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Kevin Kevin Ahern: Alright!
Good deal.
Let's see.
A couple things.
Somebody asked if I would send,
or several people asked if I would send matching,
problems for the 6th edition of the textbook
compared to the 7th
and I did that and hopefully that's helpful to you.
As I noted in my e-mail,
sometimes the problems change a little bit
so I can't tell you they're exactly
the same problems but at least
they're the ones I assigned the students
last year so you can see approximately where those are.
I have put a couple of books on reserve in the library.
They're not yet available I think.
The library takes a little while
to get stuff out there,
so, when I get the word from the library they're available,
they'll be available to you.
And I've got a bunch of 6th edition textbooks
sitting around as well so if there's interest
in those and you want to take a look
at one of those come see me,
I can work with that also.
Okay, not yesterday,
Wednesday, Monday, whatever it was...
okay, Monday,
I got through talking,
about just sort of an introduction
about chemistry and I started talking
about solutions and pH
and today I'll spend a fair amount of time
talking about pH and buffers.
It's important that we all get on the same,
page with respect to this.
Now somebody asked me a question
just before class about,
is it true I don't let you use calculators on an exam?
And that's true...
you don't use calculators on exam.
But you also won't have to come up
with a number on an exam, okay?
You're not going to have to do a logarithm
in your head for example,
okay, it's not something like that.
But you will have to know how to use equations
and how those equations tell you information.
I'll give you some examples of that today.
So even though there are problems
you may be actually using a calculator to do them,
all the problems that I will assign you,
you will not need a calculator to do.
Yes, ma'am?
Student: So we should commit all the equations to memory?
Kevin Ahern: Should you commit all the equations to memory?
That's a very good question.
In fact, I will give you every equation
that you need to know on the exam.
Ahh
[class laughing]
right?
Okay, so you won't need to memorize any equations.
You won't have to crunch a number as such, okay?
You will have to know how to use equations
and get information from those equations
by their very nature.
So that's something you will need to know how to do.
The TAs will be helping you
with that process and of course I'm available
to help you with that process as well,
so don't sweat the calculator component, okay?
Alright, well usually in my lectures
what I have are a series of figures
I go through and show you various principles and so forth,
and this is one place today
and today's an unusual lecture
in that I don't really have much
in the way of figures I'll be going through
and showing you,
simply because your book actually
I think is rather short-sighted.
It's one of the few places where the book
is not very good in its coverage of the subject.
It's not very good in covering pH and buffers.
And so it's important to understand
these concepts because,
as we shall see,
understanding how the pH
of a solution affects molecules,
we understand better how it affects
charges of molecules and ultimately
how that affects these molecules that,
that are in proteins.
Proteins, we are going to come back to many times,
are essential obviously for cellular life,
but their structure is essential for their function.
And one of the things that we will see
when we start talking about proteins,
and I'll actually...
[buzzing]
you know, when they redesigned this room,
I was told that was going to go away.
Space aliens are still here I guess.
Okay, when they,
proteins get different charges,
they adopt different shapes.
And so we change the ability of a protein
to function as we change the pH of a solution.
So, what I'm going to do today...
[buzzing]
oh, don't do that...
[rumbling continues]
okay.
What I'm going to go through today
is talking about pH and buffers,
okay, so it's very important to understand that.
Alright, well last time I introduced
the topic I said pH of course
we know is a negative log with a hydrogen ion
concentration and pOH is a negative log
with a hydroxide ion concentration.
Freshman chemistry, okay?
pH plus pOH is equal to 14, okay?
And by the way,
I think you should be able to add
14 and subtract 14 without a calculator so there are,
I can't tell you,
you won't have to do simple math,
but you won't have to do any crunching.
Another concept I want to introduce here,
I talked last time a little about
the fact there's a difference
between strong acids and weak acids.
And we will be mostly concerned
in this course with weak acids.
That is acids that do not completely dissociate in water,
okay, at a certain pH, alright?
Now a prime example of that,
acedic acid.
As I said,
acedic acid ionizes to a very limited extent.
If I put it into water,
I might get one molecule
in a thousand that are ionizing, okay?
I put HCl in water and I get every
one of them ionizing,
meaning they come apart.
So there is a fundamental difference
between weak acids and strong acids.
If I know how much HCl I start with,
I know how many protons there are there.
I don't necessarily know that if I have acetic acid.
