
don't have to be using a

00:09:22.130 --> 00:09:24.150
weak acid or a weak base.

00:09:24.150 --> 00:09:31.070
Conjugate acid, and then you
have a conjugate base.

00:09:31.070 --> 00:09:34.440
And even though something might
be a conjugate acid or

00:09:34.440 --> 00:09:38.340
conjugate base, it doesn't
necessarily mean that they're

00:09:38.340 --> 00:09:41.740
very basic, for example,
or very acidic.

00:09:41.740 --> 00:09:45.870
If I have hydrogen chloride, we
know this is a strong acid.

00:09:45.870 --> 00:09:48.990
Hydrogen chloride.

00:09:48.990 --> 00:09:52.460
Its conjugate base, we
essentially just get rid of

00:09:52.460 --> 00:09:55.090
one of these hydrogen protons--
it doesn't take its

00:09:55.090 --> 00:09:56.480
electron with it.

00:09:56.480 --> 00:10:00.720
So it's just going to be the
chlorine negative ion.

00:10:00.720 --> 00:10:02.300
This is its conjugate.

00:10:02.300 --> 00:10:04.280
If I gave you a chlorine
negative ion and said, what's

00:10:04.280 --> 00:10:07.640
its conjugate base-- what's its
conjugate acid? you'd say

00:10:07.640 --> 00:10:08.880
it's hydrochloric acid.

00:10:08.880 --> 00:10:11.590
If I gave you hydrochloric acid
and I say, what's its

00:10:11.590 --> 00:10:15.300
conjugate base, you get rid of
a hydrogen proton only, and

00:10:15.300 --> 00:10:18.100
you're left with the chlorine
negative ion-- You said that

00:10:18.100 --> 00:10:19.280
its conjugate base.

00:10:19.280 --> 00:10:22.410
Now, with that said, we know
that when you put hydrochloric

00:10:22.410 --> 00:10:25.190
acid, we know this reaction.

00:10:25.190 --> 00:10:28.000
This was I think the first
reaction we looked at in

00:10:28.000 --> 00:10:29.780
aqueous solution.

00:10:29.780 --> 00:10:37.140
It disassociates completely to
form hydrogen protons plus

00:10:37.140 --> 00:10:39.230
chlorine anions-- everything,
of course,

00:10:39.230 --> 00:10:41.840
in an aqueous solution.

00:10:41.840 --> 00:10:43.900
Now, the fact that it
disassociates completely, that

00:10:43.900 --> 00:10:47.700
this is not an equilibrium
reaction, this implies that

00:10:47.700 --> 00:10:52.170
this guy is more basic
than water.

00:10:52.170 --> 00:10:55.630
He has no temptation-- no, no--
let me say that he is

00:10:55.630 --> 00:10:57.230
less basic than water.

00:10:57.230 --> 00:11:03.710
He has no temptation to grab
these hydrogen protons from

00:11:03.710 --> 00:11:07.710
the surrounding medium to reform
hydrochloric acid.

00:11:07.710 --> 00:11:11.150
This reaction does not
go in this direction.

00:11:11.150 --> 00:11:14.990
So even though this chlorine
anion, or negative ion of

00:11:14.990 --> 00:11:24.560
chlorine, is a quote unquote
conjugate base of HCL, that

00:11:24.560 --> 00:11:26.790
doesn't necessarily mean
it's that basic.

00:11:26.790 --> 00:11:28.980
This is less basic than water.

00:11:28.980 --> 00:11:32.140
It wants the hydrogen
protons less

00:11:32.140 --> 00:11:34.180
than, let's say, hydronium.

00:11:34.180 --> 00:11:39.890
So if you put some chlorine plus
some hydronium, or let's

00:11:39.890 --> 00:11:43.880
say you have some H plus, you're
not going to reform

00:11:43.880 --> 00:11:45.250
hydrochloric acid.

00:11:45.250 --> 00:11:50.260
So this is not really basic even
though it's considered a

00:11:50.260 --> 00:11:52.620
conjugate base.

00:11:52.620 --> 00:11:54.760
And that's generally the case
whenever you're dealing with

00:11:54.760 --> 00:11:58.920
strong acids, like in the case
of hydrochloric acid.

00:11:58.920 --> 00:12:05.610
If I had a big solution of just
chlorine anions in water,

00:12:05.610 --> 00:12:08.550
so I just had tons of a super
high concentration of chlorine

00:12:08.550 --> 00:12:12.420
anions and water, because it's
not going to do anything to

00:12:12.420 --> 00:12:16.550
change the actual hydrogen or
hydroxide concentration in the

00:12:16.550 --> 00:12:22.510
water because it's less basic
than the water itself-- it

00:12:22.510 --> 00:12:26.050
doesn't want to take or give
anything to the water-- the

00:12:26.050 --> 00:12:30.380
PH, so if you have a soup,
the PH would be 7.

00:12:30.380 --> 00:12:34.730
If you have chlorine minus in
an aqueous solution-- and I

00:12:34.730 --> 00:12:38.440
don't care what its
concentration is, you could

00:12:38.440 --> 00:12:41.480
have 10 molar of it-- the PH is
still going to be 7 because

00:12:41.480 --> 00:12:43.700
it's not going to change
its solution.

00:12:43.700 --> 00:12:46.640
It's not going to change the
PH, just this by itself.

00:12:46.640 --> 00:12:48.970
Obviously, if you put
hydrochloric acid in an

00:12:48.970 --> 00:12:51.400
aqueous solution this will
change it, because you're

00:12:51.400 --> 00:12:54.390
going to be dumping all of these
hydrogen protons into

00:12:54.390 --> 00:12:55.420
the solution.

00:12:55.420 --> 00:12:58.310
So in general-- I mean, you can
kind of remember it, but I

00:12:58.310 --> 00:13:05.080
think it's maybe common sense--
a strong acid's

00:13:05.080 --> 00:13:10.940
conjugate base is neutral
in water.

00:13:10.940 --> 00:13:12.160
Neutral.

00:13:12.160 --> 00:13:14.560
So that means no impact on PH.

00:13:21.670 --> 00:13:27.400
You say chlorine plus H2O, I
mean, you're essentially still

00:13:27.400 --> 00:13:29.520
going to have chlorine
plus H2O.

00:13:29.520 --> 00:13:31.760
You're not really changing
the concentration.

00:13:31.760 --> 00:13:35.480
Now, on the other hand, when
you're dealing with weak

00:13:35.480 --> 00:13:38.960
acids, so this reaction will
go in the other direction.

00:13:38.960 --> 00:13:44.690
If you put some fluorine in
water, it will grab some

00:13:44.690 --> 00:13:47.710
hydrogen-- not necessarily a
ton of it, but it will grab

00:13:47.710 --> 00:13:49.910
some hydrogen from the
surrounding water-- and

00:13:49.910 --> 00:13:52.470
increase the hydroxide
concentration.

00:13:52.470 --> 00:13:54.500
It's increasing the
concentration of this thing

00:13:54.500 --> 00:13:55.220
right here.

00:13:55.220 --> 00:13:57.990
So it is making, in this
case, it is making the

00:13:57.990 --> 00:14:01.130
solution more basic.

00:14:01.130 --> 00:14:04.030
It's increasing the PH
of the solution.

00:14:04.030 --> 00:14:18.610
So whenever you have a weak acid
its conjugate base will

00:14:18.610 --> 00:14:19.860
be a weak base.

00:14:22.220 --> 00:14:23.430
And you could make
this statement

00:14:23.430 --> 00:14:24.560
the other way around.

00:14:24.560 --> 00:14:27.950
If you have a conjugate-- let
me switch colors, this is

00:14:27.950 --> 00:14:36.710
getting annoying-- conjugate
base-- sorry, a weak base, its

00:14:36.710 --> 00:14:40.560
conjugate acid is going
to be a weak acid.

00:14:40.560 --> 00:14:42.450
So hopefully you get
the idea here.

00:14:42.450 --> 00:14:44.650
It's actually not that
fancy of an idea.

00:14:44.650 --> 00:14:48.860
It's just that if you have an
acid, its conjugate base is

00:14:48.860 --> 00:14:50.535
just that acid minus
a hydrogen.

00:14:53.060 --> 00:14:57.220
If you have a base, its
conjugate acid is just that

00:14:57.220 --> 00:14:59.410
thing plus a hydrogen.

00:14:59.410 --> 00:15:05.440
Actually, let me just do a bunch
of problems here just to

00:15:05.440 --> 00:15:10.640
really hit the point home of
what we're talking about.

00:15:10.640 --> 00:15:11.810
Let's just do a bunch of them.

00:15:11.810 --> 00:15:19.940
So if this is the acid and this
is its conjugate base, so

00:15:19.940 --> 00:15:24.010
if I have-- I mean, you don't
even have to know the words.

00:15:24.010 --> 00:15:26.130
If I have that, the conjugate
base, well, I'm just going to

00:15:26.130 --> 00:15:28.270
get rid of a hydrogen proton.

00:15:28.270 --> 00:15:30.300
So NO3 minus.

00:15:30.300 --> 00:15:32.620
I didn't get rid of the whole
neutral hydrogen molecule.

00:15:32.620 --> 00:15:34.950
Remember, I just took a proton
away, the electrons stay the

00:15:34.950 --> 00:15:38.360
same, so I have a
negative charge.

00:15:38.360 --> 00:15:43.460
Let's say we did H2SO4.

00:15:43.460 --> 00:15:46.890
So its conjugate base, get
rid of a hydrogen.

00:15:46.890 --> 00:15:49.760
HSO4 minus.

00:15:49.760 --> 00:15:55.440
If I have hydrogen bromide,
get rid of a hydrogen.

00:15:55.440 --> 00:15:57.420
It's BR minus.

00:15:57.420 --> 00:16:00.150
And this is a strong acid.

00:16:00.150 --> 00:16:03.010
So this is going to be a
neutral-- if you put this in

00:16:03.010 --> 00:16:05.440
water, it's really not going to
do anything even though you

00:16:05.440 --> 00:16:08.720
are calling it hydrogen
bromide's conjugate base.

00:16:08.720 --> 00:16:10.460
Now, if we go the other way.

00:16:10.460 --> 00:16:13.420
If we give you the base, if I
give you OH minus, what's its

00:16:13.420 --> 00:16:14.570
conjugate acid?

00:16:14.570 --> 00:16:18.320
Well, you add a proton
to it, you get H2O.

00:16:18.320 --> 00:16:22.090
If I have H2O-- we already did
that-- you add a proton to it,

00:16:22.090 --> 00:16:24.410
you get H3O plus.

00:16:24.410 --> 00:16:28.540
If you have-- I mean, we
could just keep going.

00:16:28.540 --> 00:16:29.880
Let's say I have that.

00:16:29.880 --> 00:16:32.090
If I add a hydrogen
to it, I have H2.

00:16:34.850 --> 00:16:35.640
There you go.

00:16:35.640 --> 00:16:37.550
And it's neutral now because
I added a proton.

00:16:37.550 --> 00:16:40.640
Anyway, hopefully I haven't
beaten this horse to death and

00:16:40.640 --> 00:16:45.520
you understand what conjugate
acid and bases are all about.


pH of a Weak Base

WEBVTT
Kind: captions
Language: en

00:00:01.090 --> 00:00:04.300
We've dealt with the weak acid,
so let's try an example

00:00:04.300 --> 00:00:05.340
with the weak base.

