What happens to the pH of a buffer solution after you add a small amount of strong acid HCL, well, because you're adding an acid, the pH is going to go down, but not by as much as you might think, especially considering it's a strong acid because buffers do resist changes in pH. Now, the equation we use to calculate the pH of buffers is usually the henderson-hasselbalch equation. pH equals PKA, plus the log of base over acid. Now, lots of people will use the formula and concentration terms for base over. Acid, but the secret of the henderson-hasselbalch equation is that you can also use the number of moles of base and acid in this equation as well we're going to start with 100 milliliters of a buffer, that's, 0.1, molar, acetic acid and 0.1 molar of its conjugate base.
I chose sodium acetate, but it could have been potassium acetate or anything else. Now, acetic acid has a PKA of 4.75 4.76 is sometimes listed, and we are going to need to know the number of moles of base and acid. Now we are starting with. The same concentration of both base and acid. The number of moles is concentration times volume. The concentration is 0.1 moles per liter.
And we have 0.1 liters of it as well, which means that we're starting with .01 moles of both base and acid 0.01 0.01 now, that's a number of moles. Remember now here comes the change that we're about to make when we add HCL we're going to add in this case, one milliliter of one molar HCL. The number of moles that that represents is one times point zero. Zero, one it's. Zero point zero, zero, one, moles of the acid that we're, adding you are going to need the number of moles for this equation and that's. Because this is the number of moles of acid that we are adding. What is the effect of that on the acid base equilibrium here?
Well that is going to take away from some base. And it is going to contribute the same amount to the amount of acid does that make sense you have your starting amount of base your starting amount of acid. And when you add some strong acid, You take away from the base because you're neutralizing it, and you're creating the conjugate acid in the process. So the base concentration or number of moles goes down while the acid concentration or number of moles goes up. So how much is this pH perturbed by well by the way? Because I started with the same concentration of acid and base when I started before, I added the HCL, my pH equaled, the PKA, it was 4.75.
And here I'm actually going to need my calculator. I've got to do 0.009 over 0.011 let's. Go see what happens when I do that 0.009 divided by 0.011 gives me 0.8, and I'm going to take the logarithm of that gives me a negative answer. Now that makes sense to me because I increased the denominator here, and I decreased the numerator. So my pH is 4.75, minus 0.09 I'm, just rounding that to two decimal places.
And when I combine those two terms, I end up with 4.66. So despite the fact that I added an admittedly small amount of a strong acid, my pH didn't even change by 0.1, the effect of adding a. Strong acid to a buffer is usually that the pH goes down, because you are adding acid, but not by much, of course, if you add enough, strong, acid, you'll, overwhelm the buffer entirely.
And then at some point, you might not even have the conjugate base in solution. And then you have a buffer at all. But as long as you have some of both the solution should be a little resistant to pH changes. Thanks for being with me and best of luck.