The number of protons that I will have free
in a solution where there's acetic acid
will depend on the pH of that solution, okay?
The ionization of acetic acid varies with pH,
that's number one.
The ionization of a weak acid varies with pH.
Okay.
Now we'll see that mathematically
and I hope you'll think in mathematical terms
instead of memorization terms.
I can tell you for example if the pH is higher,
we're going to have more ionization
than if we have the pH being low, okay?
When I say ionized with respect to a weak acid,
I'm talking about a proton coming off.
That's what ionizing is all about.
For HAc I can write
HAc goes to H+ plus Ac-.
Okay.
That is an ionization right there.
I'm making two ions.
Now, this equation you're going to see
over and over and over
because it's important that we understand
what's happening in that ionization.
Alright.
Well how do we determine,
are all weak acids the same?
They're not all the same.
In fact, we see enormous variety in the strength of weak acids.
If I were to define strength of weak acid to you,
I would say that at a given pH,
a relatively strong weak acid
will ionize more than a relatively weak, weak acid.
Now we've got strong weak acids
and weak, weak acids, alright.
Nothing like confusing the picture
on the second day of class.
Okay.
That's important, okay.
Now, how do we compare those?
Well we compare those where there's a measure
of a strength of an acid it's a constant known as Ka.
We won't even concern ourselves with that.
We're going to be concerned with the negative
log of Ka which is the pKa.
So just like ph is the negative log
of a hydrogen ion concentration,
pOH is the negative log of the hydroxide ion concentration,
pKa is the negative log of the Ka.
We probably won't talk about Ka again after today.
We will talk a lot about pKa.
So what is pKa?
pKa is the measure of the strength of an acid.
A strong acid like HCl has no pKa,
it completely comes apart, okay.
A weak acid like,
acetic acid has a pKa of 4.76, okay.
What does that mean?
Okay, I'll tell you what that means in a second.
But I'm going to compare the pKa of acetic acid
with that of formic acid, okay.
Formic acid is also a weak acid but its pKa is 3.75.
And no, you don't need to memorize these numbers,
you'll be given them if you need them.
Okay.
If I compare acetic acid to formic acid,
formic acid has a pka of 3.75,
acetic acid has a pKa of 4.76,
formic acid is stronger of a weak acid than acetic acid is.
Okay?
Are we clear on that?
So when comparing the two,
I compare the two pKas,
the one that has the lower pKa is the stronger acid.
Now we can go through,
we can derive all the math,
as necessary to do that,
but we don't really need
to do that in this class, okay.
Simple concepts about what Ka is.
Well, what pKa is, I'm sorry.
Okay.
I said HCl, strong acid, doesn't have pKa.
Okay.
We don't, we don't even consider it.
Because it just completely comes apart.
We can't mess with that.
Alright.
So, the lower the pKa, the stronger the acid.
Alright?
[buzzing]
I don't know why it does that.
Male student: Do you have your phone in your pocket?
Kevin Ahern: I do have my phone in my pocket,
but I don't think that should be doing that.
Should I, should I mess with it?
Male student #1: Yeah
Male student #2: Yeah, that will get feedback across.
Kevin Kevin Ahern: I'll put it on airplane mode.
I've tried this before but it didn't seem to make go away.
[Buzzing continues]
Female student: [inaudible]
Kevin Kevin Ahern: I'll just jump.
Okay.
Now, I apologize for that noise.
I think it's something that this room
is haunted or something,
it just doesn't,
it's stuck there.
Alright, let's think about that,
that weak acid,
let's think about acetic acid for a moment
and let's think about what happens
with it in a solution.
I said if I just dump it into water,
okay, a pretty small percentage
of that HAc becomes H+ and Ac-.
When I'm concerned about pH,
the only thing I'm concerned about is the H+,
not the Ac-, right?
If I have it sitting there
and let's say I put a million molecules
of acetic acid into that aqueous solution
of water and one thousand of those,
protons come off,
I'm going to have one thousand protons
from there come off,
I'm going to have one thousand Ac minuses, right?
And I'm going to have 999 thousand HAcs left behind, right?
Everybody with me?
Let's say I add some sodium hydroxide to that solution.
Sodium hydroxide of course is a strong base,
and like a strong acid,
a strong base completely dissociates in water,
so if I put a million molecules
of NaOH into a solution of water,
I get a million molecules of Na+
and a million molecules of OH-.
Completely dissociates.
So strong acid is equivalent
to strong base in terms of its strength.
