0:01
Titration is a procedure for determining
0:03
the concentration of a solution.
0:06
And so let's say we're starting with an acidic solution.
0:08
So in here let's say we have some hydrochloric acid.
0:11
So we have come HCl.
0:13
And we know the volume of HCL,
0:15
let's say we're starting with 20.0 milliliters of HCl.
0:20
But we don't know the concentration right?
0:22
So question mark here for the concentration of HCl.
0:25
We can find out that concentration by doing a titration.
0:29
Next we need to add a few drops of an acid base indicator.
0:33
So to this flask we're also going to add
0:36
a few drops of an acid base indicator.
0:38
We're gonna use phenolphthalein.
0:40
And phenolphthalein is colorless in acid
0:43
but turns pink in the presence of base.
0:46
And since we have our phenolphthalein in acid
0:48
right now we have a clear solution.
0:51
There's no color to it.
0:52
Up here we're gonna have our standard solution right?
0:55
We're gonna have a known concentration of sodium hydroxide.
0:59
So let's say we have a solution of sodium hydroxide
1:02
and the concentration is zero point one zero zero molar.
1:07
And we're ready to start our titration.
1:09
So we allow the sodium hydroxide to drip
1:13
into our flask containing our HCl and our indicator.
1:18
And the acid in the base will react, right?
1:20
So we get an acid base neutralization reaction.
1:23
HCl plus NaOH right?
1:28
If we think about the products, this would be OH minus,
1:31
this would be H plus,
1:32
H plus and OH minus give us H2O.
1:36
And our other product we would have Na plus and Cl minus,
1:40
which give us NaCl, or sodium chloride.
1:44
So let's say we add a certain volume of base right?
1:47
So now this would be higher,
1:49
and we see our solution turn light pink.
1:54
Alright so let's say we see our solution turn light pink
1:57
and it stays light pink.
1:58
That means that all of the acid
2:02
has been neutralized by the base.
2:04
And we have a tiny amount of excess base present,
2:08
and that's causing the acid base indicator to remain pink.
2:12
So a tiny excess of base
2:14
means we've neutralized all of the acid present.
2:17
And where the indicator changes color,
2:20
this is called the end point of a titration, alright?
2:23
So when our solution changes color,
2:27
that's the end point of our titration.
2:29
And here we stop and we check and see the volume
2:32
of base that we used in our titration.
2:35
So if we started right here,
2:36
if we started with that much base,
2:38
let's say we ended down here, alright?
2:40
So we still have a little bit of base left.
2:42
And this would be the volume
2:44
of base that we used in the titration.
2:47
Alright so we have a change in volume here,
2:51
and let's say that it's 48.6 milliliters.
2:56
So it took 48.6 milliliters of our base
2:59
to completely neutralize the acid that we had present.
3:04
And so we can now calculate the concentration of the HCl.
3:09
Alright so let's go ahead and do that,
3:11
and let's start with the concentration of sodium hydroxide.
3:15
Alright we know that we started with
3:18
point one zero zero molar solution of sodium hydroxide.
3:23
So point one zero zero molar.
3:25
And molarity is equal to mols over liters.
3:30
Alright so this is equal to mols over liters.
3:35
And our goal is to figure out how many mols of base
3:38
that we used to neutralize the acid that was present.
3:43
Alright so we can take our volume here, 48.6 mililiters
3:47
and we can convert that into liters.
3:50
Alright so just move your decimal place
3:52
three places to the left.
3:56
So that's point zero four eight six liters.
4:00
So this is equal to mols over
4:02
zero point zero four eight six liters.
4:07
And so let's get some more space.
4:10
Alright let me just rewrite this really quickly.
4:12
Zero point one zero zero is equal to
4:15
X over zero point zero four eight six.
4:18
So we're just solving for X, and X represents
4:21
the mols of sodium hydroxide that were necessary
4:25
to neutralize the acid that we had present.
4:28
Alright so when you solve for X,
4:30
you get zero point zero zero four eight six mols
4:35
of sodium hydroxide used in our titration.
4:40
Next you look at the balanced equation for what happened .
4:44
So if I look at my balanced equation
4:47
alright there's a one here
4:49
and there's a one here.
4:51
So we have a one to one mol ratio.
4:54
And the equivalence point is where
4:57
just enough of your standard solution has been added
5:00
to completely react with the solution that's being titrated.
5:05
And at the equivalence point,
5:07
all of the acid has been neutralized.
5:09
Right? So it's completely reacted.
5:11
And since we have a one to one mol ratio,
5:14
if I used this many mols of sodium hydroxide,
5:18
that must be how many mols of HCl that we had present
5:21
in our original solution.
5:23
So therefore, I can go ahead and write that I must have had
5:27
zero point zero zero four eight six mols
5:31
of HCl present in the flask before we started our titration.
5:37
Right and I knew that because of the one to one mol ratio.
5:42
Remember our goal was to find the concentration of HCl.
5:46
The original concentration.
5:49
molarity is equal to mols over liters.
5:52
So now I know how many mols of HCl I had,
5:55
and my original volume of HCl
5:58
was 20 milliliters right?
6:01
So right up here we had 20 milliliters.
6:03
So I need to convert that into liters.
6:06
So I move my decimal place one two three.
6:09
So I get point zero two liters.
6:12
So now our final step here to calculate
6:15
the concentration of HCl,
6:17
right so the concentration of HCl is equal to
6:20
how many mols of HCl we have,
6:21
which is zero point zero zero four eight six mols,
6:25
over liters of solution.
6:28
And we had 20 milliliters which is equal to
6:31
zero point zero two zero zero liters.
6:35
Alright so now we can take out our calculator
6:38
and do this calculation to find
6:40
the concentration of HCl that we started with.
6:43
Point zero zero four eight six,
6:46
all right and we're gonna divide that by
6:49
point zero two zero zero.
6:52
And we get zero point two four three for our answer.
6:56
So the concentration of HCl is equal to
7:00
zero point two four three molar.
7:05
So we've solved for the original concentration of HCl.
7:09
There's a shortcut way to do this problem,
7:12
and the shortcut way would be to do the molarity
7:16
times the volume of the acid
7:19
is equal to the molarity times the volume of the base used.
7:23
So MV is equal to MV.
7:26
So let's say we have the acid over here on the left,
7:28
and the base over here on the right.
7:31
So the molarity of the acid is what we're trying to find.
7:34
So I'll just make that X.
7:36
The volume of the acid that we started with,
7:38
you can just leave this in milliliters if you want,
7:41
20 point zero milliliters is
7:43
how much of the acid we started with.
7:45
And for the base, we knew the concentration
7:48
of the base that we used in our titration right?
7:50
It was zero point one zero zero molar.
7:54
And we also knew the volume of base
7:56
that we used to completely neutralize the acid.
8:00
We used 48.6 milliliters.
8:04
And notice how the mLs would cancel out here.
8:07
Right and we can just go ahead and do the math
8:10
So we get out the calculator, and we need to multiply
8:14
48.6 times point one zero zero.
8:19
Alright and so we get four point eight six obviously.
8:22
And then if we divide by 20
8:25
we will get our answer of zero point two four three.
8:29
So X is equal to zero point two four three molar.
8:35
And this shortcut way works pretty well
8:38
when you're dealing with a strong acid and a strong base
8:41
and a one to one molar relationship.
8:44
Alright in the next video we'll do a problem where
8:46
the mol ratio is no longer one to one.
