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Henderson-Hasselbalch Equation Example 4

Jules Bruno
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Hey, guys, In this new video, we're gonna take a look and see what's the connection between an ideal buffer and Ph. So the question says which weak acid conjugate based combination would be ideal to form a buffer with a pH of 4.74 Now, when we use the word ideal Ah, buffer his idea when the weak acid is equal to the conjugate base and we know we're dealing with the buffer because we've been talking about it pH equals p K plus log of conjugate base over weak acid. Now, if both of those are equal to each other, then this simply becomes log of one. And if you plug log of one into your calculator, you'll get back zero. So basically all of this gets canceled out. So an ideal buffer pH equals p K. And we're gonna realize here is that PK a just simply means negative log of K. Now, if we wanna isolate RK by itself, we're gonna divide out the negative one here to get rid of this sign here. So negative pH equals log off k a toe. Isolate Jess K. We're gonna divide both sides by log. So K equals 10 to the negative pH. Now, this is the equation that you would use for a question like this. If they ever ask you which one of these buffers is best at this pH, you just simply say K equals 10 to the negative. PH. Your professors don't talk about this in lecture, and it's not really in a book. It's just us deriving this formula based on the relationships that we see so again, any time they ask you what's the best buffer at this particular pH? Simply use that equation Plug in the pH, which is 4.74 though on the buffer that gives us the closest to this answer is going to be our answer. So when we do this, it gives us 1.8 times 10 to the negative five. Here. These two are negative five. The one that's closer to that number is C acetic acid and sodium acetate. So again, anytime they ask you what's the best buffer at this pH. Just simply use that equation the K value get look to see which one matches up closest to it, and that will be your answer