They completely come apart.
Let's imagine, if you will,
that I put a thousand of molecules,
let's make it fun,
let's put 449,000 molecules
of OH- into that solution.
Okay?
What's going to happen to that solution?
I'm going to have an excess of OH-, right?
Is the pH going to go way up?
It's gonna go up.
But it's not going to go up as much as you might think.
Why not?
Go ahead.
Female student: The buffer's to the limit?
Kevin Ahern: Well, you're getting ahead of me
in terms of definitions of what a buffer is,
but yes, buffers,
there are buffers,
this is acting as a buffer,
but I don't even want to use that term yet.
Why doesn't the pH just go through the roof?
Female student: The acedic acid dissociates?
Kevin Ahern: Acetic acid can dissociate.
So I had 499, I'm sorry,
I had 999 molecules of that Hac
that was sitting there, right?
Some of those could give out protons, right?
And what's acedic,
what's OH going to do when it hits a proton,
well it's gonna hit with a proton and make water,
thereby neutralizing it, right?
So I had a thousand molecules of H+
and I had a thousand molecules of Ac-,
and I add 449,000 molecules,
why am I think 449,
499 thousand molecules of NaOH,
499 thousand molecules of that Hac
is going to give up protons.
It's going to give up protons.
And at that point,
I'm gonna have 500,000
molecules of Ac- and I'm going
to have 500,000 of HAc.
Right?
1,000 plus 499,000, right?
With me?
You can say, "yes, Kevin."
No you can't obviously.
Okay.
At that point I have 500,000 molecules of HAc,
I have 500,000 molecules of Ac-,
and I've got a higher pH.
That higher pH turns out to have an important name.
When I have equal numbers of Ac- and HAc,
I've reached the pKa.
pH equals pKa when salt equals acid.
With me?
pH equals pKa when salt equals acid.
So pKa is simply a pH.
It's a special pH.
It's the pH at which salt equals acid.
Now, I told you that acetic acid had a pH of 4.76, right?
That tells you something,
it tells you that pKa,
I said pH, has a pKa of 4.76.
That tells you that pKa is a constant.
It's a constant for a given acid.
Whenever I have acedic acid,
it will always be 4.76.
It will never change.
I can change the protons,
I can change the hydroxide,
but the pKa will be a constant.
The pH will change, but the pKa won't.
Alright?
Well it turns out there's an equation
that relates these things I'm telling
you just conceptually at the moment, okay?
The equation is an equation
you're going to hear a lot about,
it's called the Henderson Hasselbalch equation.
The Hendrson Hasselbach equation states that,
here we go, pH equals pKa plus log of Ac- over HAc.
More commonly we will say that pH equals pKa plus log,
the concentration of salt,
divided by the concentration of acid.
I called the thing that has lost the proton the salt.
And I think you'll find it much easier
to understand if you call it the salt instead of the base.
Okay?
So, pH equals pKa plus log,
the salt, over acid.
Acid is the thing that has the proton.
Acid is not the proton.
The acid is the thing that has the proton.
The difference between the salt
and the acid is a single proton.
There it is right there.
Okay?
Now, everybody understand the terms I'm talking about?
Alright?
Salt, acid, Henderson Hasselbalch equation.
We're going to use the equation in just a second.
How do I make something into a salt?
I take protons, I take protons off of an acid, right?
I can convert acid into salt by taking protons off
and in the example I just gave you,
how did I take protons off the acid?
I added a strong base.
Okay?
If I wanted to put protons onto that salt,
how do you suppose I would do it?
I would add a strong acid.
Okay?
How would I protons on?
Well, if I start dumping protons into the reaction right here,
what's going to happen to this equation
on the basis of the principle of Levoisier?
Male student: It's going to push it left.
Kevin Ahern: It's going to move to the left,
which means I'm going to make this
and I'm going to lose this.
Right?
You already saw I went to the right
when I started taking protons away,
the solution starts trying to make them up.
Starts trying to replace them, starts making more Ac-.
There's a one to one relationship.
For every molecule of strong base I added,
I lost one of these and I made one of these.
The same holds true if I go the other way.
If I go the other way
and I add protons to this solution,
if I add 500 molecules of ACl,
I'm going to lose 500 of these,
I'm going to make 500 of these.
Understanding that is the most thing students
screw up on buffer problems.
The single most common thing they screw up on.
There's a one to one relationship
between adding and subtracting acids and bases.