00:00:05.340 --> 00:00:06.590
Let's say we had ammonia.

00:00:09.730 --> 00:00:12.350
That's nitrogen with
three hydrogens.

00:00:12.350 --> 00:00:16.370
And it's a weak base because
it likes to accept hydrogen

00:00:16.370 --> 00:00:18.750
from water, leaving the water
with just a hydroxide.

00:00:18.750 --> 00:00:21.190
So it increases the hydroxide
concentration.

00:00:21.190 --> 00:00:23.940
So if you have some ammonia
in an aqueous

00:00:23.940 --> 00:00:28.150
solution, plus water.

00:00:28.150 --> 00:00:29.430
I'll throw the water in there.

00:00:29.430 --> 00:00:35.470
Plus water in an aqueous
solution.

00:00:35.470 --> 00:00:36.940
It's a weak base.

00:00:36.940 --> 00:00:40.530
So this reaction doesn't go
in just one direction.

00:00:40.530 --> 00:00:41.890
It's an equilibrium reaction.

00:00:45.670 --> 00:00:49.980
And since this is a weak base,
it-- and this is where the

00:00:49.980 --> 00:00:53.140
Bronsted-Lowry definition
really kind of pops out.

00:00:53.140 --> 00:00:57.040
Is that it's a proton acceptor
instead of a donor.

00:00:57.040 --> 00:01:03.960
So it turns into ammonium,
or an ammonia cation.

00:01:03.960 --> 00:01:06.970
Ammonium has another hydrogen
on it, so now

00:01:06.970 --> 00:01:08.915
it has another proton.

00:01:08.915 --> 00:01:11.195
So it's the plus charge.

00:01:11.195 --> 00:01:12.450
An it's an aqueous.

00:01:12.450 --> 00:01:15.250
And it took that hydrogen
from the water.

00:01:15.250 --> 00:01:20.050
So plus OH minus aqueous.

00:01:20.050 --> 00:01:22.430
And remember, if you look at
it from the Bronsted-Lowry

00:01:22.430 --> 00:01:25.110
definition , it was
a proton acceptor.

00:01:25.110 --> 00:01:26.600
So that made it a base.

00:01:26.600 --> 00:01:28.210
Or if you look at the Arrhenius
definition, it

00:01:28.210 --> 00:01:32.010
increased the concentration of
OH in the solution, so that

00:01:32.010 --> 00:01:33.860
makes it an Arrhenius base.

00:01:33.860 --> 00:01:37.060
But anyway, given that
we have-- let me

00:01:37.060 --> 00:01:37.970
pick a random number.

00:01:37.970 --> 00:01:48.890
Let's say we have 0.2
molar of NH3.

00:01:48.890 --> 00:01:51.950
What is going to be the pH?

00:01:51.950 --> 00:01:55.710
So what's going to be our pH of
the solution, considering

00:01:55.710 --> 00:01:58.260
that it's 0.2 molar of NH3.

00:01:58.260 --> 00:01:59.350
So the first thing
we need to do.

00:01:59.350 --> 00:02:01.930
We need to figure out the
equilibrium constant for this

00:02:01.930 --> 00:02:03.220
base reaction.

00:02:03.220 --> 00:02:10.820
And I just went to Wikipedia--
I wanted to say liquidpedia,

00:02:10.820 --> 00:02:13.370
I'm talking about
liquids so much.

00:02:13.370 --> 00:02:14.130
And equilibrium.

00:02:14.130 --> 00:02:15.170
Equipedia.

00:02:15.170 --> 00:02:19.070
But I went to Wikipedia, and
they have a little chart for

00:02:19.070 --> 00:02:20.630
almost any compound
you look for.

00:02:20.630 --> 00:02:21.880
And they give you pKb.

00:02:26.120 --> 00:02:27.760
Which is, you see
that p there.

00:02:27.760 --> 00:02:32.090
That just means it's the
minus log base 10 of

00:02:32.090 --> 00:02:33.565
the equilibrium constant.

00:02:38.870 --> 00:02:42.700
And they give that
as being 4.75.

00:02:42.700 --> 00:02:45.330
So we can just do a little bit
of math here to solve for the

00:02:45.330 --> 00:02:47.000
equilibrium constant.

00:02:47.000 --> 00:02:47.620
So let's see.

00:02:47.620 --> 00:02:53.220
If we multiply both sides by
negative, you get log base 10

00:02:53.220 --> 00:02:57.460
of our equilibrium constant
for this base reaction.

00:02:57.460 --> 00:02:58.570
That's why the b is there.

00:02:58.570 --> 00:03:03.030
Is equal to minus 4.75,
or 10 to the minus

00:03:03.030 --> 00:03:05.400
4.75 should be Kb.

00:03:05.400 --> 00:03:11.540
So Kb is equal to 10
to the minus 4.75.

00:03:11.540 --> 00:03:14.040
That's not an easy exponent to
figure out in your head, so

00:03:14.040 --> 00:03:16.530
I'll bring out the calculator
for that.

00:03:16.530 --> 00:03:27.850
So if we take 10 to the 4.75
minus, it equals, let's just

00:03:27.850 --> 00:03:31.460
say 1.8 times 10 to
the negative 5.

00:03:31.460 --> 00:03:37.320
This is equal to 1.8 times
10 to the minus 5.

00:03:37.320 --> 00:03:40.090
So now we can use this
information and we can do a

00:03:40.090 --> 00:03:42.260
mathematical thing very
similar to we

00:03:42.260 --> 00:03:45.010
did in the last video.

00:03:45.010 --> 00:03:46.480
It's going to be hard to
figure out the hydrogen

00:03:46.480 --> 00:03:47.760
concentration directly, right?

00:03:47.760 --> 00:03:50.520
Because our equilibrium reaction
only has hydroxide.

00:03:50.520 --> 00:03:53.500
But if we know the hydroxide
concentration, then we can

00:03:53.500 --> 00:03:55.770
back into the hydrogen
concentration, knowing that

00:03:55.770 --> 00:04:01.510
this plus the hydrogen
concentration has to equal 10

00:04:01.510 --> 00:04:02.440
to the minus 14.

00:04:02.440 --> 00:04:06.620
Or if you figure out the pOH,
that plus the pH has to be 14.

00:04:06.620 --> 00:04:09.000
And we did that a couple
of videos ago.

00:04:09.000 --> 00:04:14.060
So this equilibrium constant
or this formula

00:04:14.060 --> 00:04:14.880
would look like this.

00:04:14.880 --> 00:04:24.940
1.8 times 10 to the minus 5
will be equal to-- in the

00:04:24.940 --> 00:04:27.880
denominator, we have our
concentration of reactants.

00:04:27.880 --> 00:04:30.060
And remember, you don't
include the solvent.

00:04:30.060 --> 00:04:31.980
So you only include the NH3.

00:04:31.980 --> 00:04:35.480
We have 0.2 molars is what we
put in, but some of it, let's

00:04:35.480 --> 00:04:37.980
say X of it, is going to be
converted into this stuff on

00:04:37.980 --> 00:04:39.670
the right-hand side.

00:04:39.670 --> 00:04:43.380
So in the denominator, we're
going to have 0.2 minus

00:04:43.380 --> 00:04:45.870
whatever gets converted into
the right-hand side.

00:04:45.870 --> 00:04:49.510
And so then in the right-hand
side, we're going to have x of

00:04:49.510 --> 00:04:51.470
NH4 and x of OH.

00:04:54.300 --> 00:04:58.140
This is the concentration
of ammonia.

00:04:58.140 --> 00:05:00.300
And then we have x times x.

00:05:00.300 --> 00:05:07.830
This is the concentration of
NH4 plus-- that's a 4.

00:05:07.830 --> 00:05:10.320
And then this is the
concentration,

00:05:10.320 --> 00:05:15.330
right here, of OH minus.

00:05:15.330 --> 00:05:15.870
Right?

00:05:15.870 --> 00:05:17.860
And we just solve for x.

00:05:17.860 --> 00:05:19.850
Let's do that.

00:05:19.850 --> 00:05:21.080
Solve for x.

00:05:21.080 --> 00:05:23.390
And once we have x, we know
the concentration of OH.

00:05:23.390 --> 00:05:25.310
We'll be able to figure out
the pOH, and then we'll be

00:05:25.310 --> 00:05:27.900
able to figure out the pH.

00:05:27.900 --> 00:05:28.750
OK.

00:05:28.750 --> 00:05:32.300
Multiply this times both
sides of this equation.

00:05:32.300 --> 00:05:35.060
And just so you know, that same
simplification step that

00:05:35.060 --> 00:05:36.380
we did in the previous thing.

00:05:36.380 --> 00:05:44.260
When this is several orders of
magnitude smaller than this

00:05:44.260 --> 00:05:49.830
number right here-- I want to
give you-- heuristics are just

00:05:49.830 --> 00:05:51.290
kind of rules of thumb
that sometimes work.

00:05:51.290 --> 00:05:53.170
Let's just do the quadratic
equation.

00:05:53.170 --> 00:05:56.150
But you can kind of think about
sometimes when you can

00:05:56.150 --> 00:05:57.000
get rid of that middle term.

00:05:57.000 --> 00:05:57.760
But let's just multiply it.

00:05:57.760 --> 00:06:03.450
0.2 two times 1.8 is 0.36.

00:06:03.450 --> 00:06:07.980
0.36 times 10 to the
minus 5, right?

00:06:07.980 --> 00:06:11.650
2 times 1.8 would be
3.6, this is 0.36.

00:06:11.650 --> 00:06:20.660
Minus 1.8 times 10 to the
minus 5 x, right?

00:06:20.660 --> 00:06:22.942
Is equal to that.

00:06:22.942 --> 00:06:24.900
x squared.

00:06:24.900 --> 00:06:29.260
Let's put everything on the
same side of the equation.

00:06:29.260 --> 00:06:31.220
I'm going to move all of these
the right-hand side, so you

00:06:31.220 --> 00:06:35.090
get 0 is equal to x squared.

00:06:35.090 --> 00:06:37.990
Add this to both sides
of the equation.

00:06:37.990 --> 00:06:44.520
Plus 1.8 times 10 to
the minus 5 x.

00:06:44.520 --> 00:06:48.870
1.8 times 10 to the minus 5.

00:06:48.870 --> 00:06:50.940
Just so you can see the
coefficients separate from the

00:06:50.940 --> 00:06:53.010
x terms.

00:06:53.010 --> 00:06:59.860
Minus 0.36 times 10
to the minus 5.

00:06:59.860 --> 00:07:00.990
So let's solve this.

00:07:00.990 --> 00:07:04.610
And once again, if you wanted
to kind of do it, you could

00:07:04.610 --> 00:07:06.720
eliminate this term and then
just figure out the straight

00:07:06.720 --> 00:07:07.540
up square root.

00:07:07.540 --> 00:07:08.360
But we won't do that.

00:07:08.360 --> 00:07:09.890
We'll actually use a
quadratic equation.

00:07:09.890 --> 00:07:12.610
So a is 1.

00:07:12.610 --> 00:07:13.950
b is this.

00:07:13.950 --> 00:07:14.530
That's b.

00:07:14.530 --> 00:07:15.200
And this is c.

00:07:15.200 --> 00:07:17.340
And you just supply than in
the quadratic equation.

