Acid–base indicators

WEBVTT
Kind: captions
Language: en

00:00:00.320 --> 00:00:03.000
- [Instructor] Acid-base
indicators are used in titrations

00:00:03.000 --> 00:00:06.060
to determine when the
equivalence point is reached.

00:00:06.060 --> 00:00:08.650
Let's look at a hypothetical indicator.

00:00:08.650 --> 00:00:13.456
In the protonated form, the
indicator has the formula HIn.

00:00:13.456 --> 00:00:18.010
So this would be the acidic
proton on this protonated form.

00:00:18.010 --> 00:00:20.210
When base is added, the protonated form

00:00:20.210 --> 00:00:23.490
is converted into the deprotonated form.

00:00:23.490 --> 00:00:26.040
So we lose an H and we
lose a positive charge.

00:00:26.040 --> 00:00:28.580
So the deprotonated form is represented by

00:00:28.580 --> 00:00:30.860
In with a negative charge.

00:00:30.860 --> 00:00:34.240
And if we add an H plus
to the deprotonated form,

00:00:34.240 --> 00:00:36.110
we would make the protonated form

00:00:36.110 --> 00:00:37.990
of the acid-base indicator.

00:00:37.990 --> 00:00:40.280
For this hypothetical acid-base indicator,

00:00:40.280 --> 00:00:42.390
the protonated form is red

00:00:42.390 --> 00:00:45.730
and the deprotonated form is yellow.

00:00:45.730 --> 00:00:47.740
Next, let's think about
the color of a solution

00:00:47.740 --> 00:00:51.750
containing this acid-base
indicator at different pH values.

00:00:51.750 --> 00:00:55.160
Let's say that the pKa
value for the acidic proton

00:00:55.160 --> 00:00:58.400
on the protonated form is equal to three

00:00:58.400 --> 00:01:00.800
at 25 degrees Celsius.

00:01:00.800 --> 00:01:05.500
And let's say the pH of the
solution is equal to two.

00:01:05.500 --> 00:01:07.300
In that case, the pH of the solution

00:01:07.300 --> 00:01:12.090
is less than the pKa value
for our acidic proton.

00:01:12.090 --> 00:01:15.870
So if you think about the
protonated form being a weak acid

00:01:15.870 --> 00:01:19.610
and the deprotonated form
being the conjugate base,

00:01:19.610 --> 00:01:22.350
when the pH is less than the pKa value

00:01:22.350 --> 00:01:24.680
for a conjugate acid-base pair,

00:01:24.680 --> 00:01:27.760
the concentration of the
weak acid is greater than

00:01:27.760 --> 00:01:30.660
the concentration of the conjugate base.

00:01:30.660 --> 00:01:33.610
At a pH of two, we have a lot
more of the protonated form

00:01:33.610 --> 00:01:36.830
which is red than the
deprotonated form which is yellow.

00:01:36.830 --> 00:01:40.220
Therefore, the color of the
solution that a pH of two

00:01:40.220 --> 00:01:42.530
would appear to be red.

00:01:42.530 --> 00:01:44.930
Next, let's think about
the color of the solution

00:01:44.930 --> 00:01:48.480
if the pH of the solution
is equal to four.

00:01:48.480 --> 00:01:50.710
Since the pKa value is equal to three,

00:01:50.710 --> 00:01:52.660
if the pH is equal to four,

00:01:52.660 --> 00:01:56.690
the pH is greater than the pKa value.

00:01:56.690 --> 00:01:58.640
And for a conjugate acid-base pair,

00:01:58.640 --> 00:02:02.230
when the pH of the solution
is greater than the pKa value,

00:02:02.230 --> 00:02:04.030
the concentration of the conjugate base

00:02:04.030 --> 00:02:07.160
is greater than the
concentration of the weak acid.

00:02:07.160 --> 00:02:08.580
Or as I have written here,

00:02:08.580 --> 00:02:10.980
the concentration of the
weak acid is less than

00:02:10.980 --> 00:02:13.460
the concentration of the conjugate base.

00:02:13.460 --> 00:02:15.490
So at a pH of four, we have a lot more

00:02:15.490 --> 00:02:18.000
of the deprotonated form which is yellow

00:02:18.000 --> 00:02:20.010
than the protonated form which is red.

00:02:20.010 --> 00:02:22.240
And since we have more yellow than red,

00:02:22.240 --> 00:02:24.480
the solution would be yellow.

00:02:24.480 --> 00:02:26.380
Finally, let's think about the situation

00:02:26.380 --> 00:02:29.700
where the pH is equal to three.

00:02:29.700 --> 00:02:32.980
Since the pKa value is
also equal to three,

00:02:32.980 --> 00:02:36.270
the pH is equal to the pKa value.

00:02:36.270 --> 00:02:38.210
And for a conjugate acid-base pair,

00:02:38.210 --> 00:02:40.560
when the pH is equal to the pKa,

00:02:40.560 --> 00:02:42.900
the concentration of the weak acid

00:02:42.900 --> 00:02:46.740
is equal to the concentration
of the conjugate base.

00:02:46.740 --> 00:02:49.120
And if we have equal amounts
of the protonated form

00:02:49.120 --> 00:02:51.790
and the deprotonated form,
we have equal amounts

00:02:51.790 --> 00:02:55.500
of red and yellow,
therefore at a pH of three,

00:02:55.500 --> 00:02:58.310
the solution would be orange.

00:02:58.310 --> 00:03:01.400
Next let's think the pH
range of the color change

00:03:01.400 --> 00:03:03.630
for this hypothetical indicator.

00:03:03.630 --> 00:03:06.560
The approximate pH range
over which an indicator

00:03:06.560 --> 00:03:11.560
changes color is equal to the
pKa value plus or minus one.

00:03:12.160 --> 00:03:14.990
The pKa value for our
hypothetical indicator

00:03:14.990 --> 00:03:18.080
was equal to three,
therefore three plus one

00:03:18.080 --> 00:03:21.150
is equal to four and three
minus one is equal to two,

00:03:21.150 --> 00:03:24.700
and two to four is the
approximate pH range

00:03:24.700 --> 00:03:27.410
over which our indicator changes color.

00:03:27.410 --> 00:03:28.910
And we already know our solution

00:03:28.910 --> 00:03:33.400
with the acid-base indicator
in it at a pH of two is red

00:03:33.400 --> 00:03:35.870
and at a pH of three
the solution is orange

00:03:35.870 --> 00:03:38.580
and at a pH of four
the solution is yellow.

00:03:38.580 --> 00:03:42.410
Therefore, if we were to
change the pH from two to four,

00:03:42.410 --> 00:03:45.130
we would see the color of the solution go

00:03:45.130 --> 00:03:48.010
from red to orange to yellow.

00:03:48.010 --> 00:03:50.820
Now let's see how to choose
the right acid-base indicator

00:03:50.820 --> 00:03:52.210
for a titration.

00:03:52.210 --> 00:03:55.450
Our goal is to choose an indicator
whose color change occurs

00:03:55.450 --> 00:03:59.720
as close as possible to the
pH of the equivalence point.

00:03:59.720 --> 00:04:02.400
For a weak acid strong based titration,

00:04:02.400 --> 00:04:06.250
the equivalence point occurs
at a pH greater than seven.

00:04:06.250 --> 00:04:09.460
First, let's look at the
acid-base indicator methyl red.

00:04:09.460 --> 00:04:13.950
Methyl red has a pH range of
approximately four to six.

00:04:13.950 --> 00:04:17.090
So a little bit over four
methyl red starts to change

00:04:17.090 --> 00:04:20.290
from red to orange, and
then eventually to yellow

00:04:20.290 --> 00:04:22.900
by the time you hit a pH of about six.

00:04:22.900 --> 00:04:25.750
However, the pH at the equivalence
point for this titration

00:04:25.750 --> 00:04:28.680
looks to be somewhere
between eight and 10.

00:04:28.680 --> 00:04:31.480
Therefore, if we used
methyl red and we stopped

00:04:31.480 --> 00:04:33.930
the titration when the color changed,

00:04:33.930 --> 00:04:36.130
we'd be stopping the titration too early.

00:04:36.130 --> 00:04:38.220
So we might be stopping
it somewhere in here

00:04:38.220 --> 00:04:40.840
before we reach the equivalence point.

00:04:40.840 --> 00:04:43.340
So methyl red would not be a good choice

00:04:43.340 --> 00:04:46.490
as an acid-base indicator
for this titration.

00:04:46.490 --> 00:04:49.440
Another way to think about
this is with pKa values.

00:04:49.440 --> 00:04:53.150
For methyl red, the pKa
value is approximately five.

00:04:53.150 --> 00:04:55.180
And the goal is to match the pKa value

00:04:55.180 --> 00:04:58.490
as closely as possible to the
pH at the equivalence point.

00:04:58.490 --> 00:05:00.490
But since the pH at the equivalence point

00:05:00.490 --> 00:05:04.390
is between eight and 10,
that's too far away from five.

00:05:04.390 --> 00:05:07.390
So by looking at the pKa
value, we know that methyl red

00:05:07.390 --> 00:05:09.863
is not a good fit for this titration.

00:05:10.750 --> 00:05:12.650
Phenolphthalein is an example of another

00:05:12.650 --> 00:05:16.090
acid-base indicator, and it
has a different pH range.

00:05:16.090 --> 00:05:18.840
At a pH of about eight,
phenolphthalein is colorless.

00:05:18.840 --> 00:05:22.160
However, as the pH
changes from eight to 10,

00:05:22.160 --> 00:05:25.020
phenolphthalein goes
from colorless to pink.

00:05:25.020 --> 00:05:27.930
Because the color of the indicator
changes in the same range

00:05:27.930 --> 00:05:29.730
where we would find the equivalence point,

00:05:29.730 --> 00:05:33.090
phenolphthalein is a good
choice as an acid-base indicator

00:05:33.090 --> 00:05:34.600
for this titration.

