Acids and bases
Weak Acid Titration Equivalence point when titrating a weak acid
Weak Acid Titration
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- 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.
- 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.
- 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
- 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
- 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.
- 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.
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