00:07:17.340 --> 00:07:20.500
So you get minus b.

00:07:20.500 --> 00:07:26.230
So you minus 1.8 times 10
to the minus 5 power.

00:07:26.230 --> 00:07:27.060
Plus or minus.

00:07:27.060 --> 00:07:28.790
We'll only have to do the plus
because if we do the minus,

00:07:28.790 --> 00:07:30.250
we'll end up with a negative
concentration.

00:07:30.250 --> 00:07:34.440
So plus, the square root-- we
have to do a lot of math

00:07:34.440 --> 00:07:36.970
here-- b squared.

00:07:36.970 --> 00:07:39.220
So it's 1.8 times 10
to the negative 5.

00:07:39.220 --> 00:07:41.550
So it's 1.8.

00:07:41.550 --> 00:07:45.210
If you square it, it's 3.24.

00:07:45.210 --> 00:07:51.150
So it's 3.24 times-- if you
square 10 to the minus 5-- 10

00:07:51.150 --> 00:07:57.630
to the minus 10 minus 4
times a, which is 1,

00:07:57.630 --> 00:07:59.380
times c, which is minus.

00:07:59.380 --> 00:08:04.860
So it's 4 times-- the minuses
cancel out-- times 0.36 times

00:08:04.860 --> 00:08:08.220
10 to the minus 5.

00:08:08.220 --> 00:08:20.140
Which is 4 times 0.36
is equal to 1.44.

00:08:20.140 --> 00:08:21.435
I should have been able
to do that in my head.

00:08:21.435 --> 00:08:25.440
Now you have 1.44 e minus 5.

00:08:25.440 --> 00:08:30.120
Times 10 to-- let
me write that.

00:08:30.120 --> 00:08:32.549
So this is 1.44.

00:08:32.549 --> 00:08:36.260
And of course all of
this is over 2a.

00:08:36.260 --> 00:08:37.390
So let's see.

00:08:37.390 --> 00:08:38.789
This is my x value.

00:08:38.789 --> 00:08:41.860
My concentration of OH.

00:08:41.860 --> 00:08:42.350
So let's see.

00:08:42.350 --> 00:08:51.380
I have 3.24 times 10
to the minus 10.

00:08:51.380 --> 00:08:53.020
That's that.

00:08:53.020 --> 00:09:03.870
Plus 1.44 times 10 to the minus
5 is equal to that.

00:09:03.870 --> 00:09:05.530
So that's this whole thing
under the radical.

00:09:05.530 --> 00:09:08.180
And I want to take the
square root of that.

00:09:08.180 --> 00:09:12.520
And so that is to
the 0.5 power.

00:09:12.520 --> 00:09:16.680
So I get 0.00379.

00:09:16.680 --> 00:09:19.590
So I'll switch colors.

00:09:19.590 --> 00:09:25.820
So I get x is equal to a minus
1.8 times 10 to the minus 5

00:09:25.820 --> 00:09:33.320
plus 0.003794.

00:09:33.320 --> 00:09:35.660
All of that over 2.

00:09:35.660 --> 00:09:36.330
Do the math.

00:09:36.330 --> 00:09:42.660
So to that I'm going to subtract
minus this point

00:09:42.660 --> 00:09:43.060
right here.

00:09:43.060 --> 00:09:43.750
I have this value.

00:09:43.750 --> 00:09:44.990
I'm just subtracting this.

00:09:44.990 --> 00:09:53.770
Minus 1.8 e 5 negative
is equal to that.

00:09:53.770 --> 00:09:54.750
This is the whole numerator.

00:09:54.750 --> 00:09:57.470
And now I need to just
divide it by 2.

00:09:57.470 --> 00:10:03.680
Divided by 2 is equal
to 0.001.

00:10:03.680 --> 00:10:04.430
Let me write that.

00:10:04.430 --> 00:10:05.910
So x.

00:10:05.910 --> 00:10:08.140
So I'll switch colors
arbitrarily again.

00:10:08.140 --> 00:10:18.480
x is equal to 0.001888-- I mean,
then there's a 3 and so

00:10:18.480 --> 00:10:19.690
forth and so on.

00:10:19.690 --> 00:10:21.740
But if you remember from
our original equation.

00:10:21.740 --> 00:10:22.490
What was x?

00:10:22.490 --> 00:10:26.760
It was what's both the ammonium
concentration and the

00:10:26.760 --> 00:10:27.855
hydroxide concentration.

00:10:27.855 --> 00:10:30.530
We care about the hydroxide
concentration.

00:10:30.530 --> 00:10:36.060
So this is equal to my
concentration of hydroxide.

00:10:36.060 --> 00:10:40.580
Now if I want to figure out my
pOH, I just take the minus log

00:10:40.580 --> 00:10:44.610
of this number, which
is equal to--

00:10:44.610 --> 00:10:46.830
So let's just take
the log of it.

00:10:46.830 --> 00:10:49.180
The log is that, and then I
take the minus of that.

00:10:49.180 --> 00:10:55.180
So it's 2.72.

00:10:55.180 --> 00:11:00.420
And now if we want to figure out
the pH, my concentration

00:11:00.420 --> 00:11:03.370
of hydrogen ions-- just
remember, when you're in an

00:11:03.370 --> 00:11:10.740
aqueous solution at 25 degrees
Celsius, your pK of water is

00:11:10.740 --> 00:11:15.870
equal to your pOH
plus your pH.

00:11:15.870 --> 00:11:19.730
This at 25 degrees is 14.

00:11:19.730 --> 00:11:22.815
Because you have 10 to the minus
14 molar concentration--

00:11:22.815 --> 00:11:24.580
well no, actually, I don't
want to go into that.

00:11:24.580 --> 00:11:26.720
You have 10 to the minus
7 of each of these.

00:11:26.720 --> 00:11:28.510
But anyway, this is equilibrium
constant for the

00:11:28.510 --> 00:11:30.310
disassociation of water.

00:11:30.310 --> 00:11:37.170
This, when water's neutral is 7
or a concentration of OH of

00:11:37.170 --> 00:11:38.280
10 to the minus 7.

00:11:38.280 --> 00:11:39.760
We can take the minus
log, this becomes 7.

00:11:39.760 --> 00:11:44.070
But now we know we have a much
higher concentration of OH.

00:11:44.070 --> 00:11:45.080
2.72.

00:11:45.080 --> 00:11:47.550
Remember, that minus log
kind of flips it.

00:11:47.550 --> 00:11:52.670
So a lower pOH means a higher
concentration of pOH.

00:11:52.670 --> 00:11:53.230
Right?

00:11:53.230 --> 00:11:57.200
And a lower pOH, if this
is lower, right?

00:11:57.200 --> 00:11:58.480
This is a lower pOH.

00:11:58.480 --> 00:12:00.290
That means your pH is higher.

00:12:03.570 --> 00:12:04.870
So what is your pH
going to be?

00:12:04.870 --> 00:12:15.024
So your pH is going to be
equal to 14 minus 2.72.

00:12:15.024 --> 00:12:21.710
So let me do the minus
plus 14 is equal to--

00:12:21.710 --> 00:12:23.570
let's just say 11.3.

00:12:23.570 --> 00:12:26.650
So your pH is equal to 11.3.

00:12:26.650 --> 00:12:30.850
Which makes sense, because we
said this was a weak base.

00:12:30.850 --> 00:12:33.680
Ammonia is a weak base.

00:12:33.680 --> 00:12:35.070
So it's basic.

00:12:35.070 --> 00:12:39.120
So it should increase your
pH above the neutral 7.

00:12:39.120 --> 00:12:42.805
So the pH should be greater than
7, but as you compare it

00:12:42.805 --> 00:12:45.280
to some of the strong bases
before that took our pH when

00:12:45.280 --> 00:12:49.410
you added a molar to 14, this
took our pH-- although we only

00:12:49.410 --> 00:12:54.010
did add 0.2 molar
of it to 11.3.

00:12:54.010 --> 00:12:56.970
Anyway, this is more of a math
problem than chemistry, but

00:12:56.970 --> 00:12:59.800
hopefully it clarified
a few things as well.


pH of a Weak Acid

WEBVTT
Kind: captions
Language: en

00:00:00.000 --> 00:00:04.220
Sal: We now know all
about strong acids.

00:00:04.220 --> 00:00:08.700
For example, hydrochloric acid
in an aqueous solution.

00:00:08.700 --> 00:00:12.500
It's strong because it
disassociate completely into

00:00:12.500 --> 00:00:15.920
hydrogen ions or hydronium
molecules, depending on how

00:00:15.920 --> 00:00:17.590
you want to view it.

00:00:17.590 --> 00:00:20.890
And chloride anions.

00:00:20.890 --> 00:00:25.410
And it's an acid in the
Arrhenius definition, because

00:00:25.410 --> 00:00:28.326
once again, this is an
aqueous solution.

00:00:28.326 --> 00:00:32.159
In the Arrhenius definition
because it is increasing the

00:00:32.159 --> 00:00:34.590
hydrogen concentration
in the water.

00:00:34.590 --> 00:00:38.490
It's an acid in the
Bronsted-Lowry definition

00:00:38.490 --> 00:00:42.950
because this thing is donating
these hydrogen protons.

00:00:42.950 --> 00:00:46.810
And it's a Lewis acid because
you can kind of view it as

00:00:46.810 --> 00:00:48.480
hydrochloric acid.

00:00:48.480 --> 00:00:52.180
By giving away this, it's taking
these-- the chloride,

00:00:52.180 --> 00:00:53.970
as you can kind of say,
it's taking the

00:00:53.970 --> 00:00:55.190
electrons for itself.

00:00:55.190 --> 00:00:56.740
So it's an electron acceptor.

00:00:56.740 --> 00:01:00.100
So that's another reason
why it's a Lewis acid.

00:01:00.100 --> 00:01:03.190
But in every case, it completely
disassociates.

00:01:03.190 --> 00:01:05.150
So it's a strong acid.

00:01:05.150 --> 00:01:08.980
Now, by implication, when I call
this a strong acid, that

00:01:08.980 --> 00:01:11.600
must mean that there must be
things called weak acids.

00:01:15.940 --> 00:01:18.170
And you could probably
guess what they do.

00:01:18.170 --> 00:01:19.850
They don't disassociate
completely.

00:01:19.850 --> 00:01:27.620
So let's say you have
hydrofluoric acid in an

00:01:27.620 --> 00:01:30.380
aqueous solution.

00:01:30.380 --> 00:01:35.370
It does disassociate somewhat,
so you are going to have some

00:01:35.370 --> 00:01:38.320
hydrogen break-off of the
fluorine molecules.

00:01:38.320 --> 00:01:40.360
So a hydrogen ion, and
that's of course

00:01:40.360 --> 00:01:42.740
in an aqueous solution.

00:01:42.740 --> 00:01:46.830
I was doing aqueous in blue.

00:01:46.830 --> 00:01:51.315
And then that's going to
disassociate with the

00:01:51.315 --> 00:01:56.170
fluoride, the fluorine cation.

00:01:56.170 --> 00:01:57.780
But this is a weak
acid, so it's

00:01:57.780 --> 00:01:59.490
not just in one direction.

00:01:59.490 --> 00:02:01.210
It actually goes in both.

00:02:01.210 --> 00:02:03.070
This is an equilibrium
reaction.

00:02:03.070 --> 00:02:05.280
We know all about these now.