00:05:34.600 --> 00:05:37.320
And thinking about the pKa
value for phenolphthalein

00:05:37.320 --> 00:05:40.330
which is approximately nine,
that falls in the range

00:05:40.330 --> 00:05:43.020
of eight to 10 where we
find our equivalence point.

00:05:43.020 --> 00:05:45.860
So we could think about it
either in terms of the pH range

00:05:45.860 --> 00:05:47.683
or the pKa value.

00:05:48.570 --> 00:05:49.890
Next let's choose an indicator

00:05:49.890 --> 00:05:52.870
for a weak based strong acid titration.

00:05:52.870 --> 00:05:54.810
For a weak based strong acid titration,

00:05:54.810 --> 00:05:58.950
the equivalence point occurs
at a pH less than seven.

00:05:58.950 --> 00:06:01.600
If we try to use phenolphthalein
for this titration,

00:06:01.600 --> 00:06:04.710
remember phenolphthalein
changes from eight to 10,

00:06:04.710 --> 00:06:07.410
or in this case, it'd be
changing from 10 to eight.

00:06:07.410 --> 00:06:10.130
So we'd start at this
relatively high pH here,

00:06:10.130 --> 00:06:11.880
and if we tried to use phenolphthalein

00:06:11.880 --> 00:06:13.700
and we stopped at when
the color change occurred,

00:06:13.700 --> 00:06:15.920
we'd be stopping the titration too early.

00:06:15.920 --> 00:06:19.020
So we might be stopping the
titration somewhere in here.

00:06:19.020 --> 00:06:21.380
So phenolphthalein is not a good choice

00:06:21.380 --> 00:06:25.620
as an acid-base indicator for
this particular titration.

00:06:25.620 --> 00:06:27.670
Thinking about using pKa values,

00:06:27.670 --> 00:06:30.570
the pKa value for phenolphthalein
is approximately nine,

00:06:30.570 --> 00:06:33.330
which is not a good fit for
the pH at the equivalence point

00:06:33.330 --> 00:06:37.510
which appears to be between four and six.

00:06:37.510 --> 00:06:40.110
Methyl red has a pH range
of about four to six

00:06:40.110 --> 00:06:41.360
over which it changes color

00:06:41.360 --> 00:06:43.820
and a pKa value of approximately five.

00:06:43.820 --> 00:06:46.160
And since the equivalence point,

00:06:46.160 --> 00:06:48.950
the pH of the equivalence
point is between four and six,

00:06:48.950 --> 00:06:51.100
methyl red would be a good choice

00:06:51.100 --> 00:06:54.520
as an acid-based indicator
for this titration.

00:06:54.520 --> 00:06:57.190
So to summarize, when choosing
an acid-base indicator

00:06:57.190 --> 00:06:59.360
for a titration, choose an indicator

00:06:59.360 --> 00:07:02.330
whose color change occurs
as closely as possible

00:07:02.330 --> 00:07:04.453
to the pH at the equivalence point.


Weak base–strong acid titrations

WEBVTT
Kind: captions
Language: en

00:00:00.400 --> 00:00:03.100
- [Instructor] Ammonia is
an example of a weak base

00:00:03.100 --> 00:00:06.730
and hydrochloric acid is an
example of a strong acid.

00:00:06.730 --> 00:00:10.700
And if we're doing a weak
base-strong acid titration,

00:00:10.700 --> 00:00:12.950
that means that ammonia is the analyte,

00:00:12.950 --> 00:00:14.600
the substance we're analyzing,

00:00:14.600 --> 00:00:17.800
and we're titrating ammonia
with hydrochloric acid

00:00:17.800 --> 00:00:20.840
and therefore hydrochloric
acid is the titrant.

00:00:20.840 --> 00:00:24.650
And when ammonia reacts
with hydrochloric acid,

00:00:24.650 --> 00:00:28.700
the product is an aqueous
solution of ammonium chloride.

00:00:28.700 --> 00:00:32.020
For our complete or
overall ionic equation,

00:00:32.020 --> 00:00:33.920
since ammonia is a weak base,

00:00:33.920 --> 00:00:38.050
we show it as NH3 in our
complete ionic equation.

00:00:38.050 --> 00:00:40.830
However, since hydrochloric
acid is a strong acid

00:00:40.830 --> 00:00:42.530
that ionizes 100%,

00:00:42.530 --> 00:00:44.690
we show it as breaking up into its ion,

00:00:44.690 --> 00:00:45.523
so H+ and CL-.

00:00:47.270 --> 00:00:50.480
Ammonium chloride is a soluble salt,

00:00:50.480 --> 00:00:52.750
therefore we would show ammonium chloride

00:00:52.750 --> 00:00:55.070
in aqueous solution as ammonium cations

00:00:55.070 --> 00:00:57.000
and chloride anions.

00:00:57.000 --> 00:00:58.560
To write the net ionic equation,

00:00:58.560 --> 00:01:00.390
we leave out spectator ions.

00:01:00.390 --> 00:01:02.840
And since we have chloride
anions on the left side

00:01:02.840 --> 00:01:04.200
and on the right side,

00:01:04.200 --> 00:01:06.250
chloride anions are the spectator ions.

00:01:06.250 --> 00:01:08.930
And leaving those out, we
get the net ionic equation,

00:01:08.930 --> 00:01:13.930
which is ammonia NH3 plus H+ goes to NH4+.

00:01:14.000 --> 00:01:17.130
So this is one way to write
to the net ionic equation

00:01:17.130 --> 00:01:20.370
for this weak base-strong acid titration.

00:01:20.370 --> 00:01:22.280
Next, let's look at the titration curve

00:01:22.280 --> 00:01:25.220
for our weak base-strong acid titration.

00:01:25.220 --> 00:01:29.360
pH is on the y-axis and milliliters
of acid is on the x-axis

00:01:29.360 --> 00:01:33.750
because we're adding our strong
acid to our aqueous solution

00:01:33.750 --> 00:01:35.390
of our weak base.

00:01:35.390 --> 00:01:37.900
Looking at the first point
on our titration curve,

00:01:37.900 --> 00:01:40.030
the pH is relatively basic.

00:01:40.030 --> 00:01:43.410
So this is before any
strong acid has been added.

00:01:43.410 --> 00:01:45.900
The reason why the pH is
basic is because we have

00:01:45.900 --> 00:01:48.290
an aqueous solution of
our weak base, ammonia,

00:01:48.290 --> 00:01:51.240
which reacts with water to
produce ammonium cations

00:01:51.240 --> 00:01:52.820
and hydroxide anions.

00:01:52.820 --> 00:01:55.610
And it's these hydroxide
anions that cause the pH

00:01:55.610 --> 00:01:57.390
to be relatively high.

00:01:57.390 --> 00:02:00.210
However, the equilibrium
favors the reactants

00:02:00.210 --> 00:02:01.330
for this reaction.

00:02:01.330 --> 00:02:04.960
So we have mostly ammonia
and very little ammonium

00:02:04.960 --> 00:02:07.370
at this point in the titration curve.

00:02:07.370 --> 00:02:09.760
Next, we think about adding some acid

00:02:09.760 --> 00:02:12.740
to our aqueous solution of ammonia.

00:02:12.740 --> 00:02:15.060
And from our net ionic equation,

00:02:15.060 --> 00:02:16.530
when ammonia reacts with H+,

00:02:16.530 --> 00:02:20.490
that forms the ammonium cation, NH4+.

00:02:20.490 --> 00:02:21.910
Looking at the titration curve,

00:02:21.910 --> 00:02:23.760
as we add more and more acid,

00:02:23.760 --> 00:02:26.070
the pH starts to decrease.

00:02:26.070 --> 00:02:30.410
However, in this range, there's
a slow decrease in the pH.

00:02:30.410 --> 00:02:31.860
As more acid is added,

00:02:31.860 --> 00:02:35.310
more ammonia is turned
into the ammonium cation.

00:02:35.310 --> 00:02:36.900
Eventually, we reach a point

00:02:36.900 --> 00:02:39.530
where all of the initial
ammonia has been neutralized

00:02:39.530 --> 00:02:41.400
by the addition of the acid.

00:02:41.400 --> 00:02:44.690
This point is called
the equivalence point.

00:02:44.690 --> 00:02:46.260
And the way to find the equivalence point

00:02:46.260 --> 00:02:47.620
on our titration curve

00:02:47.620 --> 00:02:51.600
is to first look for this
sharp decrease in the pH.

00:02:51.600 --> 00:02:54.400
And then we can draw a straight line here.

00:02:54.400 --> 00:02:57.380
And approximately halfway
down that straight line

00:02:57.380 --> 00:03:00.170
is a good estimate of
the equivalence point

00:03:00.170 --> 00:03:02.030
for this titration.

00:03:02.030 --> 00:03:04.500
To find the pH of the solution
at the equivalence point,

00:03:04.500 --> 00:03:07.470
we simply go over to where
the equivalence point is

00:03:07.470 --> 00:03:09.170
on the y-axis.

00:03:09.170 --> 00:03:10.520
And so for this pH,

00:03:10.520 --> 00:03:12.850
we can see the pH at the equivalence point

00:03:12.850 --> 00:03:14.210
is less than seven.

00:03:14.210 --> 00:03:15.640
So let me go ahead and
write that down here.

00:03:15.640 --> 00:03:17.840
The pH is less than seven

00:03:17.840 --> 00:03:21.120
for a weak base-strong acid titration.

00:03:21.120 --> 00:03:23.240
The reason why the pH is less than seven

00:03:23.240 --> 00:03:24.420
at the equivalence point

00:03:24.420 --> 00:03:26.730
is because all the ammonia
that we started with

00:03:26.730 --> 00:03:28.540
has been completely neutralized

00:03:28.540 --> 00:03:32.160
and turned into the ammonium cation, NH4+.