That's all there is to it.
A very simple concept, okay?
Now, let's go back to our equation.
Our equation said pH equals pKa plus the log,
the concentration of salt,
divided by the concentration of the acid, alright?
In the example I gave you,
we had 500,000 molecules of salt
and we had 500,000 molecules of acid.
Let's plug in to that equation.
They're in the same volume
so the volume cancels out,
we don't have to worry about concentration,
we can actually use numbers, okay?
you have a hand up.
Female student: Is that a dash or a negative?
Kevin Ahern: Uh, where?
Female student: [inaudible]
Kevin Ahern: This?
That's a dash.
Yeah, that's not a negative, that's a dash.
That's a good question,
I didn't notice there was a dash there.
Maybe I'll remove that dash so you don't get confused.
That's a dash, not a negative pH,
that's just a dash.
pH equals pKa plus log of salt over acid.
Now, Let's think about that.
Let's plug in our terms.
I want to find the pH of the solution I've just,
I've just defined for you.
I just made a solution of acetic acid.
I have a pKa of 4.76 because that's
a constant for acetic acid.
And I've got 500,000 molecules
of Ac - and 500,000 molecules of HAC.
What's the pH?
4.76!
You can tell by either the fact I just told
you when the two are equal,
if that makes sense,
but you can also tell it more importantly from the equation.
The log of 500,000 over 500,000
is the same as the log of 1,
and the log of 1 is equal to zero.
Yes, you have to know that,
but I'll even put that on the exam.
Okay?
That's cool.
Okay.
That's cool.
So now I see mathematically why the pKa is the pH
at which the salt equals the acid
because when the salt equals the acid,
this log term becomes zero and pH equals pKa.
Questions?
I'm kinda going through this kinda blig-da-bleh.
Am I that clear?
I know I'm not.
Am I that fiersome?
I probably am.
Yeah?
[laughing]
Oh, is there a question?
I'm sorry.
Yeah?
Male Student: You said if you want to increase
the [inaudible], you add a strong base?
Kevin Ahern: If I want to increase the amount of salt,
okay, I would have to pull protons off
of the acid using a strong base.
Okay.
Important concept and thank you for asking the question.
If I want to make acid from salt,
I've got to put protons in which means
I've got to add a strong acid.
Yes?
Male Student: So to make acid,
you have to use acid?
Kevin Ahern: In order to make acid in this system, I have to use
protons from a strong acid, that's correct.
Okay?
So if I guess there's no questions, you guys are ready for a pop quiz.
That means you've already understood it, right?
Or are there questions?
Because I'll find out really quickly if you understood it or not.
I guess if there are no questions,
you must understand it,
therefore the pop quiz is irrelevant, right?
Nobody has a question?
Male student: I have a question.
Kevin Ahern: Okay, good.
Male student: What's the Na...
Kevin Ahern: You just saved the whole class right there,
they should thank you.
[class laughing]
Male student: What's the Na doing
while it dissasociates from the OH in a strong base?
Kevin Ahern: So I add NaOH,
what's happening to the Na?
Nothing, just sits there.
Male student: Just hangs?
Kevin Ahern: Just sits there.
Female student: And that goes to say
[inaudible] strong acid [inaudible].
Kevin Ahern: Mhm, the Cl's just going to sit there.
And if you keep adding strong acid and strong base,
strong acid, strong base,
you're making NaCl,
and NaCl, and you've got a very salty solution,
and that's all that happens.
Other good questions.
So, you're saved from a pop quiz.
How about that?
I won't be so nice next time.
Alright, now, what we're starting to understand,
or what I hope to introduce next
is the concept of what is something called a buffer.
Okay?
You used the term up here of what a buffer was,
we need to talk about what a buffer is.
So I'm gonna find a buffer for you
and then we're going to go through some examples.
Alright?
A buffer.
Buffers are absolutely essential.
You'll see why the further we go along.
Alright?
Definition of a buffer,
a buffer is a system that that resists change in pH.
It resists them.
It doesn't prevent them.
It resists them.
Okay?
In the example I gave you,
we dumped a whole bunch of OH in there,
but the pH didn't go up very much
and I'll show you a graph of that in a bit, okay?
The system is acting like a buffer.
It's preventing the pH from going up as much
as it would if the buffer weren't there.
There's a couple of problems
that I've assigned in your book
that will illustrate to you
what a buffer is and how a buffer works.
