Acids and bases
Strong Acid Titration Strong acid titration and equivalence point
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- Let's have 1 molar of-- let me pick a strong acid.
- Let's say it's hydrochloric acid.
- HCl.
- And we know, of course, that since it's a strong acid it
- disassociates completely.
- So HCl in an aqueous solution-- or a solution of
- water-- disassociates completely into hydrogen ions
- in an aqueous solution, which is really hydronium.
- I've said that multiple times.
- Plus your chlorine anion in an aqueous solution.
- So if you actually have a molar of this, as soon as you
- put it in the water, you really have 1 molar of this.
- So your hydrogen concentration is really 1 molar.
- And of course we know, what's the pH of that?
- Well, the pH is just the minus log base 10 of your hydrogen
- concentration, which is 1.
- Which is the minus log of 0.
- So minus 0, which is equal to 0.
- So your pH is going to be equal to 0.
- And let's say for use in the later part of this, let's say
- I have a liter of this.
- Right.
- This just tells me how many moles per liter, and now I
- just told you that I have 1 liter.
- So you know that you have 1 mole.
- Let me make a little chart here.
- I'll make a little box here.
- And let's make this axis right here.
- This is going to be my pH.
- And right when I take that first measurement-- let's say
- this is 0 here, this is 7, and this is 14 up here.
- When I take that first measurement, my solution of
- hydrogen chloride, or hydrochloric acid, has a pH 0.
- Now, what I'm going to do is I'm going to titrate this
- aqueous solution of acid.
- And titrate just means drop something else into it in
- controlled increments to see what happens.
- And what I'm doing here is going to be
- an acid-base titration.
- I have an acid here, and what I'm actually going to titrate
- it with, or the reagent that I'm going to add to it--
- The reagent is essentially a reactant that, when you view
- it this way, that you're adding to the solution.
- So it's the titrant.
- So in this case, my solution already has
- hydrochloric acid in it.
- The pH is already 0.
- But what I'm going to do is I'm going to
- add a strong base.
- Let's say sodium hydroxide.
- That's my favorite strong base.
- And let's say I have a solution that I have prepared
- ahead of time.
- Let's say it's a 1 molar solution.
- But I'm not going to add all of it at the same time.
- I'm going to add it in increments of
- a tenth of a liter.
- So this is 1 liter that I've added.
- And then this would be 2 liters.
- And so this would be 500 milliliters.
- 500 mL.
- This is 1 liter.
- This is 1.5 liters.
- This 2 liters.
- And so on this axis, I'm going to slowly add more and more of
- this hydroxide.
- So what's going to happen?
- As I slowly add hydroxide, it's going to increase the pH.
- Why is it going to increase the pH?
- Because this OH.
- Remember, hydroxide is a strong base.
- So the NaOH disassociates completely
- in an aqueous solution.
- To OH minus plus sodium cation.
- Right?
- Now if I'm adding this to this stuff here, what do you
- think's going to happen?
- Well this stuff right here, these OHs are just going to
- sop up your hydrogens.
- So for the most part-- obviously, everything is
- probabalistic.
- And whenever we're dealing with chemistry, there's always
- some molecules that won't completely react, and there
- will always be some concentration of anything.
- And that's why you actually can never get to a 0
- concentration of hydrogen, no matter how much base you add.
- Because there will always be some hydrogen molecules that
- are just hiding just perfectly, or they're in
- perfect equilibrium so they don't react.
- But most of them will react.
- But as I add more H, these are going to disappear.
- So my hydrogen concentration is going to go down.
- The pH is a negative log of that, so your pH will go up.
- Your pH will go up slowly-- well, it will look slowly,
- because it's a log chart, so maybe it's doing
- something like this.
- Your pH concentration is going to look something like-- as
- you add more and more.
- But you might say, oh that's such a slow movement.
- Why isn't it occurring faster.
- But you have to think about something.
- This right here, if this is a pH of 1, this means-- so, at a
- pH of 0, this means your hydrogen
- concentration is 1 molar.
- At a pH of 1, this means your hydrogen concentration is at
- 10 to the minus 1.
- Which is 0.1 molar.
- So even though on a log chart it looks like you've made a
- very small movement, you've actually gone
- by a factor of 10.
- In terms of your reduction of your hydrogen concentration.
- You started off with 1 molar of your hydrogen.
- By the time you've added roughly 900 milliliters of
- your solution, you probably would have gotten
- rid of most of it.
- And obviously the denomenator's changing,
- because I'm actually adding volume here.
- Right?
- Because I'm actually adding some volume here.
- So you would have to add them.
- But you get the I general idea.
- That these actual molecules of hydroxide are going to react
- with these things.
- They're going to essentially turn into water and disappear
- into the solution.
- So these are going to disappear with those
- disappearing, for the most part.
- Which leads in an overall reduction concentration of
- your hydrogen ions, which will lead to an
- increase in your pH.
- Until some point where you have pretty much sopped up as
- many of these things as you can.
- There will always be a few of them, but you've sopped up as
- many of them as you can.