00:03:32.160 --> 00:03:34.500
The ammonium cation is a weak acid

00:03:34.500 --> 00:03:38.970
and reacts with water to
form hydronium ions, H3O+,

00:03:38.970 --> 00:03:41.220
and ammonia, an aqueous solution.

00:03:41.220 --> 00:03:45.380
At 25 degrees Celsius,
water has a pH of seven.

00:03:45.380 --> 00:03:48.940
However, since the ammonium
cation is a weak acid

00:03:48.940 --> 00:03:52.950
and we're increasing the
concentration of hydronium ions

00:03:52.950 --> 00:03:56.400
in solution, that decreases the pH,

00:03:56.400 --> 00:03:58.780
therefore the pH is less than seven

00:03:58.780 --> 00:04:00.420
at the equivalence point.

00:04:00.420 --> 00:04:02.340
In addition to ammonium ions,

00:04:02.340 --> 00:04:04.820
there are also chloride
anions in solution.

00:04:04.820 --> 00:04:07.740
However, chloride anions
do not react with water

00:04:07.740 --> 00:04:10.660
and therefore do not affect the pH.

00:04:10.660 --> 00:04:13.590
Going back to the equivalence
point on our titration curve,

00:04:13.590 --> 00:04:16.170
if we dropped down here to the x-axis,

00:04:16.170 --> 00:04:19.900
we can see the equivalence point
occurs after 50 milliliters

00:04:19.900 --> 00:04:22.180
of acid has been added.

00:04:22.180 --> 00:04:24.710
Therefore, if it took
50 milliliters of acid

00:04:24.710 --> 00:04:28.170
to neutralize all of the ammonia
that was initially present,

00:04:28.170 --> 00:04:32.320
it would take half that volume
or 25 milliliters of acid

00:04:32.320 --> 00:04:34.740
to neutralize half of the ammonia.

00:04:34.740 --> 00:04:37.890
So if we go back up here
and we draw a dashed line

00:04:37.890 --> 00:04:39.640
to our titration curve,

00:04:39.640 --> 00:04:42.340
this point on our
titration curve represents

00:04:42.340 --> 00:04:44.920
the half equivalence point.

00:04:44.920 --> 00:04:47.600
So this point represents
the half equivalence point

00:04:47.600 --> 00:04:48.960
on our titration curve.

00:04:48.960 --> 00:04:52.030
And since we've neutralized
half of the ammonia

00:04:52.030 --> 00:04:53.820
that was initially present,

00:04:53.820 --> 00:04:57.030
that means there are equal
concentrations of ammonia

00:04:57.030 --> 00:04:59.860
and the ammonium cation at this point.

00:04:59.860 --> 00:05:01.810
Let's go back to our equivalence points

00:05:01.810 --> 00:05:04.050
where all the ammonia that we started with

00:05:04.050 --> 00:05:05.710
has been neutralized.

00:05:05.710 --> 00:05:09.500
Therefore, if we add some
more acid to the solution,

00:05:09.500 --> 00:05:12.000
there's no more ammonia
for it to react with.

00:05:12.000 --> 00:05:15.640
And therefore we see the pH drop.

00:05:15.640 --> 00:05:18.050
So this portion of the titration curve

00:05:18.050 --> 00:05:21.150
is the region of excess acid.

00:05:21.150 --> 00:05:24.100
Let's go back to the
half equivalence point

00:05:24.100 --> 00:05:25.970
on our titration curve.

00:05:25.970 --> 00:05:27.160
Remember at that point,

00:05:27.160 --> 00:05:29.240
the concentration of ammonium cation

00:05:29.240 --> 00:05:31.860
is equal to the concentration of ammonia.

00:05:31.860 --> 00:05:33.930
The ammonium cation and ammonia

00:05:33.930 --> 00:05:36.140
are a conjugate acid-base pair.

00:05:36.140 --> 00:05:38.220
And when there are significant amounts

00:05:38.220 --> 00:05:41.430
of a weak conjugate acid-base pair,

00:05:41.430 --> 00:05:43.380
there's a buffer solution.

00:05:43.380 --> 00:05:45.680
Therefore, at the half equivalence point,

00:05:45.680 --> 00:05:47.090
we have a buffer solution,

00:05:47.090 --> 00:05:50.000
and we can calculate the pH at that point

00:05:50.000 --> 00:05:53.100
by using the
Henderson-Hasselbalch equation.

00:05:53.100 --> 00:05:56.330
So pH is equal to the
pKa of the weak acid,

00:05:56.330 --> 00:05:59.790
plus the log of the concentration
of the conjugate base,

00:05:59.790 --> 00:06:02.780
divided by the concentration
of the weak acid.

00:06:02.780 --> 00:06:06.100
For this example, the
base is ammonia, NH3,

00:06:06.100 --> 00:06:09.830
and the conjugate acid is
the ammonium cation. NH4+.

00:06:09.830 --> 00:06:11.540
Therefore, this pKa value

00:06:11.540 --> 00:06:13.170
in the Henderson-Hasselbalch equation

00:06:13.170 --> 00:06:16.570
is referring to the pKa value of ammonium.

00:06:16.570 --> 00:06:19.650
And because the concentrations
of ammonium and ammonia

00:06:19.650 --> 00:06:21.990
are equal at the half equivalence point,

00:06:21.990 --> 00:06:25.390
the ratio of their
concentrations is equal to one

00:06:25.390 --> 00:06:28.600
and the log of one is equal to zero.

00:06:28.600 --> 00:06:30.620
Therefore, at the half equivalence point,

00:06:30.620 --> 00:06:35.620
the pH is equal to the pKa
value of the weak acid.

00:06:35.730 --> 00:06:37.770
So if we wanted to find the pKa value

00:06:37.770 --> 00:06:38.990
for the ammonium cation,

00:06:38.990 --> 00:06:40.990
we would find the half equivalence point

00:06:40.990 --> 00:06:43.420
and we'd draw our dotted line over to

00:06:43.420 --> 00:06:45.660
where the intersects on our y-axis

00:06:45.660 --> 00:06:47.520
and whatever pH that is,

00:06:47.520 --> 00:06:50.040
that's the pKa value of ammonium.

00:06:50.040 --> 00:06:52.800
So in this case, it looks
to be a little bit over nine

00:06:52.800 --> 00:06:55.300
as a good estimate for the pKa value

00:06:55.300 --> 00:06:57.120
of the ammonium cation.

00:06:57.120 --> 00:07:00.290
Next, let's think about how
our titration curve can tell us

00:07:00.290 --> 00:07:02.200
about the relative concentrations

00:07:02.200 --> 00:07:05.010
of our weak conjugate acid-base pair.

00:07:05.010 --> 00:07:07.030
We know that at the
half equivalence points

00:07:07.030 --> 00:07:09.320
where the pH is equal to the pKa value,

00:07:09.320 --> 00:07:11.950
the concentration of ammonium cations

00:07:11.950 --> 00:07:14.500
is equal to the concentration of ammonia.

00:07:14.500 --> 00:07:16.950
So let's think about a
point just to the left

00:07:16.950 --> 00:07:18.570
of our half equivalence point,

00:07:18.570 --> 00:07:21.070
which I'm gonna call point P.

00:07:21.070 --> 00:07:24.210
At point P, the pH is greater

00:07:24.210 --> 00:07:26.220
than the pKa value.

00:07:26.220 --> 00:07:28.840
And we know the initial
point on our titration curve

00:07:28.840 --> 00:07:32.380
was almost all weak base, almost all NH3.

00:07:32.380 --> 00:07:35.410
Because point P is in
between the initial point

00:07:35.410 --> 00:07:38.040
where there was almost all NH3,

00:07:38.040 --> 00:07:40.690
and the half equivalence point
where there was equal amounts

00:07:40.690 --> 00:07:42.980
of NH3 and NH4+,

00:07:42.980 --> 00:07:46.140
at point P, there must be more NH3

00:07:46.140 --> 00:07:48.480
than NH4+.

00:07:48.480 --> 00:07:50.630
Therefore, when the pH of
the solution is greater

00:07:50.630 --> 00:07:52.430
than the pKa value,

00:07:52.430 --> 00:07:55.080
we know the concentration
of ammonia is greater than

00:07:55.080 --> 00:07:57.550
the concentration of the ammonium cation.

00:07:57.550 --> 00:07:59.810
Or you could say the
concentration of ammonium

00:07:59.810 --> 00:08:02.360
is less than the concentration of ammonia.

00:08:02.360 --> 00:08:03.940
We could have also figured this out using

00:08:03.940 --> 00:08:05.780
the Henderson-Hasselbalch equation.

00:08:05.780 --> 00:08:08.350
However, it's often
easier just to think about

00:08:08.350 --> 00:08:10.470
the shape of the titration curve

00:08:10.470 --> 00:08:12.430
and where the point in question is

00:08:12.430 --> 00:08:14.080
in relation to important points.

00:08:14.080 --> 00:08:15.250
For example, in this case,

00:08:15.250 --> 00:08:18.230
the initial point and the
half equivalence point.

00:08:18.230 --> 00:08:20.680
Next, let's think about
a point just to the right

00:08:20.680 --> 00:08:22.330
of the half equivalence point.

00:08:22.330 --> 00:08:24.970
And I'm gonna call this point Q.

00:08:24.970 --> 00:08:29.970
At point Q, the pH of the
solution is less than the pKa.

00:08:30.850 --> 00:08:34.960
Point Q is in between the
half equivalence point

00:08:34.960 --> 00:08:36.800
and the equivalence point,

00:08:36.800 --> 00:08:39.920
which is approximately here
on the titration curve.

00:08:39.920 --> 00:08:41.760
Remember at the equivalence point,

00:08:41.760 --> 00:08:43.360
all the ammonia that we started with

00:08:43.360 --> 00:08:46.810
has been converted into ammonium, NH4+.