00:02:05.280 --> 00:02:07.670
Which means that some of this
goes in that direction, some

00:02:07.670 --> 00:02:09.740
of that goes in that direction,
and both of these

00:02:09.740 --> 00:02:12.720
species exist in the molecule.

00:02:12.720 --> 00:02:16.010
And just as aside, you might
wonder hey, aren't fluorine

00:02:16.010 --> 00:02:17.690
and chlorine-- they're
both halogens.

00:02:17.690 --> 00:02:20.940
Why is this a weak acid while
this is a strong one?

00:02:20.940 --> 00:02:21.940
And this is just an aside.

00:02:21.940 --> 00:02:24.220
This is something I always
used to wonder about.

00:02:27.030 --> 00:02:32.160
So in general, hydrogen
likes to form bonds.

00:02:32.160 --> 00:02:35.390
Remember, alkali metals
we included

00:02:35.390 --> 00:02:36.850
everything below hydrogen.

00:02:36.850 --> 00:02:37.180
Right?

00:02:37.180 --> 00:02:38.540
We included all of these guys.

00:02:38.540 --> 00:02:42.470
All of these guys, they have
this 1 extra electron and

00:02:42.470 --> 00:02:44.470
they're on a valence shell, so
they just want to give it up.

00:02:44.470 --> 00:02:46.310
And they tend to be very
willing to give

00:02:46.310 --> 00:02:47.320
it up to these guys.

00:02:47.320 --> 00:02:51.500
So if you have any of these
period 2 and below with a

00:02:51.500 --> 00:02:54.600
halogen, you normally form
this ionic salt.

00:02:54.600 --> 00:02:58.730
Where this guy gives away an
electron, and one of these

00:02:58.730 --> 00:03:01.110
guys takes it, and you have
an ionic compound.

00:03:01.110 --> 00:03:04.540
Hydrogen's a little different.

00:03:04.540 --> 00:03:07.630
Hydrogen does have exactly
1 electron in

00:03:07.630 --> 00:03:08.270
it's outermost shell.

00:03:08.270 --> 00:03:09.780
It just has one shell.

00:03:09.780 --> 00:03:12.820
But the first shell is
interesting because it's

00:03:12.820 --> 00:03:15.110
complete when it only
has 2 electrons.

00:03:15.110 --> 00:03:16.340
These guys wouldn't
have been happy.

00:03:16.340 --> 00:03:18.490
If you gave 1 electron
to these guys-- they

00:03:18.490 --> 00:03:19.060
need to get to eight.

00:03:19.060 --> 00:03:19.800
So they're not happy.

00:03:19.800 --> 00:03:21.790
But if you give 1 electron
to hydrogen,

00:03:21.790 --> 00:03:23.590
hydrogen is pretty happy.

00:03:23.590 --> 00:03:27.580
So when hydrogen forms bonds
with these guys here, it

00:03:27.580 --> 00:03:30.140
doesn't just completely
give up the electron.

00:03:30.140 --> 00:03:35.630
When hydrogen forms bonds with
our halogens over here, it

00:03:35.630 --> 00:03:37.620
doesn't completely give
up the electron.

00:03:37.620 --> 00:03:39.250
Because hydrogen says hey, if
you give me some of the

00:03:39.250 --> 00:03:41.450
electron, then I get to 2.

00:03:41.450 --> 00:03:43.540
And then if you share the
electron, you get to 8.

00:03:43.540 --> 00:03:46.510
So it does definitely like the
bond with these guys, but

00:03:46.510 --> 00:03:48.250
doesn't give it completely up.

00:03:48.250 --> 00:03:49.280
Now.

00:03:49.280 --> 00:03:53.440
Hydrochloric acid, hydrobromic
acid, and hydroiodic-- I think

00:03:53.440 --> 00:03:56.100
I think I'm seeing it right--
these are all strong acids.

00:03:56.100 --> 00:03:58.620
When hydrogen bonds with
any of these guys.

00:03:58.620 --> 00:04:01.690
Which means it disassociates
completely in water.

00:04:01.690 --> 00:04:06.310
But when hydrogen bonds with
flourine, it doesn't

00:04:06.310 --> 00:04:08.440
disassociate completely
in water.

00:04:08.440 --> 00:04:11.670
And the reason why is flourine
is even more electronegative.

00:04:11.670 --> 00:04:14.770
So this bond between hydrogen
and fluorine-- so if you

00:04:14.770 --> 00:04:22.790
compare hydrogen fluorine or
hydrofluoric acid to hydrogen

00:04:22.790 --> 00:04:28.410
chloride, hydrogen fluoride to
hydrogen chloride, this bond

00:04:28.410 --> 00:04:29.660
is much stronger.

00:04:29.660 --> 00:04:30.440
Not much stronger.

00:04:30.440 --> 00:04:32.620
I mean, you can measure in the
energy of the bond, and we'll

00:04:32.620 --> 00:04:33.270
do that eventually.

00:04:33.270 --> 00:04:35.970
But this is a stronger bond
than this right here.

00:04:35.970 --> 00:04:39.650
And hydrobromic is
even weaker.

00:04:39.650 --> 00:04:40.280
Let me draw it.

00:04:40.280 --> 00:04:43.660
So this one is a little bit
weaker than that one and then

00:04:43.660 --> 00:04:45.980
hydrobromic-- I'll draw it
as one line right there.

00:04:45.980 --> 00:04:47.350
It's like that.

00:04:47.350 --> 00:04:50.850
So these are easier to break, so
when you put this in water,

00:04:50.850 --> 00:04:55.700
hydrogen sees these water
molecules that have these

00:04:55.700 --> 00:05:00.280
extra electron pairs
floating around.

00:05:00.280 --> 00:05:03.290
And and it hogged electrons
from here.

00:05:03.290 --> 00:05:06.220
So there's a partial negative
charge, it's tempting for it

00:05:06.220 --> 00:05:08.930
to disassociate with the
water molecules.

00:05:08.930 --> 00:05:11.630
Well, in this case, yeah, it's
tempting, but it still like to

00:05:11.630 --> 00:05:12.480
be hanging around.

00:05:12.480 --> 00:05:13.410
Oxygen is very

00:05:13.410 --> 00:05:15.640
electronegative, but so is fluorine.

00:05:15.640 --> 00:05:17.170
So it likes to hang
around with both.

00:05:17.170 --> 00:05:19.530
So it kind of doesn't completely
disassociate.

00:05:19.530 --> 00:05:23.240
So that's why this one is a
hydrofluoric acid, is a weak

00:05:23.240 --> 00:05:29.220
acid, while all of the other
halogenic acids are strong.

00:05:29.220 --> 00:05:32.500
Well, with that said, the point
of this isn't to just

00:05:32.500 --> 00:05:33.720
look at the periodic table.

00:05:33.720 --> 00:05:38.630
Let's figure out what the pH is
when I dissolve-- let's say

00:05:38.630 --> 00:05:50.460
I have 1.5 molar of
hydrofluoric acid.

00:05:50.460 --> 00:05:53.740
And obviously, by definition,
since I've given this as

00:05:53.740 --> 00:05:55.605
molarity, I have it
in some solutions.

00:05:55.605 --> 00:05:59.320
In this case, I have it in
an aqueous solution.

00:05:59.320 --> 00:05:59.760
Now.

00:05:59.760 --> 00:06:01.790
You could look it up.

00:06:01.790 --> 00:06:02.950
Well actually, I'll
just write it.

00:06:02.950 --> 00:06:08.285
So every equilibrium reaction
has some equilibrium constant.

00:06:08.285 --> 00:06:11.380
So the equilibrium constant for
this reaction-- I'll do it

00:06:11.380 --> 00:06:16.080
in green-- and since this is an
acidic reaction, we'll say

00:06:16.080 --> 00:06:19.960
it's the equilibrium constant,
or an acid constant, however

00:06:19.960 --> 00:06:22.230
you want to say it, for
hydrogen fluorine.

00:06:22.230 --> 00:06:23.800
Hydrofluoric acid.

00:06:23.800 --> 00:06:27.395
Hydrofluoric acid is equal
to the products.

00:06:27.395 --> 00:06:34.140
So it's the concentration of
your hydrogen ions times your

00:06:34.140 --> 00:06:40.650
concentration of your fluorine
ions divided by your

00:06:40.650 --> 00:06:43.230
concentration of your hydrogen
fluoride, or

00:06:43.230 --> 00:06:44.380
hydrofluoric acid.

00:06:44.380 --> 00:06:45.630
So HFl.

00:06:49.020 --> 00:06:53.700
And if you look this up at 25
degrees Celsius at roughly

00:06:53.700 --> 00:06:58.280
room temperature-- I believe
that is, but let's say it is--

00:06:58.280 --> 00:07:05.250
you get a constant value here of
7 times 10 to the minus 4.

00:07:05.250 --> 00:07:10.210
So if I have 1.5 molar of
hydrofluoric acid, can I

00:07:10.210 --> 00:07:11.720
figure out what the pH is?

00:07:11.720 --> 00:07:15.390
Remember, esssentially, I want
to solve for this right here.

00:07:15.390 --> 00:07:18.370
If I solve for this, then
I know the hydrogen

00:07:18.370 --> 00:07:19.110
concentration.

00:07:19.110 --> 00:07:21.490
And then if I take the negative
log of that, then I

00:07:21.490 --> 00:07:23.130
know its pH.

00:07:23.130 --> 00:07:25.400
So let's see if we
can do that.

00:07:25.400 --> 00:07:25.990
So.

00:07:25.990 --> 00:07:28.990
I'm throwing 1.5 molar
onto this side of

00:07:28.990 --> 00:07:30.880
the equation, right?

00:07:30.880 --> 00:07:34.440
But some of it's going to go
and form that stuff, right?

00:07:34.440 --> 00:07:37.180
So let's say x molar
gets disassociated.

00:07:37.180 --> 00:07:43.440
So on this side-- let me
see if I can do this.

00:07:43.440 --> 00:07:47.250
So I'm throwing 1.5 molar
on this side.

00:07:47.250 --> 00:07:51.290
Let's say x molar goes
to that side, right?

00:07:51.290 --> 00:07:54.910
And so let's say I have 1.5
molar on this side.

00:07:54.910 --> 00:07:58.460
Once I'm in equilibrium,
x goes to

00:07:58.460 --> 00:07:59.640
that side of the equation.

00:07:59.640 --> 00:08:01.700
Or x reacts to form this
stuff, right here.

00:08:01.700 --> 00:08:03.230
Disassociates.

00:08:03.230 --> 00:08:08.700
So once I'm in equilibrium, I'll
have my concentration of

00:08:08.700 --> 00:08:13.860
hydrogen fluoride will
be 1.5 molar minus x.

00:08:13.860 --> 00:08:16.610
And what's my concentration
of hydrogen?

00:08:16.610 --> 00:08:20.990
Well, if x molar, or x moles of
this, the way you can view,

00:08:20.990 --> 00:08:24.405
disassociate, then we have x
moles or x molar of that.

00:08:24.405 --> 00:08:26.590
And we'll also have
x molar of that.

00:08:26.590 --> 00:08:27.390
Right?

00:08:27.390 --> 00:08:29.610
If one of the disassociates,
then you get one of that and

00:08:29.610 --> 00:08:30.540
one of that.

00:08:30.540 --> 00:08:33.250
So these will be the equilibrium
concentrations.