And you'll compare those,
actually, it's not in the book,
it's actually one of the ones I made up
for you on the system, okay?
Where there are the ones that Kevin
made up on the site, alright?
If you click on those,
you'll see a couple of them that will illustrate
to you how a system that has a buffer differs
from a system that doesn't have a buffer
if you have the same amount of protons.
And you'll see there's a very big difference between the two.
Now, so I've defined a buffer for you.
A buffer is a system that resists change in pH.
Yes, ma'am.
Female student: Does it work the same
whether the buffer starts in the system
or whether you add the buffer later?
Kevin Ahern: Does it matter if you add the buffer
first or if you add the other stuff later.
Turns out it technically doesn't, no.
Good question.
Yes, sir?
Male Student: Approximately, what's the effective
range pH-wise of the average buffer?
Kevin Ahern: Oh this is a very good question
also and thank you for asking that.
Very much.
What's the range of a buffer?
is a buffer an infinite thing?
Can a buffer resist pH change forever?
No, okay.
Buffers have what we call capacity.
We'll see some examples,
in fact one of the problems in your book
actually illustrates capacity to you.
It's going to confuse you when you first
do it 'cause it's not gonna make any sense.
And that should be a clue that something
isn't what you think it is.
But to answer your question.
What's the range of the effectiveness of a buffer?
It's mostly a definition thing.
But effectively, most buffers are good
within one pH unit above or below their pKa.
So for acetic acid,
the effective buffering range,
that is where it's best at resisting change in pH,
is from about 3.76 up to about 5.76.
One pH unit of its pKa.
Either way.
Okay?
And I haven't given you an example of how a buffer works yet,
so I'm gonna do that in a second,
but other questions just in general about buffers?
Okay.
Alright.
I keep needing to look up also,
make sure there are people up there.
Yes?
Male student: Will a buffer work outside of its effective range?
Kevin Ahern: Will a buffer work outside its effective range?
Yes, I mean, will a buffer participate
in accepting and donating protons outside that range?
Yes, but its effect on controlling
the pH will be minimized.
So we won't see as strong a protective
effect if we get outside of that range.
Good question.
Alright.
So how does a buffer work?
Well, let's think back to this buffer that
this system that I just described to you.
I've got a solution that has 500,000 molecules
of Ac- and it has 500,000 molecules of HAc.
Equal numbers of salt and acid.
Let's imagine that I add 10,000 protons to this system.
You can do the math pretty quickly and say,
"well, you're gonna have 501,000 molecules
"of HAc because we're making Hac
"and I have 499,000 of Ac- because I lost Ac-", right?
How much would the pH change?
Well, it turns out it's not going to change very much.
How would I know?
I plug it into the Henderson Hasselbalch equation
and what I would see is that I would have
pH equals 4.76 plus the log of 499,000
divided by 501,000, right?
That's very close to one.
That means that log term is very close to zero.
Right?
The buffer is protecting this.
Where there's excess protons,
the buffer grabs them.
Where's there's excess OH,
the buffer makes protons.
Very, very important concept.
Female student: The buffer, in that example you just gave,
did you give us what your buffer was,
or did you just say you added this much buffer?
Kevin Ahern: So, acetic acid is the buffer system here.
Thank you for asking that also.
Alright.
Weak acid systems make great buffers.
Anything that has a pKa makes
a great buffer in a certain range.
Okay?
Anything that has a pKa makes
a great buffer in a certain range.
Now, in this example I just gave you,
we don't see the pH change very much.
It's a very miniscule change
that happens and that logirithm
of it actually makes it an even smaller change.
If I did this, I would ask you a reasonable
question for me to ask you
on an exam would be,
"is the pH higher than the pKa?
"Or is the pH lower than the pKa?"
Well I don't have a calculator!
You don't have to have a calculator to answer that question.
How would you answer that question?
Male student: It's lower.
Kevin Ahern: It's lower?
Why would you say it's lower?
Male student: It's been acidified and the lower pH.
Kevin Ahern: Okay.
So he says it's been acidified
and it's true that's done that.
But one of the places where students
confuse themselves is which acidification.
I want you thinking mathematically.
Mathematically, why is the pH lower than the pKa.
And it is lower than the pKa, you're right.
Female student: [inaudible]
Kevin Ahern: You're taking the log
of a number that's less than one.
Exactly!
499,000 divided by 501,000
is a number that's less than one.