00:08:46.810 --> 00:08:50.560
And because point Q is in between
the half equivalence point

00:08:50.560 --> 00:08:55.080
where the amount of NH3 is
equal to the amount of NH4+,

00:08:55.080 --> 00:08:57.900
and the equivalence
point where all the NH3

00:08:57.900 --> 00:09:00.210
has been converted into NH4+,

00:09:00.210 --> 00:09:02.140
all of the initial NH3.

00:09:02.140 --> 00:09:07.007
That means that at Q, there
must be more NH4+ than NH3.

00:09:08.020 --> 00:09:12.100
Therefore, when the pH is
less than the pKa value,

00:09:12.100 --> 00:09:15.490
we can say the concentration
of ammonium, NH4+,

00:09:15.490 --> 00:09:19.260
is greater than the
concentration of ammonia, NH3.

00:09:19.260 --> 00:09:23.300
The half equivalence
point, point P and point Q

00:09:23.300 --> 00:09:28.300
are all a part of the buffer
region on the titration curve.

00:09:28.710 --> 00:09:32.280
Remember that buffers
resist large changes in pH,

00:09:32.280 --> 00:09:35.760
and that's why we see
a slow decrease in pH

00:09:35.760 --> 00:09:39.760
as acid is added at this
part of the titration curve.

00:09:39.760 --> 00:09:41.320
At the very beginning of the titration,

00:09:41.320 --> 00:09:45.070
we had almost all ammonia
and therefore we did not have

00:09:45.070 --> 00:09:46.490
a buffer solution.

00:09:46.490 --> 00:09:50.040
However, as acid was added
and the ammonia was converted

00:09:50.040 --> 00:09:52.590
into the ammonium cation, NH4+,

00:09:53.460 --> 00:09:55.960
when significant amounts
of both are present,

00:09:55.960 --> 00:09:58.050
we do have a buffer solution.

00:09:58.050 --> 00:10:00.740
And that represents the buffer region

00:10:00.740 --> 00:10:03.490
on our titration curve, so in here.

00:10:03.490 --> 00:10:05.920
However, as more and more acid is added,

00:10:05.920 --> 00:10:08.960
we can see a sharp change in pH start

00:10:08.960 --> 00:10:10.220
to occur right about here,

00:10:10.220 --> 00:10:12.670
so we're no longer in the buffer region

00:10:12.670 --> 00:10:15.150
as we approach the equivalence point.

00:10:15.150 --> 00:10:17.410
So when we think about the titration curve

00:10:17.410 --> 00:10:20.390
of a weak base-strong acid titration,

00:10:20.390 --> 00:10:22.430
and we think about the
half equivalence point

00:10:22.430 --> 00:10:26.510
where the pH is equal to the
pKa value of the weak acid,

00:10:26.510 --> 00:10:29.290
it's important to remember
that there's a buffer region

00:10:29.290 --> 00:10:33.373
or a buffer zone around
that half equivalence point.


Weak acid–strong base titrations

WEBVTT
Kind: captions
Language: en

00:00:00.000 --> 00:00:03.560
- [Instructor] Acetic acid
is an example of a weak acid

00:00:03.560 --> 00:00:07.750
and sodium hydroxide is an
example of a strong base.

00:00:07.750 --> 00:00:09.410
If we are titrating a sample

00:00:09.410 --> 00:00:12.260
of acetic acid with sodium hydroxide,

00:00:12.260 --> 00:00:14.190
acetic acid would be the analyte,

00:00:14.190 --> 00:00:15.920
the substance that we are analyzing,

00:00:15.920 --> 00:00:18.890
and sodium hydroxide would be the titrant.

00:00:18.890 --> 00:00:21.540
When acetic acid reacts
with sodium hydroxide,

00:00:21.540 --> 00:00:23.350
the products are an aqueous solution

00:00:23.350 --> 00:00:26.050
of sodium acetate and water.

00:00:26.050 --> 00:00:29.710
Next, let's look at the complete
or overall ionic equation

00:00:29.710 --> 00:00:30.930
for this reaction.

00:00:30.930 --> 00:00:33.190
Because the acetic acid is a weak acid,

00:00:33.190 --> 00:00:36.460
it does not ionize completely
in aqueous solution.

00:00:36.460 --> 00:00:39.010
Therefore, for our
complete ionic equation,

00:00:39.010 --> 00:00:40.680
we simply write a acetic acid.

00:00:40.680 --> 00:00:43.280
We don't show it as being ionized.

00:00:43.280 --> 00:00:46.065
However, sodium hydroxide is a strong base

00:00:46.065 --> 00:00:49.680
that associates 100% in aqueous solution.

00:00:49.680 --> 00:00:52.150
Therefore, we would show it as its ions,

00:00:52.150 --> 00:00:52.983
Na+ and OH-.

00:00:54.460 --> 00:00:58.370
Since sodium acetate is a
soluble salt in aqueous solution,

00:00:58.370 --> 00:01:01.973
it would consist of sodium
cations and the acetate anion.

00:01:01.973 --> 00:01:04.980
And we would also include
water in our overall

00:01:04.980 --> 00:01:07.100
or complete ionic equation.

00:01:07.100 --> 00:01:08.730
For our net ionic equation,

00:01:08.730 --> 00:01:10.530
we leave out spectator ions.

00:01:10.530 --> 00:01:13.370
Since sodium cations are on
the left and the right side,

00:01:13.370 --> 00:01:14.440
when we take those out,

00:01:14.440 --> 00:01:16.610
we're left with the net ionic equation

00:01:16.610 --> 00:01:19.200
for this weak acid-strong base titration.

00:01:19.200 --> 00:01:22.320
So acetic acid reacts
with hydroxide anions

00:01:22.320 --> 00:01:25.020
to form the acetate anion and water.

00:01:25.020 --> 00:01:27.050
Next, let's look at the titration curve

00:01:27.050 --> 00:01:29.840
for our weak acid-strong base titration,

00:01:29.840 --> 00:01:33.280
pH is on the y-axis and since
we're adding our strong base

00:01:33.280 --> 00:01:34.970
to our solution of weak acid,

00:01:34.970 --> 00:01:38.330
milliliters of base
added is on the x-axis.

00:01:38.330 --> 00:01:40.760
Let's use particulate
diagrams to help us figure out

00:01:40.760 --> 00:01:43.130
what's going on in this titration.

00:01:43.130 --> 00:01:45.100
Keep in mind that these
particulate diagrams

00:01:45.100 --> 00:01:47.310
are only meant to
represent what's going on

00:01:47.310 --> 00:01:49.260
in the solution during the titration

00:01:49.260 --> 00:01:52.800
and water molecules will
be left out for clarity.

00:01:52.800 --> 00:01:55.010
Looking at our first particulate diagram,

00:01:55.010 --> 00:01:57.850
there's only acetic acid present,

00:01:57.850 --> 00:02:00.660
only two particles of acetic acid present.

00:02:00.660 --> 00:02:03.720
Therefore, this is before
any base has been added.

00:02:03.720 --> 00:02:05.540
So on our titration curve,

00:02:05.540 --> 00:02:08.830
we're right here at zero
milliliters of base added.

00:02:08.830 --> 00:02:10.820
Next, let's add some sodium hydroxide

00:02:10.820 --> 00:02:13.811
to our initial solution of acetic acid.

00:02:13.811 --> 00:02:15.820
So for sodium hydroxide,

00:02:15.820 --> 00:02:18.150
the pink sphere represents
the sodium cation.

00:02:18.150 --> 00:02:20.650
So we think about adding
in the sodium cation

00:02:20.650 --> 00:02:23.043
and also a hydroxide anion.

00:02:23.900 --> 00:02:25.957
Remember from our net ionic equation,

00:02:25.957 --> 00:02:29.890
hydroxide anion reacts with acetic acid

00:02:29.890 --> 00:02:33.360
to form the acetate anion and water.

00:02:33.360 --> 00:02:36.310
Since water is left out of
our particulate diagrams,

00:02:36.310 --> 00:02:39.265
we don't see it here in this
second particulate diagram.

00:02:39.265 --> 00:02:43.372
However, we do see the
acetate anion that formed

00:02:43.372 --> 00:02:46.220
when the hydroxide anion reacted

00:02:46.220 --> 00:02:48.770
with the acetic acid particle.

00:02:48.770 --> 00:02:50.560
In the second particulate diagram,

00:02:50.560 --> 00:02:53.508
we also see the sodium
cation that was added

00:02:53.508 --> 00:02:56.407
and also the particle of acetic acid

00:02:56.407 --> 00:02:58.910
that was present initially.

00:02:58.910 --> 00:03:02.040
Because we started with two
particles of acetic acid

00:03:02.040 --> 00:03:05.740
and we have only one left in
the second particulate diagram,

00:03:05.740 --> 00:03:07.960
that means that half of the initial acid

00:03:07.960 --> 00:03:09.680
has been neutralized.

00:03:09.680 --> 00:03:13.250
So this second particulate
diagram is meant to represent

00:03:13.250 --> 00:03:16.730
the half equivalence
point in the titration.

00:03:16.730 --> 00:03:19.110
Next, let's add enough sodium hydroxide

00:03:19.110 --> 00:03:21.480
to neutralize the other half of acid

00:03:21.480 --> 00:03:24.300
that was initially present in solution.

00:03:24.300 --> 00:03:27.833
The acetic acid particle
reacts with the hydroxide anion

00:03:27.833 --> 00:03:31.800
to form water and the acetate anion.

00:03:31.800 --> 00:03:34.180
Since water is left out of
our particulate diagram,

00:03:34.180 --> 00:03:37.030
we don't see water in this
third particulate diagram.

00:03:37.030 --> 00:03:40.780
However, we do see the
acetate anion that was formed.