00:08:33.250 --> 00:08:36.559
And I think you can smell some
algebra coming our way.

00:08:36.559 --> 00:08:38.840
So these are the equilibrium
concentrations.

00:08:38.840 --> 00:08:42.809
Let's see if we can
solve for x.

00:08:42.809 --> 00:08:45.360
So we know what the equilibrium
constant is.

00:08:45.360 --> 00:08:48.520
It's 7 times 10 to
the minus fourth.

00:08:48.520 --> 00:08:51.860
We know that the final
equilibrium concentration of

00:08:51.860 --> 00:08:56.710
our hydrofluoric acid is the
original 1.5 molar we put in,

00:08:56.710 --> 00:09:00.130
minus whatever disassociates
into the stuff up here.

00:09:00.130 --> 00:09:04.550
And since x disassociated this
stuff here, I have x molar of

00:09:04.550 --> 00:09:08.900
hydrogen, and I have
molar of fluoride.

00:09:08.900 --> 00:09:10.400
This is just pure algebra now.

00:09:10.400 --> 00:09:12.440
Let's see if we can
solve this.

00:09:12.440 --> 00:09:15.260
So this is equal to--
I'm going to

00:09:15.260 --> 00:09:18.090
need space to do this.

00:09:18.090 --> 00:09:22.350
So we get 7 times 10
to the minus 4.

00:09:22.350 --> 00:09:25.510
Let's multiply both sides
times 1.5 minus x.

00:09:25.510 --> 00:09:28.310
Times 1.5 minus x.

00:09:28.310 --> 00:09:28.850
Is equal to

00:09:28.850 --> 00:09:31.450
x times x, is equal
to x squared.

00:09:31.450 --> 00:09:36.290
And then if we multiply-- 7
times 10 to the minus fourth

00:09:36.290 --> 00:09:41.330
times-- 7 times 1.5,
that's 10.5, right?

00:09:41.330 --> 00:09:47.230
So we have 10.5 times 10
to the minus fourth.

00:09:47.230 --> 00:09:48.700
That's that times that.

00:09:48.700 --> 00:09:54.950
Minus 7 times 10 to
the minus fourth x

00:09:54.950 --> 00:09:58.420
is equal to x squared.

00:09:58.420 --> 00:10:00.740
This looks complicated, but
it's just a quadratic

00:10:00.740 --> 00:10:03.240
equation, and there's
many videos on that.

00:10:03.240 --> 00:10:05.440
So let's just throw all of
this stuff onto to the

00:10:05.440 --> 00:10:06.850
right-hand side because
I like having a

00:10:06.850 --> 00:10:09.530
positive coefficient here.

00:10:09.530 --> 00:10:13.290
So if I subtract this from both
sides and add this to

00:10:13.290 --> 00:10:21.590
both sides, I get 0 is equal
to x squared, plus this, so

00:10:21.590 --> 00:10:26.010
plus 7 times 10 to the minus
fourth x, right?

00:10:26.010 --> 00:10:28.370
I just add this to both sides,
so it shows up on this side of

00:10:28.370 --> 00:10:30.120
the equation, which
is right here.

00:10:30.120 --> 00:10:30.740
Minus this.

00:10:30.740 --> 00:10:33.370
I'm subtracting this from both
sides of the equation.

00:10:33.370 --> 00:10:39.850
So I get minus 10.5 times
10 to the minus 4.

00:10:39.850 --> 00:10:43.540
Now, this is a quadratic
equation here.

00:10:43.540 --> 00:10:47.170
And actually, I'll throw
you a little--

00:10:47.170 --> 00:10:48.590
We can solve this quadratic
equation.

00:10:48.590 --> 00:10:49.140
We'll get the answer.

00:10:49.140 --> 00:10:51.870
In fact, I'm about
to solve it.

00:10:51.870 --> 00:10:55.550
But there is an approximation
you can do.

00:10:55.550 --> 00:10:58.400
Because if you look at this
quadratic equation, this term

00:10:58.400 --> 00:11:03.330
here, x squared, dominates
this term right here.

00:11:03.330 --> 00:11:06.460
Even when you have relatively
small values of x, it

00:11:06.460 --> 00:11:07.890
dominates that term.

00:11:07.890 --> 00:11:10.020
So if chemists look at
this and they say

00:11:10.020 --> 00:11:11.270
hey, you know what?

00:11:14.510 --> 00:11:18.450
If this term is so much smaller
than what's going on

00:11:18.450 --> 00:11:20.810
here, in a lot of the cases,
you might be able

00:11:20.810 --> 00:11:21.590
to just ignore it.

00:11:21.590 --> 00:11:24.310
And then the problem becomes
very simple.

00:11:24.310 --> 00:11:27.250
Because if you ignore this term,
it's still a quadratic,

00:11:27.250 --> 00:11:28.850
but you don't have to use
a quadratic equation.

00:11:28.850 --> 00:11:30.420
Actually I'll do it on
the side right here.

00:11:30.420 --> 00:11:32.140
I'll just do it in a box.

00:11:32.140 --> 00:11:34.020
Because this isn't the exact
answer, but it's going to get

00:11:34.020 --> 00:11:35.640
it's pretty close to
the exact answer.

00:11:35.640 --> 00:11:40.120
So in this-- I'll make a
nice little box here.

00:11:40.120 --> 00:11:41.760
I'll make a box.

00:11:41.760 --> 00:11:47.490
So if we just ignored this term,
then we'd have x squared

00:11:47.490 --> 00:11:49.600
is equal to this.

00:11:49.600 --> 00:11:53.820
Is equal to minus-- you would
add this to the other side, so

00:11:53.820 --> 00:11:58.070
you would have plus 10.5 times
10 to the minus 4.

00:11:58.070 --> 00:12:00.865
Or x would be equal do the
square root of this.

00:12:00.865 --> 00:12:07.030
x would be equal to-- what?

00:12:07.030 --> 00:12:13.420
See, 10.5-- I see you have to
put the exponent thing here.

00:12:13.420 --> 00:12:20.120
So exponent for minus
all right.

00:12:20.120 --> 00:12:25.460
And I want to take the square
root, so to the 0.5 power is

00:12:25.460 --> 00:12:29.040
equal to 0.0324.

00:12:29.040 --> 00:12:32.880
So this is 0.0324.

00:12:32.880 --> 00:12:36.320
So if we make that simplifying
assumption, we've essentially

00:12:36.320 --> 00:12:38.400
solved four our hydrogen
concentration, right?

00:12:38.400 --> 00:12:39.980
That was x.

00:12:39.980 --> 00:12:40.780
This is x.

00:12:40.780 --> 00:12:43.380
So this is 0.0324.

00:12:43.380 --> 00:12:48.880
And if we want to figure out
the pH, we just take the

00:12:48.880 --> 00:12:50.930
negative log of this.

00:12:50.930 --> 00:12:52.230
So let's do that.

00:12:52.230 --> 00:12:53.460
So you take the log of it.

00:12:53.460 --> 00:12:56.045
If you just press log,
that's base 10.

00:12:56.045 --> 00:12:58.170
So you take the log.

00:12:58.170 --> 00:13:01.550
And we want the negative of
that, so you just say minus

00:13:01.550 --> 00:13:05.290
1.489, so about 1.49.

00:13:05.290 --> 00:13:08.710
So it's a pH of 1.49.

00:13:08.710 --> 00:13:11.400
But remember, I did that using
my little simplifying

00:13:11.400 --> 00:13:11.725
assumption.

00:13:11.725 --> 00:13:14.090
It does not solve the
quadratic equation.

00:13:14.090 --> 00:13:16.670
Now let's solve it, just to kind
of confirm in our heads

00:13:16.670 --> 00:13:18.240
that we get the right answer.

00:13:18.240 --> 00:13:19.580
Because that's actually
the right way to

00:13:19.580 --> 00:13:20.460
get the right answer.

00:13:20.460 --> 00:13:22.150
So quadratic equation.

00:13:22.150 --> 00:13:24.050
Minus b.

00:13:24.050 --> 00:13:27.220
So x is going to be equal
to-- let me do it here.

00:13:27.220 --> 00:13:29.230
x is equal to minus b.

00:13:29.230 --> 00:13:34.610
So minus 7 times 10 to the
minus 4 plus or minus the

00:13:34.610 --> 00:13:38.030
square root of b squared.

00:13:38.030 --> 00:13:45.280
So b squared is 49 times 10
to the minus 8, right?

00:13:45.280 --> 00:13:46.280
7 squared.

00:13:46.280 --> 00:13:49.200
And then 10 to the
minus 4 squared.

00:13:49.200 --> 00:13:52.040
Is minus 4 times 2 is minus 8.

00:13:52.040 --> 00:13:56.810
Minus 4 times a, which
is 1, times c.

00:13:56.810 --> 00:14:00.110
Well, c is a minus sign there--
we have this minus

00:14:00.110 --> 00:14:01.550
sign, so it becomes plus.

00:14:01.550 --> 00:14:05.360
So 4 times 10.5 times 10
to the negative 4.

00:14:05.360 --> 00:14:05.860
That's what?

00:14:05.860 --> 00:14:12.390
That's 42 times 10 to the
minus fourth power.

00:14:12.390 --> 00:14:13.690
All of that over 2a.

00:14:13.690 --> 00:14:14.730
a is just 1.

00:14:14.730 --> 00:14:19.730
All of that over 2.

00:14:19.730 --> 00:14:21.610
So let's see what we
can get in this

00:14:21.610 --> 00:14:25.200
square root right here.

00:14:25.200 --> 00:14:28.220
And notice, this is a very small
number compared to this.

00:14:28.220 --> 00:14:30.390
This is really all we should
care about with the square

00:14:30.390 --> 00:14:32.020
root, but let's just do it.

00:14:32.020 --> 00:14:42.970
So 49 e 8 negative plus, so
that's that small number--

00:14:42.970 --> 00:14:50.560
plus 42 times 10 to the
negative-- or to the 4

00:14:50.560 --> 00:14:54.200
negative, is equal to that.

00:14:54.200 --> 00:14:55.940
So that's this value
right here.

00:14:55.940 --> 00:14:57.740
And now we want to take
the square root.

00:14:57.740 --> 00:15:04.570
So to the 0.5 power
is equal to 0.06.

00:15:04.570 --> 00:15:08.680
So so far, we get x is equal
to-- I will arbitrarily switch

00:15:08.680 --> 00:15:12.050
colors-- minus 7 times
10 to the minus 4

00:15:12.050 --> 00:15:16.940
plus or minus 0.0648.

00:15:16.940 --> 00:15:21.250
All of that over 2.

00:15:21.250 --> 00:15:22.750
And it should be clear.

00:15:22.750 --> 00:15:26.950
If we subtract it right here,
we get a negative number.

00:15:26.950 --> 00:15:28.640
This is already a negative, so
if we subrtact it, we get a

00:15:28.640 --> 00:15:29.160
negative numbers.

00:15:29.160 --> 00:15:31.150
So let's just worry about the
plus sign because we can't

00:15:31.150 --> 00:15:33.180
have a negative concentration.

00:15:33.180 --> 00:15:39.300
So if we add that to this right
there, so we have 0.06,

00:15:39.300 --> 00:15:45.470
and then we have minus--

00:15:45.470 --> 00:15:49.200
Actually let me make sure
that the minus is there.