Just like 499 over 501 is less than one.
The logirithm of a number less than one is a negative number.
So if I have 4.76 plus a negative number,
I have to have the pH lower than 4.76.
Right?
What if it were higher?
What would I have to do to add
to the solution to make it be,
for example, 501,000 molecules of Ac-
and 499,000 molecules of HAc?
What would I have to add to make that happen?
I wouldn't add salt.
I would have to add HCl.
I could add salt,
I could add salt.
But not based on what I just told you.
I didn't change the total amount.
I said 499 and 501.
If I add salt,
I'm gonna have 501 and 501, right?
I'm going to have a different total amount.
When I add a strong acid,
I turn salt into acid and my total stays the same.
One million molecules.
So I add a strong,
base to that to make that happen.
Pull those protons off to switch the HAc into Ac-.
Everybody clear on that?
Okay.
If I told you I had a solution that had
a pH of 4.3 and the system had
a pKa of 4.9, more salt or more acid?
pH 4.3, pKa 4.9.
What does that say about the log term?
What does the log term have to be?
Students: It has to be negative.
Kevin Ahern: It has to be negative.
Negative log term, what do I have to have?
I have to have more acid, right?
Has to be less than one,
which means I have to have more,
and by the way,
HAc only holds for acetic acid,
so to make it general,
we call it A - and HA, okay.
So I have to have more A - than I have HA.
Now these are the kinds of things
you can work completely without a calculator.
You can manipulate that log term in your head.
***.
You got it right there.
You don't have to have a calculator.
It's important for students
to learn how to work problems,
okay, to understand the math of what's there,
not to see the confusion of the numbers.
That's why I don't want you using a calculator.
I want you to think about these things.
I want you to understand them
at a real level and not
a "a-duh-duh-duh-duh-duh-duh-duh-duh" level.
Because if those numbers that you
"duh-duh-duh-duh-duh" in don't have meaning,
then you get garbage out as well.
You have to put the right stuff in.
So what I'm trying to get you
to do is to understand how to get that right stuff in.
On my exam, all you will have to do
is get a solution to the point
where it would go into a calculator.
If you get down to this is the logirithm of 3,
then the logirithm of 3 is the answer and that's it.
You don't have to calculate that, okay?
Make sense?
Okay, um, let's see.
Other questions about that?
I'm going to show you some examples
after if you don't have any questions.
Nobody?
Okay, alright.
Let's see a buffer in action, right?
Yes?
Female student: [inaudible].
to assume that the concentration
of Ac- plus HAc is .1 more?
Kevin Ahern: Not for a buffer, no.
A buffer can have any concentration.
Female student: Okay.
That was the concentration of that specific problem.
Kevin Ahern: Okay.
Okay.
Female Student: Okay.
So just don't always assume.
Kevin Ahern: No, no.
So buffers can have any concentration.
And that's important because buffers,
I can make a buffer as concentrated as I want to.
If I say the word "buffer,"
here's something I want you guys to pop in your heads.
Alright?
When I say the word buffer,
I want you to think of two terms.
Two terms I've been using over and over, alright?
Salt and acid.
That should immediately pop those two up in your head.
When I say buffer,
if I told you I have a buffer that is .1 molar,
the first thing that should pop into your head
is salt plus acid equals .1 molar.
Because when I have a buffer,
I have to have both of those.
I have to have salt and I have to have acid.
Now the actual amounts of those you
may have to calculate, alright,
but when I say buffer,
salt plus acid equals the total amount of a buffer.
Alright?
In this case it was .1 but it could be a variety,
it could be anything I make up.
Yes?
Connie.
Connie: Just to verify,
did you say if the buffer's .1,
then salt plus acid equals 1?
Kevin Ahern: If I say a buffer is .1,
say .1 moles of buffer,
if I said I had .1 moles of buffer,
then salt plus acid would equal .1 moles, that's correct.
Okay?
Makes sense?
Okay, now.
The concentration of a buffer is important.
Let's think about a .1 molar buffer.
Let's say I have a .1 buffer of acetic acid.
My favorite acid, right?
And it's at its pKa value.
Its pH is at its pKa value.
What does that tell me about the concentration
of salt and acid in that?
They're equal and they're equal to what?
.05 each, right?
Half the total.
What if I add, let's say .1 molar Acl
to that in an equal volume.
What's going to happen to that buffer?
Based on what I just told you,
what are you going to see happen?