00:03:40.780 --> 00:03:43.710
We also see these sodium
cation that was added.

00:03:43.710 --> 00:03:45.330
And at the half equivalence point,

00:03:45.330 --> 00:03:49.410
we already had a sodium
cation and an acetate anion,

00:03:49.410 --> 00:03:50.360
so we can see those

00:03:50.360 --> 00:03:52.970
in our third particulate diagram as well.

00:03:52.970 --> 00:03:55.430
If we compare the third
particulate diagram

00:03:55.430 --> 00:03:57.560
to the first particulate diagram,

00:03:57.560 --> 00:03:59.825
all of the acid that was initially present

00:03:59.825 --> 00:04:01.516
has been neutralized.

00:04:01.516 --> 00:04:04.740
Therefore, the third particulate diagram

00:04:04.740 --> 00:04:08.290
represents the equivalence
point of the titration.

00:04:08.290 --> 00:04:11.230
We can estimate the equivalence
point on our titration curve

00:04:11.230 --> 00:04:14.750
by looking for the area where
we see a sharp increase in pH.

00:04:14.750 --> 00:04:15.940
So right about in here.

00:04:15.940 --> 00:04:17.410
And if we draw a little line,

00:04:17.410 --> 00:04:19.847
about halfway up that line approximately,

00:04:19.847 --> 00:04:23.280
is a good place to mark
the equivalence point.

00:04:23.280 --> 00:04:25.990
So this is a good estimate
of our equivalence point.

00:04:25.990 --> 00:04:29.020
And if we go over to where
that intersects in the y-axis,

00:04:29.020 --> 00:04:30.480
we can estimate the pH

00:04:30.480 --> 00:04:33.320
for this weak acid-strong base titration.

00:04:33.320 --> 00:04:35.320
It looks to be a little bit over eight,

00:04:35.320 --> 00:04:37.610
so between eight and nine somewhere.

00:04:37.610 --> 00:04:40.470
So the pH of the solution
at the equivalence point

00:04:40.470 --> 00:04:42.480
is greater than seven.

00:04:42.480 --> 00:04:44.575
The reason why the pH
is greater than seven

00:04:44.575 --> 00:04:47.050
is because at the equivalence point,

00:04:47.050 --> 00:04:51.130
there are acetate anions in
solution and acetate anion

00:04:51.130 --> 00:04:55.540
react with water to form
hydroxide anions and acetic acid.

00:04:55.540 --> 00:04:59.080
The pH of water at 25
degrees Celsius is seven,

00:04:59.080 --> 00:05:01.590
but because we've
increased the concentration

00:05:01.590 --> 00:05:04.910
of hydroxide anions in solution,

00:05:04.910 --> 00:05:08.391
the pH will be greater than
seven at the equivalence point.

00:05:08.391 --> 00:05:11.030
This is called anion hydrolysis

00:05:11.030 --> 00:05:13.600
and is the reason why the
pH is greater than seven

00:05:13.600 --> 00:05:14.700
at the equivalence point

00:05:14.700 --> 00:05:17.670
for a weak acid-strong base titration.

00:05:17.670 --> 00:05:20.640
Going back to our titration
curve to the equivalence point,

00:05:20.640 --> 00:05:23.290
if we drop down to the x-axis,

00:05:23.290 --> 00:05:25.890
we can see that the equivalence
point has been reached

00:05:25.890 --> 00:05:29.110
after 50 milliliters of
base have been added.

00:05:29.110 --> 00:05:31.390
So if it takes 50 milliliters of base

00:05:31.390 --> 00:05:33.230
to reach the equivalence point,

00:05:33.230 --> 00:05:35.100
it should take half that volume

00:05:35.100 --> 00:05:37.220
to reach the half equivalence point.

00:05:37.220 --> 00:05:39.500
Therefore, the half
equivalence point is reached

00:05:39.500 --> 00:05:42.630
after 25 milliliters
of base has been added.

00:05:42.630 --> 00:05:45.400
So if we go up here to
our titration curve,

00:05:45.400 --> 00:05:49.620
we can mark the location
or the approximate location

00:05:49.620 --> 00:05:52.010
of the half equivalence point.

00:05:52.010 --> 00:05:55.400
So let me draw a line here
from half equivalence point

00:05:55.400 --> 00:05:57.570
to the point on our titration curve.

00:05:57.570 --> 00:06:00.150
Let's go back to our
third particulate diagram

00:06:00.150 --> 00:06:01.990
which represents the equivalence point

00:06:01.990 --> 00:06:06.543
and let's add some more sodium
hydroxide into the solution.

00:06:06.543 --> 00:06:08.900
Because there's no more acid present

00:06:08.900 --> 00:06:11.320
to react with the sodium hydroxide

00:06:11.320 --> 00:06:13.130
in our fourth particulate diagram,

00:06:13.130 --> 00:06:15.380
we can see the sodium cation

00:06:15.380 --> 00:06:18.230
and the hydroxide anion that we added.

00:06:18.230 --> 00:06:19.200
At the equivalence point,

00:06:19.200 --> 00:06:20.610
we also had two sodium cations.

00:06:20.610 --> 00:06:23.670
So here they are on the
fourth particulate diagram

00:06:23.670 --> 00:06:27.350
and two acetate anions
which are also present.

00:06:27.350 --> 00:06:28.927
So the fourth particulate diagram

00:06:28.927 --> 00:06:33.240
represents excess base
past the equivalence point.

00:06:33.240 --> 00:06:35.360
As a quick summary of our titration curve,

00:06:35.360 --> 00:06:37.690
we started out with only weak acid

00:06:37.690 --> 00:06:39.930
so the pH was relatively low.

00:06:39.930 --> 00:06:43.840
As we add base, the pH increases slowly,

00:06:43.840 --> 00:06:46.430
and we get to the half equivalence point

00:06:46.430 --> 00:06:49.160
where half of the weak acid
that was initially present

00:06:49.160 --> 00:06:50.690
has been neutralized.

00:06:50.690 --> 00:06:53.040
Notice how the pH is changing very slowly

00:06:53.040 --> 00:06:55.700
in this region of the titration curve.

00:06:55.700 --> 00:06:57.690
As we continue to add more and more base,

00:06:57.690 --> 00:07:00.650
the pH keeps on increasing.

00:07:00.650 --> 00:07:02.250
And around the equivalence point,

00:07:02.250 --> 00:07:05.703
we see a dramatic increase in the pH.

00:07:05.703 --> 00:07:08.410
Once we go past the equivalence point,

00:07:08.410 --> 00:07:10.580
we're in the region of excess base

00:07:10.580 --> 00:07:12.220
and the pH keeps increasing

00:07:12.220 --> 00:07:15.087
as we add more and more hydroxide anions.

00:07:15.930 --> 00:07:18.970
During the titration of a
weak acid with a strong base,

00:07:18.970 --> 00:07:21.027
a buffer solution is actually formed.

00:07:21.027 --> 00:07:23.910
So let's look at more
detail at the buffer region

00:07:23.910 --> 00:07:26.900
or the buffer zone on our titration curve.

00:07:26.900 --> 00:07:30.250
The buffer region occurs around
the half equivalence point.

00:07:30.250 --> 00:07:32.480
And we can see on our titration curve,

00:07:32.480 --> 00:07:34.700
there are very small changes in pH

00:07:34.700 --> 00:07:38.250
as we add hydroxide anions in this region.

00:07:38.250 --> 00:07:41.680
That's because the weak acid
that's present in solution

00:07:41.680 --> 00:07:43.880
is neutralizing the added base

00:07:43.880 --> 00:07:47.520
and protecting against
a dramatic change in pH.

00:07:47.520 --> 00:07:50.750
We can calculate the pH at
the half equivalence point

00:07:50.750 --> 00:07:52.880
because we know at the
half equivalence point,

00:07:52.880 --> 00:07:55.340
half of the initial acid that was present

00:07:55.340 --> 00:07:58.960
has been neutralized and
turned into the conjugate base.

00:07:58.960 --> 00:08:00.800
Therefore, at the half equivalence point,

00:08:00.800 --> 00:08:02.413
the concentration of weak acid

00:08:02.413 --> 00:08:06.380
is equal to the concentration
of the conjugate base.

00:08:06.380 --> 00:08:08.470
We can use the
Henderson-Hasselbalch equation

00:08:08.470 --> 00:08:09.910
to find the pH.

00:08:09.910 --> 00:08:11.460
If the concentration of weak acid

00:08:11.460 --> 00:08:13.876
is equal to the concentration
of the conjugate base,

00:08:13.876 --> 00:08:17.740
then the ratio of their
concentrations is equal to one.

00:08:17.740 --> 00:08:20.850
And the log of one is equal to zero.

00:08:20.850 --> 00:08:24.800
Therefore, the pH is
equal to the pKa value

00:08:24.800 --> 00:08:26.390
of the weak acid

00:08:26.390 --> 00:08:28.900
at the half equivalence point.

00:08:28.900 --> 00:08:31.500
So if we wanted to find the pKa value

00:08:31.500 --> 00:08:33.960
of a weak acid from our titration curve,

00:08:33.960 --> 00:08:36.570
we would simply find the
half equivalence point

00:08:36.570 --> 00:08:39.610
and go over to the y-axis

00:08:39.610 --> 00:08:42.290
to estimate the pKa value.

00:08:42.290 --> 00:08:45.180
Because at this point,
the pH of the solution

00:08:45.180 --> 00:08:48.950
is equal to the pKa
value of the weak acid.

00:08:48.950 --> 00:08:51.560
Next, let's think about
how the titration curve

00:08:51.560 --> 00:08:54.070
can tell us about the
relative concentrations

00:08:54.070 --> 00:08:56.472
of weak acid and conjugate base.

