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Studying for a test? Prepare with these 3 lessons on Buffers, titrations, and solubility equilibria.
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Video transskription
- [Voiceover] In this video we're going to be talking about how you make a buffer. There are two main situations where you make a buffer, and for lack of a better way to put it, the first way is when you know you want a buffer with a specific pH, so you're making it on purpose for a specific application, and then the second time that you might make a buffer, especially in chemistry class, is by accident, which is to say that you're doing something else, you're probably a titration, for example, and you just happen to have done a reaction where it ends up with a buffer in your solution. So we're gonna talk a little bit about both of those situations. One equation that I'm going to refer to, and I'm gonna assume that you're already familiar with, is the Henderson-Hasselbalch equation. So that is pH equals the pKa of the weak acid plus the natural log of our weak base, which I will abbreviate as a minus, divided by our weak acid concentration. So if you're interested in the derivation of this, we won't go through that in this particular video, but we do have other videos that talk about it. So let's say we have a specific reaction we wanna run at, say, a pH of four. So we wanna make a pH-four buffer. In that case, we normally make a mixture of a weak acid plus a weak base, and that weak base might either just be a weak base like ammonia, or it might be a salt with the conjugate base of our weak acid. So an example would be acetic acid, CH3COOH, and that's our weak acid, and we might combine that in solution with sodium acetate. So sodium plus CH3COO-minus. And so I'm gonna draw this using the molecular structures too, because I think that makes it a little bit easier to see the relationship between these two structures. We have our acetic acid, which has a CH3 group here, and a carboxylic acid group with the proton, and this proton is acidic, so it can donate that proton, and then you get the acetate. So CH3COO-minus, and then we have a sodium cation. So if we combine our acetate and our sodium acetate salt and solution, what we get is an equilibrium between these two things, and that's how we have something that acts like a buffer. We have our weak acid, our weak acid being acetic acid, and what our weak acid does is it, let's see, let's do this in a different color. What our weak acid does is it soaks up any added weak base. So, sorry, it soaks up any added strong base, so hydroxide ions. So it'll react with your hydroxide ions and keep it from changing the pH as drastically as it would if the weak acid wasn't there. And on the other side of our equilibrium, we have our weak base, which is the conjugate base of our weak acid, and what that does is it soaks up any added H-plus ions, say, if you added some strong acid. So that's one very common scenario where you'll have a buffer. You want a specific buffer with a specific pH, so you might use the Henderson-Hasselbalch equation, and you'll say, "Okay, I want a pH of four," and then we'll look at different salts and their pKa, and then we'll adjust these concentrations to get the pH that we want. The second situation where you might get a buffer, and this one is also really common in chemistry, is by accident. (chuckles) So what this means is you've made a buffer because you combined a weak acid with a strong base. Or another way you can make a buffer, which I will write in parentheses, is you combine a weak base with a strong acid. And this is very common in titrations. So anytime you titrate a weak acid or a weak base, when you titrate it with a strong base or a strong acid, you end up making a buffer for part of your titration. So the example we'll go through here, we're gonna start with the weak acid, acetic acid, like in our other example. So we have this acetic acid, but this time we're going to react it with the strong base sodium hydroxide. So sodium-plus OH-minus is how it dissociates. And so what happens when you react these two things together is you get a neutralization reaction, and this acidic proton, so this acidic proton right here on our acetic acid can react with the hydroxide and it will make H2O. And then what that leaves us with here is our acetate becomes, sorry, our acetic acid becomes acetate. And so you can see that if you react a strong base with a weak acid, which I will label, if you react a weak acid with a strong base, you end up making the conjugate base of your weak acid. And so what this means is that even if you didn't combine your conjugate base with your weak acid up front, like in example one, you actually make it in solution, and so you can still end up making a base. So the key situation where you're going to see this is in a titration. To be more specific, you'll see it in the buffer region. And like I said earlier, this occurs when, when you're titrating a weak acid or a weak base, so a weak acid or base. So you don't make a buffer if you're titrating a strong acid or a strong base. But what this means is that whenever you're doing this kind of reaction where you might be making a buffer as the reaction goes on, that means we can use the Henderson-Hasselbalch equation right here to do calculations about the pH or the concentrations in our solution. So these are the two main situations where you'll have a buffer, and in both of these situations you'll probaby be using the Henderson-Hasselbalch equation to do different kinds of calculations.