I add HCl, I'm going to lose salt, right?
I'm going to make acid.
How much salt can I lose?
.05, but I just added twice that amount of HCl.
Uh oh.
When I start doing my subtracting,
I discover I have a negative amount of Ac-.
Can I have a negative amount of Ac-?
No.
Something's wrong here, right?
Something's wrong, Houston.
Okay?
Female student: How much [inaudible]?
Kevin Ahern: What I've done.
I added .1 molar, okay.
What I've done is I've just given you
an example where I've exceeded the capacity of the buffer.
Buffers have limited capacities.
Alright?
They're not infinite.
So one of the reasons we change
the concentration of a buffer,
okay, is so we can change its capacity.
Alright?
Capacity's important.
Alright, so you'll see one of the problems
in the book will actually exceed
the capacity of a buffer.
When you get to it,
I think you'll discover,
you'll figure out what it is.
Yes?
Male student: So regardless of the concentration of the buffer,
the capacity won't change [inaudible].
Kevin Ahern: I'm not sure [inaudible] the question.
Male student: So you have 500,000 or you have
.1 molar versus .05 molar,
the capacity's going to stay the same?
Kevin Ahern: The capacity is defined by the salt
and acid that is there.
So if I more than that of strong acid or strong base,
I'm going to exceed its capacity.
Connie?
Connie: So it's not dependant on the concentration at all?
Kevin Ahern: It is dependent on the concentration.
Absolutely.
Connie: Oh, well, because you said
it's usually within one pH unit of the pKa.
Kevin Ahern: Yep, yep.
Connie: What if you have a really
concentrated [inaudible].
Kevin Ahern: That's a mathematical question
that you're asking me to define non-mathematically.
So come see me,
I'll show you mathematically what we're talking about.
With one pH unit, you're not exceeding capacity.
The one pH unit is going to define
how much I can add to it to to get to that one pH unit.
Right?
So I'm limited by that.
Alright, okay.
So buffers have capacity.
I can't exceed those capacities because if I do,
the pH is going to go boing!
because I no longer have a buffer.
The buffer was providing me that,
that protection against massive change in pH.
When I exceed the buffering capacity,
I don't have a buffer anymore.
It's gone.
The pH is going to go sproing.
Yes, sir?
Male student: Could it be generalized that the capacity
is going to be defined both by concentration
and volume combined of the buffer?
Kevin Ahern: So, the capacity of a buffer is,
so the question that you're asking
is if I know the number of moles total a buffer,
that defines ultimately the capacity
because concentration times volume
gives me that and the answer is yes.
Okay.
Now, I know you're going to have some struggles
with this and I understand that.
Please come see me,
please come see the TAs,
work through the problems, okay?
I can guarantee you there will be help
in getting and understanding
of this bigger picture, okay?
It's important to get that bigger picture.
Gotta keep an eye on the time...
Okay, I promised to show you a buffer plot.
And this very high quality graphic
was drawn by yours truly.
[class laughing]
This was for you guys in freshman chemistry,
did a titration curve, right?
Did you like titration curves in freshman chemistry?
You did!
Okay, good.
Most people don't like them.
Alright, so this shows the relationship
between the pH and the amount of OH I added.
In this case, I started with a solution
that had essentially all acid to start with.
How do I know that?
Well, I had a low pH,
and right down here the low pH.
How would I know a low pH would
have mostly acid to start with?
How would I determine that?
Nobody?
If I said extra credit, then everybody would jump right?
Male student: What was the question?
[laughing]
Kevin Ahern: I didn't say I was giving extra credit,
I just said if I said I was giving extra credit, okay?
The question is how would I know
down here I got most things in the acid form?
Female student: [inaudible]
Kevin Ahern: But I'm not talking about protons.
Protons are not acid.
I'm talking about HA.
How would I know I have most everything in the HA form?
You have a friend.
What is the friend?
The friend is Henderson Hasselbalch equation!
Alright?
If the pH is low, below the pKa,
what happens to that ratio?
More salt, more acid?
Class: More acid.
Kevin Ahern: More acid, right?
Bingo.
Henderson Hasselbalch tells me and it tells me very quickly.
The answers to virtually every question
I'm going to ask you will be rooted in that equation.
They're gonna be rooted there, okay?
You wanna get familiar with that equation.
Now, let's see what happens to the solution.
I start out with the solution,
it's got protons on,
I start adding sodium hydroxide.