00:08:56.472 --> 00:08:59.920
We know that the pH is
equal to the pKa value

00:08:59.920 --> 00:09:01.330
at the half equivalence point

00:09:01.330 --> 00:09:03.340
right here on our titration curve.

00:09:03.340 --> 00:09:05.270
And we know at the half equivalence point,

00:09:05.270 --> 00:09:06.680
the concentration of weak acid

00:09:06.680 --> 00:09:10.080
is equal to the concentration
of conjugate base.

00:09:10.080 --> 00:09:12.110
Therefore, if we think about a point

00:09:12.110 --> 00:09:14.640
just to the left of the
half equivalence point,

00:09:14.640 --> 00:09:17.430
so right here on our titration curve,

00:09:17.430 --> 00:09:19.397
I'll call this point P,

00:09:19.397 --> 00:09:23.430
we know that the pH at that point is less

00:09:23.430 --> 00:09:27.250
than the pKa value of the weak acid.

00:09:27.250 --> 00:09:30.697
And we know that the initial
point on our titration curve

00:09:30.697 --> 00:09:33.382
was almost all weak acid.

00:09:33.382 --> 00:09:38.382
Therefore, since point P is
in between the initial point

00:09:38.520 --> 00:09:40.130
where we have almost all acid

00:09:40.130 --> 00:09:41.610
and the half equivalence point

00:09:41.610 --> 00:09:44.831
where we have equal amounts of
weak acid and conjugate base,

00:09:44.831 --> 00:09:49.831
point P must have more weak
acid than conjugate base.

00:09:50.230 --> 00:09:52.380
Therefore, we can say when the pH

00:09:52.380 --> 00:09:54.770
is less than the pKa value.

00:09:54.770 --> 00:09:56.450
The concentration of weak acid

00:09:56.450 --> 00:09:59.980
is greater than the
concentration of conjugate base.

00:09:59.980 --> 00:10:01.380
We could have also figured this out

00:10:01.380 --> 00:10:03.680
using the Henderson-Hasselbalch equation.

00:10:03.680 --> 00:10:05.310
However, it's often simpler

00:10:05.310 --> 00:10:08.180
just to think about the
shape of the titration curve.

00:10:08.180 --> 00:10:09.530
Next, let's think about a point

00:10:09.530 --> 00:10:12.380
just to the right of the
half equivalence point.

00:10:12.380 --> 00:10:15.460
So I'm gonna call this point, point Q.

00:10:15.460 --> 00:10:17.700
At point Q, the pH of the solution

00:10:17.700 --> 00:10:21.440
is greater than the pKa
value of the weak acid.

00:10:21.440 --> 00:10:24.170
So for trying to determine
the relative concentrations

00:10:24.170 --> 00:10:27.280
of weak acid and
conjugate base at point Q,

00:10:27.280 --> 00:10:30.780
remember that point Q is in between

00:10:30.780 --> 00:10:34.010
the half equivalence point
and the equivalence point,

00:10:34.010 --> 00:10:36.930
which is right about here
on our titration curve.

00:10:36.930 --> 00:10:38.370
And at the equivalence point,

00:10:38.370 --> 00:10:40.420
there's no more weak acid present.

00:10:40.420 --> 00:10:42.550
All of the weak acid has been neutralized

00:10:42.550 --> 00:10:46.350
and only the conjugate base, A-, remains.

00:10:46.350 --> 00:10:49.380
Since point Q is in between
the half equivalence point

00:10:49.380 --> 00:10:50.440
where there are equal amounts

00:10:50.440 --> 00:10:52.500
of weak acid and conjugate base,

00:10:52.500 --> 00:10:55.750
and the equivalence point where
there's only conjugate base,

00:10:55.750 --> 00:10:59.330
at point Q, there must
be more conjugate base

00:10:59.330 --> 00:11:01.150
than weak acid.

00:11:01.150 --> 00:11:04.100
Therefore, we can say when
the pH of the solution

00:11:04.100 --> 00:11:07.220
is greater than the pKa
value of the weak acid,

00:11:07.220 --> 00:11:09.010
the concentration of weak acid

00:11:09.010 --> 00:11:12.350
is less than the concentration
of the conjugate base.

00:11:12.350 --> 00:11:14.950
Or we could say the
concentration of conjugate base

00:11:14.950 --> 00:11:18.110
is greater than the
concentration of weak acid.

00:11:18.110 --> 00:11:20.950
Finally, let's talk
about why a buffer forms

00:11:20.950 --> 00:11:22.730
on this titration curve.

00:11:22.730 --> 00:11:23.790
When we first start off,

00:11:23.790 --> 00:11:25.680
we have almost all weak acid,

00:11:25.680 --> 00:11:27.950
and therefore we do not have a buffer.

00:11:27.950 --> 00:11:30.630
However, as base is added to the solution,

00:11:30.630 --> 00:11:33.520
the weak acid is converted
into its conjugate base.

00:11:33.520 --> 00:11:35.330
And when there are significant amounts

00:11:35.330 --> 00:11:37.620
of the weak acid and its conjugate base,

00:11:37.620 --> 00:11:39.350
we have a buffer solution.

00:11:39.350 --> 00:11:41.707
So that's right about here
on our titration curve.

00:11:41.707 --> 00:11:44.660
And we can see the buffer is resisting

00:11:44.660 --> 00:11:47.610
large changes to pH in this region.

00:11:47.610 --> 00:11:49.734
However, as we continue to add base,

00:11:49.734 --> 00:11:53.840
the concentrations of weak
acid and conjugate base change.

00:11:53.840 --> 00:11:56.490
And eventually, we no longer
have a buffer solution.

00:11:56.490 --> 00:11:59.840
So the pH changes more
dramatically at this point.

00:11:59.840 --> 00:12:02.210
So the buffer region or the buffer zone

00:12:03.300 --> 00:12:06.200
is only right around the
half equivalence point

00:12:06.200 --> 00:12:07.953
on our titration curve.


Strong acid–strong base titrations

WEBVTT
Kind: captions
Language: en

00:00:00.520 --> 00:00:02.600
- [Instructor] Hydrochloric
acid is an example

00:00:02.600 --> 00:00:05.420
of a strong acid and sodium hydroxide

00:00:05.420 --> 00:00:08.290
is an example of a strong base.

00:00:08.290 --> 00:00:09.570
Let's say we are titrating

00:00:09.570 --> 00:00:12.106
an unknown concentration
of hydrochloric acid

00:00:12.106 --> 00:00:15.530
with a known concentration
of sodium hydroxide.

00:00:15.530 --> 00:00:17.820
Let's say it's .20 molar.

00:00:17.820 --> 00:00:20.640
Because we know the concentration
of sodium hydroxide,

00:00:20.640 --> 00:00:22.290
we call that the titrant.

00:00:22.290 --> 00:00:23.510
And because we don't know

00:00:23.510 --> 00:00:25.349
the concentration of hydrochloric acid,

00:00:25.349 --> 00:00:27.940
we call that the analyte.

00:00:27.940 --> 00:00:30.480
And when our strong
acid, hydrochloric acid,

00:00:30.480 --> 00:00:33.870
reacts with our strong
base, sodium hydroxide,

00:00:33.870 --> 00:00:36.270
the products are an aqueous solution

00:00:36.270 --> 00:00:38.720
of sodium chloride and water.

00:00:38.720 --> 00:00:40.965
As a quick review of
how to write the overall

00:00:40.965 --> 00:00:43.320
or the complete ionic equation

00:00:43.320 --> 00:00:45.800
for a strong acid-strong base reaction,

00:00:45.800 --> 00:00:48.640
remember that sodium
hydroxide being a strong base

00:00:48.640 --> 00:00:50.960
dissociates completely in aqueous solution

00:00:50.960 --> 00:00:54.380
to form sodium cations
and hydroxide anions.

00:00:54.380 --> 00:00:56.560
Hydrochloric acid being a strong acid

00:00:56.560 --> 00:00:58.820
will ionize completely in aqueous solution

00:00:58.820 --> 00:01:02.025
to form H+ ions and chloride anions.

00:01:02.025 --> 00:01:04.900
Sodium chloride is a soluble salt,

00:01:04.900 --> 00:01:06.530
so an aqueous solution.

00:01:06.530 --> 00:01:09.330
We would have sodium
cations and chloride anions.

00:01:09.330 --> 00:01:11.760
And of course, we would also have water.

00:01:11.760 --> 00:01:13.620
When writing the net ionic equation,

00:01:13.620 --> 00:01:15.650
we cross out these spectator ions.

00:01:15.650 --> 00:01:18.600
So since we have sodium cations
in left and on the right,

00:01:18.600 --> 00:01:20.054
we can cross out the sodium cations

00:01:20.054 --> 00:01:20.953
and the same with the chloride anions.

00:01:22.320 --> 00:01:24.970
So both of those are the spectator ions.

00:01:24.970 --> 00:01:27.319
What we're left with is
our net ionic equation.

00:01:27.319 --> 00:01:30.200
So one way to write the net ionic equation

00:01:30.200 --> 00:01:32.176
for a strong acid-strong base reaction

00:01:32.176 --> 00:01:37.130
is hydroxide anions plus
H+ cations form water.

00:01:37.130 --> 00:01:40.350
And our goal for the strong
acids-strong based titration

00:01:40.350 --> 00:01:43.480
is to find the concentration
of hydrochloric acid

00:01:43.480 --> 00:01:45.630
using a titration curve.

00:01:45.630 --> 00:01:47.320
Let's say we do our titration

00:01:47.320 --> 00:01:49.800
and we come up with this
as a titration curve.

00:01:49.800 --> 00:01:50.870
For titration curves,

00:01:50.870 --> 00:01:55.350
you put the pH on the y-axis
and the titrant on the x-axis.

00:01:55.350 --> 00:01:56.183
So in this case,

00:01:56.183 --> 00:01:59.990
we're adding base to our
solution of hydrochloric acid.