00:15:49.200 --> 00:15:53.170
So it's minus 7 times 10.

00:15:53.170 --> 00:15:54.310
Right.

00:15:54.310 --> 00:15:56.450
I just want to make sure I'm
not making a mistake by

00:15:56.450 --> 00:15:57.700
putting a minus there.

00:16:00.040 --> 00:16:00.210
No.

00:16:00.210 --> 00:16:01.660
It looks right.

00:16:01.660 --> 00:16:04.830
So let's see.

00:16:04.830 --> 00:16:07.350
So we just do our math.

00:16:07.350 --> 00:16:09.120
So we take this and
we subtract that.

00:16:11.650 --> 00:16:13.620
So it's minus-- I want
to make sure you can

00:16:13.620 --> 00:16:15.540
see what I'm doing.

00:16:15.540 --> 00:16:30.370
So we have minus 7 e 4 negative
is equal to that and

00:16:30.370 --> 00:16:31.900
then we have to divide by 2.

00:16:31.900 --> 00:16:37.610
Divided by 2 is equal
to 0.032.

00:16:37.610 --> 00:16:41.100
So 0.032.

00:16:41.100 --> 00:16:43.170
And remember, this is our
hydrogen concentration.

00:16:43.170 --> 00:16:46.540
It's also going to be our
concentration of, of course,

00:16:46.540 --> 00:16:49.440
our fluorine ions.

00:16:49.440 --> 00:16:50.440
We don't care about that.

00:16:50.440 --> 00:16:52.230
We just want to figure
out the pH.

00:16:52.230 --> 00:16:54.670
So this is 0.032, which
is very close to our

00:16:54.670 --> 00:16:56.510
approximation before.

00:16:56.510 --> 00:16:58.310
But now we have the
exact number.

00:16:58.310 --> 00:17:00.180
So let's take the log
of it, and then take

00:17:00.180 --> 00:17:00.970
the minus of that.

00:17:00.970 --> 00:17:03.150
So the log of that.

00:17:03.150 --> 00:17:05.990
And then you want to put a minus
in front of the log.

00:17:05.990 --> 00:17:08.920
So the exact pH is
right there.

00:17:08.920 --> 00:17:17.310
So our pH of hydrofluoric
acid is equal to 1.49.

00:17:17.310 --> 00:17:19.869
So once again, pretty low pH.

00:17:19.869 --> 00:17:28.020
But notice, this pH--
this is 1.5 molar of

00:17:28.020 --> 00:17:29.850
hydrofluoric acid.

00:17:29.850 --> 00:17:37.060
While 1 molar of hydrochloric
acid was even more acidic.

00:17:37.060 --> 00:17:39.110
It had a pH of 0.

00:17:39.110 --> 00:17:42.190
So even though we put more acid
here, we didn't get our

00:17:42.190 --> 00:17:46.810
pH as low as when we used only
1 molar of hydrochloric acid.

00:17:46.810 --> 00:17:49.160
Anyway, see you in
the next video.


Acid Base Introduction

WEBVTT
Kind: captions
Language: en

00:00:00.000 --> 00:00:03.660
Sal: In the last video, we
learned that if you just leave

00:00:03.660 --> 00:00:06.470
water to itself,
it autoionizes.

00:00:06.470 --> 00:00:07.320
So let me see.

00:00:07.320 --> 00:00:09.330
I have molecules of water.

00:00:09.330 --> 00:00:12.420
There's some probability, and
it's an equilibrium, that one

00:00:12.420 --> 00:00:15.520
of the hydrogens on one of the
waters bumps off and just

00:00:15.520 --> 00:00:16.760
joins the other one.

00:00:16.760 --> 00:00:19.880
So you could end up with
a hydronium ion.

00:00:19.880 --> 00:00:22.460
It's an ion because it has
an extra hydrogen.

00:00:22.460 --> 00:00:24.110
So I'll put a plus
charge there.

00:00:24.110 --> 00:00:25.230
And obviously, everything
you hear is

00:00:25.230 --> 00:00:26.340
in an aqueous solution.

00:00:26.340 --> 00:00:28.440
I could write aqueous
everywhere.

00:00:28.440 --> 00:00:30.025
aq.

00:00:30.025 --> 00:00:31.320
I could write aq.

00:00:31.320 --> 00:00:32.700
It's kind of obvious.

00:00:32.700 --> 00:00:34.720
Water is obviously an
aqueous solution.

00:00:34.720 --> 00:00:37.070
It's obviously dissolved
in water.

00:00:37.070 --> 00:00:41.010
And then plus a hydroxide ion.

00:00:41.010 --> 00:00:45.300
This is the water molecule that
lost its hydrogen atom,

00:00:45.300 --> 00:00:46.370
or its proton.

00:00:46.370 --> 00:00:48.930
Because we know that a hydrogen
atom-- or at least a

00:00:48.930 --> 00:00:50.640
hydrogen ion-- if you get rid of
its electron, all you have

00:00:50.640 --> 00:00:51.810
left is a proton.

00:00:51.810 --> 00:00:54.490
And that's also in an
aqueous solution.

00:00:54.490 --> 00:00:57.030
And we learned that people
really care--

00:00:57.030 --> 00:00:58.370
We learned about
the equilibrium

00:00:58.370 --> 00:00:59.720
constant of this reaction.

00:00:59.720 --> 00:01:02.250
And we also learned that people
really care about the

00:01:02.250 --> 00:01:04.569
concentration of this
right here.

00:01:04.569 --> 00:01:06.230
Of the hydronium.

00:01:06.230 --> 00:01:08.720
And let me write this again.

00:01:08.720 --> 00:01:10.560
And I went over this
in the last video.

00:01:10.560 --> 00:01:14.140
But sometimes the same exact
autoionization reaction will

00:01:14.140 --> 00:01:15.710
be written like this.

00:01:15.710 --> 00:01:22.110
H2O, disassociating in itself,
really, into a hydrogen ion in

00:01:22.110 --> 00:01:28.570
an aqueous solution plus a
hydroxide anion in an aqueous

00:01:28.570 --> 00:01:30.700
solution or a negative ion.

00:01:30.700 --> 00:01:33.540
These are the same thing, and
I write it this way because

00:01:33.540 --> 00:01:35.200
this is actually the
state that happens.

00:01:35.200 --> 00:01:37.450
That you don't actually have
these protons just

00:01:37.450 --> 00:01:40.760
independently, they actually
do join onto another water

00:01:40.760 --> 00:01:42.470
molecule and form
a hydronium ion.

00:01:42.470 --> 00:01:45.630
But these are the same
exact concept.

00:01:45.630 --> 00:01:47.530
But we learned in the last video
that people really care

00:01:47.530 --> 00:01:49.750
about the concentration
of-- you could pick

00:01:49.750 --> 00:01:50.420
one of these two.

00:01:50.420 --> 00:01:53.250
They normally write this
one, right here.

00:01:53.250 --> 00:01:57.550
So people really care about
the concentration of your

00:01:57.550 --> 00:01:58.380
hydrogen ions.

00:01:58.380 --> 00:02:01.180
And we learned in the last video
that just pure water at

00:02:01.180 --> 00:02:05.060
25 degrees Celsius, which
is room temperature.

00:02:05.060 --> 00:02:05.750
So let me write that.

00:02:05.750 --> 00:02:08.830
At 25 degrees Celsius.

00:02:08.830 --> 00:02:13.700
The concentration of your
hydrogen protons-- because

00:02:13.700 --> 00:02:16.840
that's really what they are,
or hydrogen ions-- is 10 to

00:02:16.840 --> 00:02:19.360
the minus 7 molars.

00:02:19.360 --> 00:02:23.110
And then we even said, hey,
chemists, for whatever reason,

00:02:23.110 --> 00:02:25.010
don't like dealing with
negative exponents.

00:02:25.010 --> 00:02:35.770
So they defined the pH as equal
to the minus log base 10

00:02:35.770 --> 00:02:37.020
of your hydrogen
concentration.

00:02:40.330 --> 00:02:47.540
And of course, that's equal to
minus log base 10 of 10 to the

00:02:47.540 --> 00:02:54.550
minus 7 for pure water, at 25
degrees, which is equal to 7.

00:02:54.550 --> 00:02:55.730
Fair enough.

00:02:55.730 --> 00:02:58.670
And you can probably imagine, if
people took the trouble of

00:02:58.670 --> 00:03:02.350
constructing this pH thing and
saying it's the negative log,

00:03:02.350 --> 00:03:04.580
they must really care about
what the hydrogen

00:03:04.580 --> 00:03:06.090
concentration of water is.

00:03:06.090 --> 00:03:08.670
And it does matter to-- in a
whole bunch of-- especially

00:03:08.670 --> 00:03:11.430
actually biological systems--
all the time, people worry

00:03:11.430 --> 00:03:12.920
about the pH of this or that.

00:03:12.920 --> 00:03:14.540
And obviously I talked
in the last video

00:03:14.540 --> 00:03:18.270
about pH-balanced cosmetics.

00:03:18.270 --> 00:03:20.920
All sorts of bad things might
happen to your skin if the pH

00:03:20.920 --> 00:03:22.660
of the cosmetic is different
than your skin.

00:03:22.660 --> 00:03:25.240
I don't know if that's a fact,
but I guess that's the that's

00:03:25.240 --> 00:03:26.150
the theory behind it.

00:03:26.150 --> 00:03:29.490
So there's obviously things in
the world that change the

00:03:29.490 --> 00:03:30.940
hydrogen concentration.

00:03:30.940 --> 00:03:32.630
That change this
concentration.

00:03:32.630 --> 00:03:36.640
So, when we learned about Le
Chatelier's Principle, we said

00:03:36.640 --> 00:03:40.910
hey, maybe if we add more
hydrogen on this side, the

00:03:40.910 --> 00:03:44.620
equilibrium will change and this
concentration will go up.

00:03:44.620 --> 00:03:46.270
And so we have more hydrogen
concentration.

00:03:46.270 --> 00:03:49.160
I guess on the other side, you
could add more hydroxide and

00:03:49.160 --> 00:03:52.670
then you would have more OH
concentration, and maybe a

00:03:52.670 --> 00:03:55.220
little less hydrogen
concentration.

00:03:55.220 --> 00:03:57.700
So if you if you care about
things that are going to do

00:03:57.700 --> 00:04:00.260
that to this reaction,
you probably want to

00:04:00.260 --> 00:04:01.790
have a name for them.

00:04:01.790 --> 00:04:06.350
And the name for them
is acids and bases.

00:04:06.350 --> 00:04:09.260
And I'm sure you've heard both
of these words before.

00:04:09.260 --> 00:04:11.850
So, let me write
them both down.

00:04:14.540 --> 00:04:18.920
Acid and base.

00:04:18.920 --> 00:04:19.224
Now.

00:04:19.224 --> 00:04:23.155
Just to over-complicate your
life a little bit more, there

00:04:23.155 --> 00:04:26.010
are actually multiple
definitions of an acid and

00:04:26.010 --> 00:04:27.450
multiple definitions
of a base.

00:04:27.450 --> 00:04:30.940
And they kind of become
more and more broad

00:04:30.940 --> 00:04:32.720
in what they imply.

00:04:32.720 --> 00:04:34.870
The definition that you're
probably going to use the

00:04:34.870 --> 00:04:37.750
most-- the definition that I
always use in my head whenever

00:04:37.750 --> 00:04:43.760
someone says an acid-- is
it increases hydrogen

00:04:43.760 --> 00:04:46.900
concentration when you
place it in water.

