Weak Acid Titration : Solutions - Chemistry, Class 12 Video Lecture

Let's say I have a bucket of
aqueous solution, and it
contains some weak acid in it.
And so weak acid.
We've done this multiple
times.
It's in equilibrium with
its dissociated state.
So it would be in equilibrium.
Let me write the aqueous.
So it's a weak acid, where the
A could be any other kind of
molecular group.
That could be the conjugate
base of the weak acid.
So that's in equilibrium with
some hydrogen ions, so it'll
produce some of them, but
some of this is going to
be around as well.
Plus its conjugate base.
A minus.
Aqueous.
And just like we did with the
strong acid titration, let's
measure it's pH.
So I'll do that right here.
Actually, I want to do
it a little higher.
So let's do it right there.
Let me make a little
scale here.
So this is 0.
Let's say that right
there is 7.
That this right here is 14.
So our first measurement of the
pH of our solution-- we
have a maybe a liter of it.
Let's say we have 1 liter
of our solution.
We don't know the concentration
of this thing.
But let's say our first
measurement, we go and we say
OK, it's got a pH of--
I don't know.
Something here.
Let's say it's got a pH of 3.
So it's acidic, not ultra
acidic, but it's acidic.
So it's got a pH of
3 right there.
This whole scale on the
left-hand side.
This is all pH, just like
in the last video.
This is pH.
The pH is going up as
you go up the scale.
That's our first measurement.
Now we're going to titrate this
weak acid, just like we
titrated the strong base.
And normally when you do this--
I mean, you can, in
theory, titrate with anything.
But you normally titrate-- if
you're titrating an acid,
whether it's weak or strong--
you always titrate with a
strong base.
And if you are titrating a
base, whether it's weaker
strong, the reagent, or the
thing that you're going to
add, is going to be
a strong acid.
So in this case, even though
we're going to drop some drops
into a weak acid, we're
going to drop
drops of a strong base.
So let's use sodium
hydroxide again.
NaOH.
And just to make things
interesting, to make the math
a little interesting, let's say
it's a 0.5 molar solution.
And we're going to add it
in increments of little
100-milliliter increments.
A tenth of a liter.
But the interesting thing is
really the shape of the curve
that forms, or the titration
curve, or the pH curve.
Let me write what the
reaction of NaOH.
Just so you have that
in your head.
NaOH, sodium hydroxide,
and I did this in
the previous video.
It disassociates completely
in water.
And so it turns into Na plus--
all this is aqueous, of
course-- plus OH minus.
So when you throw even
a little bit.
Let's say you throw in-- this
is 0.5 molar solution-- 100
milliliters.
That's equal to one
tenth of a liter.
This 0.5 molar, that's
its molarity.
So it says hey, I'm going
to have half a
mole of this stuff.
Or really, of this stuff.
I'm going to have half
a mole of hydroxide
ions for every liter.
So in one tenth of a liter,
that means that I'm
going to have what?
I'm going to have
half of that.
So I'm going have 1/20,
or 0.05 moles.
So let's say this point right
here's is 100 milliliters.
I've added 100 milliliters
of our solution.
That's going to have
0.5 moles of OH.
Because remember, we could say
it's 0.5 moles of this, but
whenever this is in solution,
it disassociates
completely into this.
So if you have 0.5 moles of
this, you really have 0.5
moles of this.
This will be in this form
completely, because it's a
strong base.
So if I have 0.5 moles of this
and I throw it into the
solution, what's
it going to do?
It's going to start sopping up
some of our hydrogen ions.
So it's going to increase the
pH, and it's going to keep
increasing.
Right?
And this is a buffer.
And this is a key thing
to remember.
This is going to sop some
of this thing up.
But this is a buffer.
This is a weak acid.
So it's in equilibrium.
So it has this reserve over
here of the acid form.
So when you start lowering this
by adding a strong base
to it, to sop this up, what's
going to happen?
The reaction is going to move
in this direction to kind of
backfill, to fill or
to almost replace.
It won't get quite to where it
was before, but it's going to
dampen the stress on
the equilibrium.
So what's going to happen is the
concentration of your weak
acid is going to go down.
It's going to try to backfill
this stuff.