00:01:59.990 --> 00:02:01.840
And before we use our titration curve

00:02:01.840 --> 00:02:04.100
to find the concentration
of hydrochloric acid,

00:02:04.100 --> 00:02:05.890
let's go through the titration curve

00:02:05.890 --> 00:02:08.380
and look at some particulate diagrams.

00:02:08.380 --> 00:02:10.190
And as we look at particulate diagrams,

00:02:10.190 --> 00:02:12.310
keep in mind they're just
to help us understand

00:02:12.310 --> 00:02:14.360
what's going on in the actual solution,

00:02:14.360 --> 00:02:16.190
so they're are just a representation.

00:02:16.190 --> 00:02:20.190
And we're also gonna
leave out water molecules

00:02:20.190 --> 00:02:21.610
for clarity purposes.

00:02:21.610 --> 00:02:24.500
So let's think about this
point on our titration curve.

00:02:24.500 --> 00:02:27.290
So that's zero milliliters of base added,

00:02:27.290 --> 00:02:30.440
meaning we're starting only
with hydrochloric acid.

00:02:30.440 --> 00:02:33.300
So there are two H+ particles,

00:02:33.300 --> 00:02:37.660
and there are two
chloride anion particles.

00:02:37.660 --> 00:02:39.860
Next, we add in some sodium hydroxide.

00:02:39.860 --> 00:02:42.820
So the sodium cation is this
purple sphere right here,

00:02:42.820 --> 00:02:45.540
and the hydroxide anion
is over here as well.

00:02:45.540 --> 00:02:47.570
So we add in some sodium hydroxide

00:02:47.570 --> 00:02:49.926
to our solution of hydrochloric acid

00:02:49.926 --> 00:02:52.860
and for the hydroxide
anion that's neutralized

00:02:52.860 --> 00:02:55.830
by one of the H+ ions that's present.

00:02:55.830 --> 00:02:59.800
So the OH- and the H+ react to form H2O.

00:02:59.800 --> 00:03:02.920
And since we're leaving H2O out
of our particulate diagrams,

00:03:02.920 --> 00:03:06.210
we don't see it in this
second particulate diagram.

00:03:06.210 --> 00:03:09.120
What we do see in this
second particulate diagram

00:03:09.120 --> 00:03:11.850
are the chloride anions
that were initially present.

00:03:11.850 --> 00:03:12.980
So here they are.

00:03:12.980 --> 00:03:16.520
And the other H+ ion that
was initially present

00:03:16.520 --> 00:03:19.000
and the added sodium cation.

00:03:19.000 --> 00:03:22.020
And since there's still an
H+ cation left in solution,

00:03:22.020 --> 00:03:24.150
we haven't neutralized all of the acid

00:03:24.150 --> 00:03:26.010
that was initially present.

00:03:26.010 --> 00:03:28.460
So next, we add some
more sodium hydroxide.

00:03:28.460 --> 00:03:30.350
So here we're adding a sodium cation,

00:03:30.350 --> 00:03:32.990
and we're also gonna
add this hydroxide anion

00:03:32.990 --> 00:03:34.150
to our solution.

00:03:34.150 --> 00:03:36.860
The added hydroxide
anion will be neutralized

00:03:36.860 --> 00:03:39.480
by the H+ ion that's already present.

00:03:39.480 --> 00:03:40.640
They will form water.

00:03:40.640 --> 00:03:41.930
And since we're leaving water

00:03:41.930 --> 00:03:43.502
out of the particulate diagrams,

00:03:43.502 --> 00:03:47.320
we don't see water in this
third particulate diagram.

00:03:47.320 --> 00:03:50.030
What we do see in our third
particulate diagram here

00:03:50.030 --> 00:03:52.940
are the two chloride anions
that have always been with us

00:03:52.940 --> 00:03:53.778
in the titration.

00:03:53.778 --> 00:03:56.440
We had one sodium cation already present,

00:03:56.440 --> 00:03:59.210
and then we added one more sodium cation.

00:03:59.210 --> 00:04:01.570
So this third particulate
diagram represents

00:04:01.570 --> 00:04:03.970
the equivalence point of our titration.

00:04:03.970 --> 00:04:06.710
All of the acid that we
initially had present

00:04:06.710 --> 00:04:09.190
has been neutralized by the added base

00:04:09.190 --> 00:04:13.090
and we're left with an aqueous
solution of sodium chloride.

00:04:13.090 --> 00:04:14.690
So there are only sodium cations

00:04:14.690 --> 00:04:16.940
and chloride anions in solution.

00:04:16.940 --> 00:04:18.540
At 25 degrees Celsius,

00:04:18.540 --> 00:04:21.720
the pH of water is equal to seven.

00:04:21.720 --> 00:04:24.440
And since neither the sodium cation

00:04:24.440 --> 00:04:25.890
nor the chloride anion

00:04:25.890 --> 00:04:28.803
will react with water to change the pH,

00:04:28.803 --> 00:04:30.860
the pH at the equivalence point

00:04:30.860 --> 00:04:34.900
of a strong acid-strong
base titration is seven.

00:04:34.900 --> 00:04:37.980
And we can find the equivalence
point on our titration curve

00:04:37.980 --> 00:04:41.070
by going over to a pH of about seven here.

00:04:41.070 --> 00:04:43.570
And if we draw a little dash line,

00:04:43.570 --> 00:04:46.090
wherever that dash line
hits our titration curve,

00:04:46.090 --> 00:04:48.158
represents the equivalence point.

00:04:48.158 --> 00:04:50.230
So the first particulate diagram

00:04:50.230 --> 00:04:51.890
was before any base was added.

00:04:51.890 --> 00:04:54.790
So that's this point
on our titration curve.

00:04:54.790 --> 00:04:57.489
The third particulate
diagram is meant to represent

00:04:57.489 --> 00:04:59.410
the equivalence point,

00:04:59.410 --> 00:05:01.630
so this point on our titration curve.

00:05:01.630 --> 00:05:03.752
And our second particulate diagram

00:05:03.752 --> 00:05:06.220
was when we haven't neutralized

00:05:06.220 --> 00:05:07.710
all of the acid that's present.

00:05:07.710 --> 00:05:09.840
So that's in between those two points

00:05:09.840 --> 00:05:11.456
on the titration curve.

00:05:11.456 --> 00:05:13.620
Also notice how steep

00:05:13.620 --> 00:05:18.340
the graph is around the equivalence
point of this titration.

00:05:18.340 --> 00:05:20.200
Therefore, adding very small amounts

00:05:20.200 --> 00:05:22.240
of base around the equivalence point

00:05:22.240 --> 00:05:25.920
causes large changes in
the pH of the solution.

00:05:25.920 --> 00:05:28.330
Let's go back to our particulate diagram

00:05:28.330 --> 00:05:29.690
at the equivalence point,

00:05:29.690 --> 00:05:32.450
and let's add some more sodium hydroxide.

00:05:32.450 --> 00:05:34.380
So we add one more sodium cation

00:05:34.380 --> 00:05:37.120
and one more hydroxide anion.

00:05:37.120 --> 00:05:39.150
This time, since there's no more acid

00:05:39.150 --> 00:05:42.140
to neutralize the added hydroxide anion,

00:05:42.140 --> 00:05:43.870
in our fourth particulate diagram,

00:05:43.870 --> 00:05:46.100
here's our hydroxide anion.

00:05:46.100 --> 00:05:47.720
And one of these sodium cations

00:05:47.720 --> 00:05:49.720
is the one that we just added.

00:05:49.720 --> 00:05:53.330
And we still have the chloride
anions and the sodium cations

00:05:53.330 --> 00:05:56.110
that were present at
the equivalence point.

00:05:56.110 --> 00:06:00.719
So this fourth particulate
diagram represents the titration

00:06:00.719 --> 00:06:04.670
after the equivalence point
when we're adding excess base.

00:06:04.670 --> 00:06:06.550
And we can see on our titration curve,

00:06:06.550 --> 00:06:10.571
as we continue to add excess
base, the pH keeps increasing.

00:06:10.571 --> 00:06:13.610
Now that we've gone through
the particulate diagrams

00:06:13.610 --> 00:06:15.750
for our strong acid-strong base titration,

00:06:15.750 --> 00:06:18.080
let's get back to our original problem

00:06:18.080 --> 00:06:21.010
which was to find the
initial concentration

00:06:21.010 --> 00:06:22.710
of hydrochloric acid.

00:06:22.710 --> 00:06:24.740
And we can do that by figuring out

00:06:24.740 --> 00:06:27.180
how many milliliters of base were added

00:06:27.180 --> 00:06:29.830
to get to the equivalence point.

00:06:29.830 --> 00:06:32.810
So if we just drop down
here on our titration curve,

00:06:32.810 --> 00:06:35.660
we can see that after
50 milliliters of base

00:06:35.660 --> 00:06:36.670
have been added,

00:06:36.670 --> 00:06:39.040
the equivalence point has been reached.

00:06:39.040 --> 00:06:41.270
Therefore, it took 50 milliliters

00:06:41.270 --> 00:06:44.760
of our .20 molar solution
of sodium hydroxide

00:06:44.760 --> 00:06:46.600
to completely neutralize

00:06:46.600 --> 00:06:49.740
the hydrochloric acid that
was originally present.

00:06:49.740 --> 00:06:51.810
And if we know the initial volume

00:06:51.810 --> 00:06:53.330
of the hydrochloric acid solution,

00:06:53.330 --> 00:06:55.460
let's say it was 100 milliliters.

00:06:55.460 --> 00:06:57.730
We can calculate the concentration

00:06:57.730 --> 00:07:01.490
using the MV is equal to MV equation.

00:07:01.490 --> 00:07:02.520
So for our equation,

00:07:02.520 --> 00:07:04.670
let's think about acid
being on the left side.