00:04:50.710 --> 00:04:56.940
And a base is something that
increases your hydroxide

00:04:56.940 --> 00:04:59.950
concentration in water.

00:04:59.950 --> 00:05:02.010
This is the definition that
you're probably going to see

00:05:02.010 --> 00:05:04.680
in 90%-- especially in a first
year chemistry class.

00:05:04.680 --> 00:05:08.410
And this is called the Arrhenius
acid or base.

00:05:08.410 --> 00:05:10.050
I hope I get his
spelling right.

00:05:10.050 --> 00:05:12.200
You should never take any
of the my spelling

00:05:12.200 --> 00:05:13.860
words at face value.

00:05:13.860 --> 00:05:17.210
You should look them up because
I'm clearly not a

00:05:17.210 --> 00:05:18.500
master speller.

00:05:18.500 --> 00:05:20.130
Arrhenius.

00:05:20.130 --> 00:05:22.230
Arrhenius, with an H.

00:05:22.230 --> 00:05:24.150
These are Arrhenius acid,
Arrhenius base.

00:05:27.940 --> 00:05:29.190
Let me give you an example.

00:05:32.340 --> 00:05:38.280
If I were to take some
hydrochloric acid-- that's

00:05:38.280 --> 00:05:40.150
actually what it's called, but
you could say hydrogen

00:05:40.150 --> 00:05:41.840
chloride-- in an aqueous
solution.

00:05:44.380 --> 00:05:46.140
It actually disassociates
completely.

00:05:46.140 --> 00:05:47.830
This is not an equilibrium
reaction.

00:05:47.830 --> 00:05:51.130
So let me put it in
this nice-- let me

00:05:51.130 --> 00:05:52.610
use a bolder color.

00:05:52.610 --> 00:05:56.000
This nice one-way arrow, which
says, hey, this isn't one of

00:05:56.000 --> 00:05:58.780
these wishy-washy reactions that
go in both directions.

00:05:58.780 --> 00:06:00.965
This is a strong acid.

00:06:00.965 --> 00:06:05.740
It completely disassociates in
water into-- let me put the

00:06:05.740 --> 00:06:08.010
hydrogen in a different color.

00:06:08.010 --> 00:06:12.210
It completely disassociates in
water into a hydrogen ion.

00:06:12.210 --> 00:06:20.440
An aqueous solution
plus chloride.

00:06:20.440 --> 00:06:23.000
So it's just a chloride
negative ion

00:06:23.000 --> 00:06:24.050
in an aqueous solution.

00:06:24.050 --> 00:06:25.070
And they just float in there.

00:06:25.070 --> 00:06:28.410
So you could imagine if you
place hydrogen chloride in

00:06:28.410 --> 00:06:32.470
water, you're going to increase
its overall hydrogen

00:06:32.470 --> 00:06:33.450
concentration.

00:06:33.450 --> 00:06:35.560
And the same exact reaction
could have been

00:06:35.560 --> 00:06:36.760
written like this.

00:06:36.760 --> 00:06:38.570
Just to hit the point home.

00:06:38.570 --> 00:06:45.510
Hydrogen chloride in an
aqueous solution.

00:06:45.510 --> 00:06:49.504
And then you can say plus
h-- let me do it

00:06:49.504 --> 00:06:50.420
in a different color.

00:06:50.420 --> 00:06:54.085
Plus H2O.

00:06:54.085 --> 00:06:55.690
It's a one-way reaction.

00:06:58.750 --> 00:07:01.106
And then you end up with, and
I think you understand.

00:07:05.940 --> 00:07:11.460
Obviously not all 3 of them
are-- H3O, that's too dark.

00:07:11.460 --> 00:07:16.220
H3O plus-- it donated
its hydrogen proton

00:07:16.220 --> 00:07:18.390
to this water molecule.

00:07:18.390 --> 00:07:23.610
And then you have plus
the chloride anion

00:07:23.610 --> 00:07:25.690
in an aqueous solution.

00:07:25.690 --> 00:07:29.560
And these acids that completely
disassociate-- in

00:07:29.560 --> 00:07:32.140
future videos, we're going to
look at ones that are a little

00:07:32.140 --> 00:07:33.150
bit more wishy-washy.

00:07:33.150 --> 00:07:35.220
That don't completely
diaassociate.

00:07:35.220 --> 00:07:37.930
But these acids that completely
disassociate are

00:07:37.930 --> 00:07:40.300
called strong acids.

00:07:40.300 --> 00:07:43.170
And strong isn't just
a nice word

00:07:43.170 --> 00:07:44.100
that they use in chemistry.

00:07:44.100 --> 00:07:47.770
It literally means-- when
someone says a strong acid,

00:07:47.770 --> 00:07:50.000
something that completely
disassociates into water.

00:07:50.000 --> 00:07:52.310
It's a one-way reaction.

00:07:52.310 --> 00:07:55.340
And all of a strong acids-- you
could probably even guess

00:07:55.340 --> 00:07:58.020
based on their chemical
make-up.

00:07:58.020 --> 00:08:00.830
But hydrochloric acid
is one of them.

00:08:00.830 --> 00:08:04.190
You also have hydrogen
bromide.

00:08:04.190 --> 00:08:05.430
Another strong acid.

00:08:05.430 --> 00:08:06.680
Hydrogen iodide.

00:08:09.260 --> 00:08:10.380
You have nitric acid.

00:08:10.380 --> 00:08:12.300
I think I showed you that
in a previous--

00:08:12.300 --> 00:08:15.090
All of these, when you when you
put them into water, this

00:08:15.090 --> 00:08:16.910
little h is going to pop off.

00:08:16.910 --> 00:08:19.490
You join another water molecule,
form hydronium.

00:08:19.490 --> 00:08:24.070
And then this molecule right
here, in these cases-- well,

00:08:24.070 --> 00:08:27.160
in the case of chloride--
these are halogens.

00:08:27.160 --> 00:08:29.260
But they're going to go and
form negative ions.

00:08:29.260 --> 00:08:31.250
Whatever's left over.

00:08:31.250 --> 00:08:32.320
Nitric acid.

00:08:32.320 --> 00:08:34.049
Then you have sulfuric acid.

00:08:34.049 --> 00:08:35.230
You've probably heard of that.

00:08:35.230 --> 00:08:37.870
Very strong acid.

00:08:37.870 --> 00:08:40.820
Sulfuric acid and then
perchloric acid.

00:08:40.820 --> 00:08:42.370
These are the strong acids.

00:08:45.460 --> 00:08:46.330
O4.

00:08:46.330 --> 00:08:48.660
And these are good to know,
because if you see them on a

00:08:48.660 --> 00:08:51.590
chemistry exam, you know that
these are going to completely

00:08:51.590 --> 00:08:52.480
disassociate.

00:08:52.480 --> 00:08:54.710
And remember, we keep using
this word acid.

00:08:54.710 --> 00:08:55.830
What does acid mean?

00:08:55.830 --> 00:08:57.650
It means that these
are strong acids.

00:08:57.650 --> 00:08:59.530
These completely disassociate.

00:08:59.530 --> 00:09:04.990
If you like Arrhenius's
definition of what an acid is,

00:09:04.990 --> 00:09:07.860
they increase your hydrogen
concentration.

00:09:07.860 --> 00:09:10.730
Because obviously when you throw
this in some water, all

00:09:10.730 --> 00:09:14.320
of these new hydronium ions are
going to be formed, and

00:09:14.320 --> 00:09:16.720
they're going to overall
increase your

00:09:16.720 --> 00:09:17.580
concentration of them.

00:09:17.580 --> 00:09:19.650
And that's why they're
called an acid.

00:09:19.650 --> 00:09:22.720
And now if you went on the
strong base side, by

00:09:22.720 --> 00:09:29.350
Arrhenius's definition, the base
is something that creates

00:09:29.350 --> 00:09:34.360
hydroxide ions, or
anions in water.

00:09:34.360 --> 00:09:39.130
If you look at the periodic
table, any of your group 1

00:09:39.130 --> 00:09:43.230
elements, and group 1 elements
are your alkali earth metals.

00:09:43.230 --> 00:09:46.880
Your alkali earth metals are
bonded with hydroxide.

00:09:46.880 --> 00:09:49.990
If you put them in water,
the hydroxide pops off.

00:09:49.990 --> 00:09:54.130
So let me just-- has to do
with lithium or sodium.

00:09:54.130 --> 00:09:56.030
So for example, these
are strong bases.

00:09:56.030 --> 00:10:06.750
So if I take some lithium
the hydroxide

00:10:06.750 --> 00:10:08.060
in an aqueous solution.

00:10:11.080 --> 00:10:14.290
This completely disassociates.

00:10:14.290 --> 00:10:16.490
No equilibrium here.

00:10:16.490 --> 00:10:19.100
This is a strong base.

00:10:19.100 --> 00:10:20.790
So what's it going to
disassociate to?

00:10:20.790 --> 00:10:24.990
Well the little hydroxide
ion is going to pop off.

00:10:24.990 --> 00:10:26.010
That's going to be a minus.

00:10:26.010 --> 00:10:28.850
It's in water still.

00:10:28.850 --> 00:10:36.170
Plus a lithium ion.

00:10:36.170 --> 00:10:37.300
Right there.

00:10:37.300 --> 00:10:39.320
So this will always disassociate
in water.

00:10:39.320 --> 00:10:42.085
The same thing is if
you have sodium.

00:10:47.270 --> 00:10:50.910
Sodium's going to do
the same thing.

00:10:50.910 --> 00:10:53.830
Sodium hydroxide in an
aqueous solution.

00:10:53.830 --> 00:10:54.860
One-way reaction.

00:10:54.860 --> 00:11:00.950
You produce hydroxide and
some sodium cations.

00:11:00.950 --> 00:11:03.040
And so you imagine, when you
put these in water, this

00:11:03.040 --> 00:11:04.400
concentration is
going to go up.

00:11:04.400 --> 00:11:06.960
It's just like what we did in
Le Chetalier's Principal.

00:11:06.960 --> 00:11:09.720
Where we said, oh, what if you
add some of this or some of

00:11:09.720 --> 00:11:11.720
that to an equilibrium
reaction.

00:11:11.720 --> 00:11:12.980
Well, how do you add it?

00:11:12.980 --> 00:11:14.290
Well, in this case, you
can add a strong

00:11:14.290 --> 00:11:16.200
acid or a strong base.

00:11:16.200 --> 00:11:19.200
Now, everything I've done so
far-- this is the Arrhenius

00:11:19.200 --> 00:11:22.760
definition-- where an acid
increases your hydrogen

00:11:22.760 --> 00:11:27.130
concentration, a base increases
your hydroxide

00:11:27.130 --> 00:11:28.480
concentration.

00:11:28.480 --> 00:11:31.680
Now that is what you're going
to see 90% of the time.

00:11:31.680 --> 00:11:36.410
But there is a slightly broader
definition out there.

00:11:36.410 --> 00:11:37.930
And I don't know the correct--
I always say

00:11:37.930 --> 00:11:40.640
just Bronsted, but.

00:11:40.640 --> 00:11:42.840
If you don't have fancy fonts,
it's spelled like this.

00:11:42.840 --> 00:11:50.720
Bronsted-Lowry acid or base.