What happens to the pH?
The pH rises.
It rises relatively rapidly at first.
Why does it rise relatively rapidly at first?
Male student: It's outside the buffer zone.
Kevin Ahern: It's outside the buffering region.
The buffering region being plus of minus 1.
In this case, the pKa I've got for this is about 2.5.
So I'm below 2.5,
it rises relatively rapidly
and then it starts to level off.
And it's leveling off because it's acting as a buffer.
It's resisting the change.
I'm adding a lot more hydroxide,
but the pH is not going up very much.
Once I started getting away
from that region by more than 1 unit,
I all of a sudden see the pH start to go boing.
Every buffer plot is going to look like this.
This is a visual image of what a buffer is doing.
It's resisting a change in pH at a certain range.
The maximum resistance is right here where pH equals pKa,
and that's another important concept.
Not only is pKa the pH at which the buffer
has equal salt and acid,
it's also the place where there's maximum
resistance to change in pH.
We have maximum buffering capacity.
There's a question here?
Yeah.
Male student: Molecularly, why does it do that?
Why doesn't it just continue on the more you add,
the more it disassociates?
Kevin Ahern: It is associating.
Male student: I know, but what does it plateau?
What doesn't it just continue
the breaking of the bonds?
I don't know why does it pause.
Kevin Ahern: Why does it go up and then flatten?
Male student: Yeah
Kevin Ahern: Okay, well two reasons.
Alright?
One, plug it in mathematically.
Plug in a whole bunch of different values of salt
and acid in that ratio
and you'll see that mathematically,
that's exactly what it does,
so mathematically, that's the answer to your question.
Male student: I'm talking more molecularly.
Like bond-wise.
You know what I mean?
Kevin Ahern: Well you're talking about ionization.
Right?
Okay.
So ionization is happening
because the absence of presence of protons.
It's Lavoisier's equation
with the HAc going to H+ and Ac-.
We put pressure on that equation one way or another.
That favors the ionization.
Okay?
Question?
Male student: Wasn't it Le Chatelier's Principle?
Kevin Ahern: What did I say?
Lavoisier's...
Le Chatelier's Principle!
God, I do that all the time.
It is Le Chatelier's principle.
Sorry.
Lavoisier was the practical inventor of chemistry,
not Le Chatelier.
Yes, but thank you,
it is Le Chatelier's principle.
Male student: Are there ever systems where multiple
buffers that have overlapping ranges
of effect are used where you could
have one say 1.5, 2.5, 3.5?
Kevin Ahern: Yeah.
So can you have multiple buffers in there?
The answer is you can.
We're going to do buffers one at a time.
I figure you've got enough to think about.
But yes, you can.
And multiple buffers do complicate the picture a lot.
We'll see starting on Friday,
I'll be talking about amino acids.
And amino acids have multiple buffering regions.
And so you'll see a flattening,
a flattening, a flattening.
That can happen.
And so that is a simply example of what you're talking about.
Okay, now I promised you guys,
any questions about this?
We've gotten through a good number of things.
I want you to,
the most important message I want you to take across,
get across from this,
is that the ionization is going
to be related to pH and its relationship to the pKa.
pH, the ionization of a substance is related
to the pH of the solution it's in and its pKa value.
The more the pH is above the pKa,
the more the protons will be gone.
The more it's below there,
the more the protons will be on.
And you don't need to memorize that.
Henderson Hasselbalch tells you that.
What I'm saying in words to you
is what Henderson Hasselbalch is telling you.
That's a lot of stuff,
why don't we finish with some fun?
I thought we might celebrate Henderson Hasselbalch
with a song to the tune of "My Country Tis of Thee."
I've never sung in front of a class
before so let's do this.
[professor and class singing]
Lyrics: Henderson Hasselbalch
You put my brain in shock
Oh woe is me
The pKa's can make
Me lie in bed awake
They give me really bad headaches
Oh hear my plea
Sale minus acid ratios
Help keep the pH froze
By buffering
They show tenacity
Compelte audacity
If used within capacity
To maintain things
I know when H's fly
A buffer will defy
Them actively
Those protons cannot waltz
When they get bound to salts
With this the change in pH halts
All praise to thee
Thus now that I've addressed
This topic for the test
I've got know-how
The pH I can say
Equals the pKa
In sum with log of S o'er A
I know it now
Kevin Ahern: Okay, good place to stop.
Thank you
[END]