00:07:04.670 --> 00:07:07.410
So we don't know the
concentration, the molarities,

00:07:07.410 --> 00:07:08.503
we make that x.

00:07:08.503 --> 00:07:10.470
And for the volume of the acid,

00:07:10.470 --> 00:07:11.890
it's 100 milliliters.

00:07:11.890 --> 00:07:14.010
So we plugged that into our equation.

00:07:14.010 --> 00:07:14.860
On the right side,

00:07:14.860 --> 00:07:16.580
let's think about this being the base.

00:07:16.580 --> 00:07:18.280
So we know the molarity of the base,

00:07:18.280 --> 00:07:19.820
it's .20 molar.

00:07:19.820 --> 00:07:22.010
And we also know the volume of base

00:07:22.010 --> 00:07:24.400
that was necessary to
reach the equivalence point

00:07:24.400 --> 00:07:26.050
which is 50 milliliters.

00:07:26.050 --> 00:07:30.570
Solving for x, we find that
x is equal to .10 molar.

00:07:30.570 --> 00:07:33.490
So that was the initial concentration

00:07:33.490 --> 00:07:35.920
of our hydrochloric acid solution.

00:07:35.920 --> 00:07:37.930
And notice on our titration curve,

00:07:37.930 --> 00:07:40.090
remember we started with
only hydrochloric acid.

00:07:40.090 --> 00:07:42.130
So we start with a very low pH

00:07:42.130 --> 00:07:45.300
if we're titrating a strong
acid with a strong base.

00:07:45.300 --> 00:07:47.420
As we add more and more
of the strong base,

00:07:47.420 --> 00:07:49.500
the pH increases.

00:07:49.500 --> 00:07:51.670
We've just looked at the titration curve

00:07:51.670 --> 00:07:54.060
of a strong acid with a strong base.

00:07:54.060 --> 00:07:56.940
Let's compare that
titration curve to this one

00:07:56.940 --> 00:08:01.410
which is the titration of a
strong base with a strong acid.

00:08:01.410 --> 00:08:03.090
Since we're starting with a strong base,

00:08:03.090 --> 00:08:06.010
notice how the initial pH is very high.

00:08:06.010 --> 00:08:09.630
And since we're adding acid
notice down here on the x-axis,

00:08:09.630 --> 00:08:12.930
it now says milliliters of acid added.

00:08:12.930 --> 00:08:14.140
As acid is added,

00:08:14.140 --> 00:08:16.120
we can see the pH dropping slowly.

00:08:16.120 --> 00:08:18.270
And as we approach the equivalence point,

00:08:18.270 --> 00:08:22.600
we see a large drop in pH
with small additions of acid.

00:08:22.600 --> 00:08:24.660
However, the pH at the equivalence point

00:08:24.660 --> 00:08:26.110
is still equal to seven.

00:08:26.110 --> 00:08:28.800
So if we find a pH of seven on the y-axis

00:08:28.800 --> 00:08:31.190
and go over to our titration curve,

00:08:31.190 --> 00:08:33.570
this point represents
the equivalence point.

00:08:33.570 --> 00:08:35.810
And we can drop down to the x-axis

00:08:35.810 --> 00:08:39.010
and we would see it took
20 milliliters of acid

00:08:39.010 --> 00:08:42.180
to neutralize the base
that was initially present.

00:08:42.180 --> 00:08:44.330
Finally, once we go past
the equivalence point,

00:08:44.330 --> 00:08:46.160
we can see as we add more and more acid,

00:08:46.160 --> 00:08:48.420
the pH keeps getting lower.

00:08:48.420 --> 00:08:51.615
So this titration curve of a
strong base with a strong acid

00:08:51.615 --> 00:08:53.210
is essentially the reverse

00:08:53.210 --> 00:08:55.190
of the first titration curve that we saw

00:08:55.190 --> 00:08:58.933
which was the titration of a
strong acid with a strong base.


Opportunities for high school and college tutors

WEBVTT
Kind: captions
Language: en

00:00:00.850 --> 00:00:03.110
- Hi everyone, Sal Khan
here from Khan Academy,

00:00:03.110 --> 00:00:05.110
and many of you all know
about another project,

00:00:05.110 --> 00:00:06.940
another not-for-profit that
I've been involved with

00:00:06.940 --> 00:00:08.700
known as Schoolhouse.world,

00:00:08.700 --> 00:00:10.930
which is all about giving
folks free tutoring,

00:00:10.930 --> 00:00:14.520
and we do that by finding
amazing volunteer tutors.

00:00:14.520 --> 00:00:16.280
Now, the tutors we've been finding,

00:00:16.280 --> 00:00:17.650
they found a lot of reward

00:00:17.650 --> 00:00:19.520
just being able to tutor other folks,

00:00:19.520 --> 00:00:22.510
but we wanna make sure that
they also get other rewards,

00:00:22.510 --> 00:00:24.030
because if you can tutor something,

00:00:24.030 --> 00:00:26.300
not only does that mean
that you know the material,

00:00:26.300 --> 00:00:28.080
but it also means that
you can communicate,

00:00:28.080 --> 00:00:28.913
you have empathy,

00:00:28.913 --> 00:00:31.310
you wanna spend your time
doing good for others.

00:00:31.310 --> 00:00:33.940
So we were excited last year
when University of Chicago

00:00:33.940 --> 00:00:36.820
said that they were gonna use
someone's tutor reputation

00:00:36.820 --> 00:00:39.370
as a way to evaluate students applying

00:00:39.370 --> 00:00:40.790
for University of Chicago.

00:00:40.790 --> 00:00:43.890
And just a few weeks
ago, MIT also announced

00:00:43.890 --> 00:00:47.060
that they were putting
Schoolhouse.world profiles

00:00:47.060 --> 00:00:49.820
as an optional thing that
students could submit,

00:00:49.820 --> 00:00:51.470
and the school will look at them.

00:00:51.470 --> 00:00:54.210
To get a sense of the
amazing tutors we're finding,

00:00:54.210 --> 00:00:55.170
this right over here

00:00:55.170 --> 00:00:58.100
is the tutor profile and
the transcript for Karen Ji,

00:00:58.100 --> 00:00:59.480
who's a high school student,

00:00:59.480 --> 00:01:02.540
and you can see that she's
hosted a ton of sessions,

00:01:02.540 --> 00:01:06.570
543 learners impacted,
33 countries reached,

00:01:06.570 --> 00:01:09.350
864 positive ratings,

00:01:09.350 --> 00:01:11.387
and you can see what people
are saying about her,

00:01:11.387 --> 00:01:13.570
"My friend and I have been trying to go

00:01:13.570 --> 00:01:16.050
to every single one of your
sessions for the past few weeks,

00:01:16.050 --> 00:01:17.190
and we literally love you."

00:01:17.190 --> 00:01:20.390
And we can go after page after page

00:01:20.390 --> 00:01:22.970
of what everyone is saying about her,

00:01:22.970 --> 00:01:25.580
and then what also
fellow tutors are saying.

00:01:25.580 --> 00:01:26.740
So she's getting feedback

00:01:26.740 --> 00:01:28.430
to become a better and better tutor,

00:01:28.430 --> 00:01:30.470
and we also have the
certification mechanism,

00:01:30.470 --> 00:01:33.270
which is qualifying her
to tutor in specific units

00:01:33.270 --> 00:01:36.200
to show all of the things that
she has gotten mastery in.

00:01:36.200 --> 00:01:40.200
This right over here is the
tutor profile for Kate Pearce.

00:01:40.200 --> 00:01:41.140
So once again,

00:01:41.140 --> 00:01:44.270
another incredibly amazing
high school student,

00:01:44.270 --> 00:01:48.090
326 sessions hosted, over a
thousand positive ratings,

00:01:48.090 --> 00:01:51.337
and what people are saying
about Kate right over here,

00:01:51.337 --> 00:01:54.840
"She was very reassuring
and checks in with us

00:01:54.840 --> 00:01:57.280
whenever we needed help
or got a question wrong.

00:01:57.280 --> 00:02:00.170
Very helpful and patient
and broke things down."

00:02:00.170 --> 00:02:01.600
These are things that you can't get

00:02:01.600 --> 00:02:03.600
just through traditional transcripts,

00:02:03.600 --> 00:02:05.180
the ability of someone like Kate

00:02:05.180 --> 00:02:07.790
to communicate and
empathize with other folks.

00:02:07.790 --> 00:02:10.440
So if you're a university or an employer,

00:02:10.440 --> 00:02:12.930
who's really interested in
these types of students,

00:02:12.930 --> 00:02:14.620
I encourage you to click on that form

00:02:14.620 --> 00:02:18.120
in the link on this post,
and we can reach out to you,

00:02:18.120 --> 00:02:20.340
and figure out how we
can build a relationship,

00:02:20.340 --> 00:02:22.920
so that you can discover amazing talent

00:02:22.920 --> 00:02:25.180
from the tutors at Schoolhouse.world.

00:02:25.180 --> 00:02:26.510
So any young folks out there,

00:02:26.510 --> 00:02:28.330
especially high school
students or college students,

00:02:28.330 --> 00:02:29.650
who are interested in tutoring,

00:02:29.650 --> 00:02:32.030
I encourage you to check
out Schoolhouse.world,

00:02:32.030 --> 00:02:35.110
and I am going to work
with you and work for you

00:02:35.110 --> 00:02:36.850
to ensure that the rest of the world,

00:02:36.850 --> 00:02:38.290
college admissions officers,

00:02:38.290 --> 00:02:41.210
and eventually internships
and employers and scholarships

00:02:41.210 --> 00:02:43.070
recognize that one of the best ways

00:02:43.070 --> 00:02:45.580
to prove what you know
and give back to others

00:02:45.580 --> 00:02:47.683
is to tutor on Schoolhouse.world.