00:11:50.720 --> 00:11:52.720
If you if you have good fonts,
there's usually this little

00:11:52.720 --> 00:11:55.480
cross at the o.

00:11:55.480 --> 00:11:58.130
So I don't know.

00:11:58.130 --> 00:11:59.060
Well, Bronsted-Lowry acid.

00:11:59.060 --> 00:12:02.480
And all of these would also
be Bronsted-Lowry acid.

00:12:02.480 --> 00:12:08.810
But the broadening of the
definition is an acid is a

00:12:08.810 --> 00:12:10.060
proton donor.

00:12:12.450 --> 00:12:22.630
And a base is a proton
acceptor.

00:12:25.690 --> 00:12:28.290
So let's look at this definition
in the context of

00:12:28.290 --> 00:12:29.720
everything we've just
done so far.

00:12:29.720 --> 00:12:33.370
So if you live buy the
Bronsted-Lowry definition,

00:12:33.370 --> 00:12:36.310
what is a proton donor here?

00:12:36.310 --> 00:12:39.070
Well, this hydrochloric acid
is-- let me look at this

00:12:39.070 --> 00:12:40.390
reaction right here.

00:12:40.390 --> 00:12:43.600
This hydrochloric acid is
donating a proton to this

00:12:43.600 --> 00:12:44.980
water molecule, right?

00:12:44.980 --> 00:12:47.510
A hydrogen atom is
just a proton.

00:12:47.510 --> 00:12:50.300
That's an important thing to
remember, because it had-- a

00:12:50.300 --> 00:12:51.250
hydrogen ion.

00:12:51.250 --> 00:12:53.700
Because if you get rid of its
electron, it has no neutron.

00:12:53.700 --> 00:12:56.300
It's just a proton
sitting in space.

00:12:56.300 --> 00:12:59.650
So this hydrochloric acid is
donating a proton to this

00:12:59.650 --> 00:13:02.560
water molecule to make a
hydronium molecule, and it

00:13:02.560 --> 00:13:03.490
gave it away.

00:13:03.490 --> 00:13:06.050
So this is a Bronsted-Lowry
acid as well.

00:13:06.050 --> 00:13:08.190
Just as it was an
Arrhenius acid.

00:13:08.190 --> 00:13:12.120
Let's see the Bronsted-Lowry
version of a base.

00:13:12.120 --> 00:13:12.500
OK.

00:13:12.500 --> 00:13:16.870
A base is someone who--
proton acceptor.

00:13:16.870 --> 00:13:20.820
So this guy, he had this nice
little relationship with this

00:13:20.820 --> 00:13:23.050
hydroxide ion.

00:13:23.050 --> 00:13:27.380
The hydroxide ion got
disassociated, and it becomes

00:13:27.380 --> 00:13:29.750
a little bit fungible
right here.

00:13:29.750 --> 00:13:33.450
You can kind of say hey,
this guy accepted

00:13:33.450 --> 00:13:36.250
this positive charge.

00:13:36.250 --> 00:13:39.110
And, you know, it's a little
bit wishy-washy here.

00:13:39.110 --> 00:13:44.340
Because it's not like someone
gave it a proton.

00:13:44.340 --> 00:13:45.690
You can kind of think
of it as he

00:13:45.690 --> 00:13:48.160
actually gave away electrons.

00:13:48.160 --> 00:13:51.080
But if you just look at
the final product.

00:13:51.080 --> 00:13:53.990
OK, he's got a positive charge
when everything was done.

00:13:53.990 --> 00:14:00.010
So you say OK, maybe that's
a Bronsted-Lowry base.

00:14:00.010 --> 00:14:02.940
Now you're saying, OK well, why
did people even make the

00:14:02.940 --> 00:14:06.880
trouble of defining a
Bronsted-Lowry base when all

00:14:06.880 --> 00:14:09.540
of the Arrhenius acids and
bases could also be

00:14:09.540 --> 00:14:11.020
Bronsted-Lowry.

00:14:11.020 --> 00:14:12.690
Well that's because Arrhenius
is always--

00:14:12.690 --> 00:14:13.850
You're always dealing
with water.

00:14:13.850 --> 00:14:15.380
Everything's in an
aqueous solution.

00:14:15.380 --> 00:14:19.290
But I drew an example here
of a Bronsted-Lowry base.

00:14:19.290 --> 00:14:21.270
It doesn't have to be
an aqueous solution.

00:14:21.270 --> 00:14:23.870
So if you have acetic
acid right here--

00:14:23.870 --> 00:14:25.180
that's what's in vinegar.

00:14:25.180 --> 00:14:30.020
Plus ammonia-- it doesn't have
to be in an aqueous solution--

00:14:30.020 --> 00:14:31.160
what happens?

00:14:31.160 --> 00:14:33.410
This hydrogen gets
donated to the

00:14:33.410 --> 00:14:35.470
ammonia ion to make ammonium.

00:14:35.470 --> 00:14:37.290
So this becomes positive,
this because negative.

00:14:37.290 --> 00:14:42.360
It donated this proton.

00:14:42.360 --> 00:14:45.400
And Arrhenius has nothing
to say for this.

00:14:45.400 --> 00:14:48.320
Because everything he deals with
is hydronium and water.

00:14:48.320 --> 00:14:50.710
But the Bronsted-Lowry
definition

00:14:50.710 --> 00:14:53.310
works in the situation.

00:14:53.310 --> 00:14:58.940
Now the broadest definition--
although they're all these

00:14:58.940 --> 00:15:01.370
definitions, but you're going to
be 90% of the time good if

00:15:01.370 --> 00:15:03.380
you just know the Arrhenius
definition and just know the

00:15:03.380 --> 00:15:06.280
Bronsted-Lowry and what
I'm about to say

00:15:06.280 --> 00:15:07.430
right now also exists.

00:15:07.430 --> 00:15:09.000
And that's Lewis acids
and bases.

00:15:14.820 --> 00:15:16.780
Now Lewis cares about
electrons.

00:15:16.780 --> 00:15:19.340
Bronsted-Lowry cared
about protons.

00:15:19.340 --> 00:15:25.090
So Lewis, instead of saying an
acid is a proton donor, Lewis

00:15:25.090 --> 00:15:27.320
acid says it's an electron
acceptor.

00:15:32.480 --> 00:15:37.712
And a base is an
electron donor.

00:15:37.712 --> 00:15:40.090
Now, let's us look at in the
context of everything we've

00:15:40.090 --> 00:15:41.340
done so far.

00:15:44.010 --> 00:15:45.770
If this is really an
acid, it should

00:15:45.770 --> 00:15:48.320
be an electron acceptor.

00:15:48.320 --> 00:15:50.190
And the way you can kind of
think of it is you had

00:15:50.190 --> 00:15:56.840
hydrochloric acid before, when
this guy gave away this atom

00:15:56.840 --> 00:15:58.370
or this proton right there.

00:15:58.370 --> 00:16:01.200
He kind of kept his elecron.

00:16:01.200 --> 00:16:02.850
So he was kind of an
electron acceptor.

00:16:02.850 --> 00:16:05.580
It's a little bit of
a grey area there.

00:16:05.580 --> 00:16:08.250
It's not like he took an
electron from somebody else.

00:16:08.250 --> 00:16:10.740
But this would be considered
still a Lewis acid.

00:16:10.740 --> 00:16:15.380
And if you think about a Lewis
base, a Lewis base is what?

00:16:15.380 --> 00:16:19.495
It's an electron donor, right?

00:16:25.630 --> 00:16:28.890
So if you have it right
here, Lewis base

00:16:28.890 --> 00:16:30.235
is an electron donor.

00:16:30.235 --> 00:16:33.870
This lithium hydroxide, when it
goes into water-- There's a

00:16:33.870 --> 00:16:34.840
couple of ways to think of it.

00:16:34.840 --> 00:16:38.800
You can say hey, it's donating
this OH, which has these 2

00:16:38.800 --> 00:16:44.520
extra-- this negative charge,
so it's an electron donor.

00:16:44.520 --> 00:16:45.150
I don't lik it.

00:16:45.150 --> 00:16:48.050
It's kind of squishy that way,
so I'm not a big fan of that

00:16:48.050 --> 00:16:49.230
definition.

00:16:49.230 --> 00:16:51.270
In my mind, Arrhenius makes
the most sense.

00:16:51.270 --> 00:16:52.770
It's kind of the purist thing.

00:16:52.770 --> 00:16:54.980
Are you creating hydronium
or not?

00:16:54.980 --> 00:16:56.990
Or are you creating
hydroxide or not?

00:16:56.990 --> 00:17:01.870
But just to show that the Lewis
acid base definition is

00:17:01.870 --> 00:17:03.070
the broadest definition.

00:17:03.070 --> 00:17:06.930
Here's a case of a Lewis
acid base reaction.

00:17:06.930 --> 00:17:12.170
That would not be considered
either a Bronsted-Lowry or an

00:17:12.170 --> 00:17:13.150
Arrhenius base.

00:17:13.150 --> 00:17:14.720
Because this doesn't have
to happen in water.

00:17:14.720 --> 00:17:18.349
And what you have here is boron
trifluoride, with a

00:17:18.349 --> 00:17:19.960
fluoride anion.

00:17:19.960 --> 00:17:20.920
It has a negative charge.

00:17:20.920 --> 00:17:23.140
It has this little extra
electron here.

00:17:23.140 --> 00:17:28.150
In this situation, this fluoride
right here, or this

00:17:28.150 --> 00:17:32.940
fluorine anion can donate
these 2 electrons.

00:17:32.940 --> 00:17:36.810
So let's say that this
one right here is

00:17:36.810 --> 00:17:37.780
this one right here.

00:17:37.780 --> 00:17:43.370
And what it does is, it donates
these 2 electrons to

00:17:43.370 --> 00:17:46.280
this boron complex right here.

00:17:46.280 --> 00:17:49.770
And so if it's the electron
donor, what is it?

00:17:49.770 --> 00:17:51.550
The fluoride is the base.

00:17:51.550 --> 00:17:56.510
So this is Lewis base.

00:17:56.510 --> 00:17:58.970
And then the electron acceptor
is the Lewis acid.

00:17:58.970 --> 00:18:02.860
So this right here is
the Lewis Acid.

00:18:02.860 --> 00:18:05.210
It's good to know that these
definitions exist, and

00:18:05.210 --> 00:18:08.040
especially so you don't get
confused in the future.

00:18:08.040 --> 00:18:10.730
But for a first-year chemistry
class, if you know the

00:18:10.730 --> 00:18:15.560
Arrhenius definition
really well, you'll

00:18:15.560 --> 00:18:16.690
do very, very well.

00:18:16.690 --> 00:18:18.840
And frankly in my mind, that's
the easiest one to

00:18:18.840 --> 00:18:19.570
contemplate.

00:18:19.570 --> 00:18:21.750
And that's the one that's going
to matter in most of the

00:18:21.750 --> 00:18:26.590
reactions where you deal with
the changes in pH to water.

00:18:26.590 --> 00:18:28.680
Because that's what all the
acids and bases do, especially

00:18:28.680 --> 00:18:30.020
in the Arrhenius case.

00:18:30.020 --> 00:18:32.040
Anyway, see you in the next
video and watch as we do some

00:18:32.040 --> 00:18:35.970
math, figuring out the changes
in pH due to some acid.