- And so any incremental hydroxide you add will
- actually go to build the hydroxide concentration.
- Where the hydroxide concentration is going to be
- greater than your hydrogen concentration.
- But even better than that, there's going to be some point
- where you've added just enough of this that you have an equal
- amount of this and this.
- You have an equal amount of hydroxide and hydrogen.
- Right?
- What happens, first of all, when there's an equal amount
- of hydroxide and hydrogen?
- Well, then you're neutral.
- Right?
- If your hydroxide concentration is equal to your
- hydrogen concentration, you're neutral.
- This is just like water.
- You're at a pH of 7.
- So there's some point where your hydroxide concentration
- is equal to your hydrogen concentration, and your pH is
- going to be 7.
- And then above that, you're just adding
- more and more hydroxide.
- The hydroxide concentration is overwhelming your hydrogen
- concentration.
- And so you're going to get really, really basic.
- And since this is a strong base, you're eventually going
- to get to a pH of 14.
- Because you're essentially going to have-- well, you're
- not going to have 1 molar.
- Let me erase that point.
- You're going to have half a molar of OHs
- at the end of this.
- So your graph is going to look something like this.
- Now, I mean, you're still going to have hydrogen
- concentration, but it's going to go really small.
- It's going to get really, really small.
- Now.
- What's interesting about what just happened here?
- At this point right here, your OH is equal to your hydrogen.
- And the way I think about it is the great majority of your
- moles of hydrogen were sopped up at that point.
- So in order to stop up 1 mole of hydrogen, you essentially,
- for the most part-- and there's always things that
- don't get completely sopped up-- but you'll essentially
- have to add 1 mole of OH to sop up
- your 1 mole of hydrogen.
- Remember, this is 1 molar and you had 1 liter of it.
- So you had 1 mole of hydrogen to sop up.
- So you need 1 mole of OH to sop up, and how do you get 1
- mole of OH?
- Well, we said the concentration of the solution
- was 1 molar.
- So once you put a liter in there-- because that's 1 mole
- per liter-- once you put a liter in there, you would have
- put 1 mole of hydroxide cations in there
- to sop up the hydrogen.
- So this is the equivalence point.
- Where have essentially, you have an equal amount and very
- low concentrations of both hydroxide and hydrogen.
- You've essentially sopped up all of the hydrogen, but you
- also have sopped up all of the hydroxide that
- you've added so far.
- After this point, any hydroxide you add will
- essentially contribute to increasing the pH even more.
- Or it'll kind of overwhelm your hydrogen concentration.
- Now what's interesting about this?
- In this example, I told you, oh, I have hydrochloric acid
- and I gave you its concentration ahead of time.
- But let's say I didn't tell you that.
- Let's say it was some mystery substance, and you just
- measured the mystery substance's pH.
- At this point, you said OK, it's pH is 0.
- And you titrated it with this sodium hydroxide.
- And you said, well, gee, when I added 1 mole-- because you
- figured out at a liter of the solution-- of hydroxide ions.
- At that point, I've essentially eliminated all of
- the hydrogen ions that were in my original solution.
- So I must have had 1 mole of whatever acid
- this was over here.
- So this would have told you that my initial concentration
- was 1 mole of H plus.
- And since we know it's easy to see how much volume you have,
- you see I have a liter.
- So you say my original concentration of
- this acid is 1 molar.
- And you could also know that what you're dealing with is a
- strong acid.
- And I encourage you to watch the next to do I do the same
- thing with a weak acid.
- But because it's a strong acid and a strong base reacting
- with each other, when you have the same number moles of both,
- you get exactly to a pH of 7.
- They completely neutralize each other, and you have a
- completely neutral solution.
- If you had a strong base with a weak acid, the strong base--
- once you have the same amount as the weak acid, it would've
- neutralized it.
- But you would have also had some of the
- conjugate weak base left.
- So you would have actually ended up with a basic pH.
- And I'm going to do that in the next video.
- But if you see that the equivalence point occurs at a
- pH of 7, you'd know that you were titrating-- what you were
- dealing with was a strong acid.
- Because it was completely neutralized, not more than
- neutralized, by the strong base.
- And if you want to eye-ball where is equivalence point,
- you look for the steepest point in the graph.
- Because that's the point where you're going from having a
- majority of hydrogen versus hydroxide, to having a
- majority of hydroxide versus hydrogen.
- So it creates this inflection in the graph, and you just
- would have to look at this chart.
- And say, I added a liter of this solution.
- The concentration of this solution is 1 mole per liter.
- It's 1 molar.
- It has a molarity of 1.
- So I must have added 1 mole of hydroxide
- to get to this point.
- So I must have neutralized 1 mole hydrogen.
- So there must have been 1 mole of hydrogen to begin with.
- And since I had 1 liter of solution, I had 1 mole
- hydrogen and 1 liter of solution.
- So my original molarity was 1.
- Anyway, I don't want to confuse you too much.
- In the next video, I'm going to try to do the same thing,
- but I'm going to do it with a weak acid.
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