But every time the reaction goes
in this direction, this
is going to go up.
Right?
Remember, nothing is sopping
up the conjugate
base up here, right?
We're just sopping up
the hydrogen ions.
As we sop up more and more
hydrogen ions, this reaction
goes more and more in the
rightward direction.
But as it goes more and more
in the rightward direction,
we're just adding more
and more to this.
And the base just takes more
and more of this out.
So essentially when the base
is taking this out, it's
really taking this out-- or it's
taking half of this out.
It's taking the H part
of this out.
Because that converts to that,
and it gets sopped up by the
by the reagent.
The strong base.
And all of this ends
up on this side, as
the conjugate base.
So at some point, when we have
added the exact same amount of
OH as there is of this stuff, as
there is of this stuff plus
the hydrogen.
When we've sopped up all
of the hydrogen,
what's going to happen?
Well, right when you get above
that point, when you start
adding even more, you're going
to start getting very basic.
It's just going to skyrocket
up the pH.
Because then you're adding OHs,
and they're not being
canceled out by any hydrogen
over there.
And there's nothing to kind
of backfill the hydrogen.
So you're going to get to
a very high reaction.
When we add enough of the sodium
hydroxide that this
completely neutralizes this
and this, and all we
have left is that.
Well, you might say, oh, we have
a pH of 7 because all of
the acid is neutralized.
But we don't have a pH of 7 like
we did with the strong
base, because here, as we
sopped up stuff, we kept
adding conjugate
base over here.
Remember, and it's
a real base.
It actually has basic
properties.
If this is a strong acid, if
this was hydrogen chloride,
I'd still have the chloride
over here.
But chloride, even though it's
the conjugate base, it has no
basic properties.
It does not increase the
pH of a solution.
But this stuff right here does
increase the pH of a solution.
And we've done that in
previous videos.
So once all of this is sopped
up, your equivalence point, or
the point at which the number
of moles of this is equal to
the number of moles of this,
is going to have a high pH.
So it's going to be like
right over there.
And then as you add more and
more of your reagent, or your
strong base, your pH is going
to get higher and higher and
approach 14 or even
go above it.
So your titration curve
is going look
something like this.
And once again, you look
at the steepest
point on the curve.
It's right there.
And you say, OK, that's
the equivalence point.
That's the point at which
I had an equal amount of
hydroxide having sopped up all
of the hydrogen, and I've run
out of all of this stuff to
backfill the hydrogen.
So you look at your
equivalence point.
And you say, OK, if that
equivalence point was when I
added, I don't know.
Maybe that's when you added
2 liters of your reagent.
So you added 2 liters
of your reagent.
Your basic reagent,
right here.
Or your titrant.
Or titrator.
I think that's another
word for it.
How many moles have you added?
Well it's 2 liters, it's 0.5
molar, so you've added 1 mole.
So here you've added
1 mole of OH.
And when you added 1 mole of OH,
you sopped up all of the
whatever mole you had of this.
So you clearly had
1 mole of that.
So over here, your original
concentration of your-- I
mean, you really could say your
concentration of your HA
plus your initial equilibrium
hydrogen is equal to 1 molar.
Now in most chemistry classes,
this number is way bigger than
this number for most
weak acids.
And you could look at that.
Because if you look at some of
the pKa's for some weak acids,
you'll see that this is some--
well, I won't go into the
math, but this number's
a lot lower.
So this is an indication,
essentially, of your initial
concentration of
your weak acid.
So you essentially just keep
titrating it, figure out the
inflection point, you say, the
inflection point happened when
I added 2 liters of the
titrator to it, which
corresponded to 1 mole.
So therefore I must have had 1
mole of my original acid in
the equation.
Another interesting thing.
So if you said OK, I had 1 mole
of my original acid in
the equation.
So you say, this was 1 molar
originally, before I started
titrating it all.
And if I said that this is a 1
liter solution-- this is 1
mole, not 1 molar.
But if I know it's a 1-liter
solution, now I know that
originally I had 1 mole
in 1 liter, so I had
a molarity of 1.
Now, let me go over something
else that's interesting about
a basic reaction.
Actually, let me do that in
the next video, because I
think this one's getting
a little long.