Finding pH of a buffer solution

Question

Suppose you were asked to calculate the pH of a buffer solution containing 0.30 M acetate and 0.20 M acetic acid. If you were to find the pH using the ICE table, how do you know what equation to write at the top of the ice table?

Now most people write the acid dissociation equation at the top of the ice table such as this:

CH3COOH + H2O ➡️ CH3COO- + H3O+

with this equation you would need to use Ka

But I was wondering if you could also rewrite the equation with the conjugate base reacting with water like this:

CH3COO- + H20 ➡️ CH3COOH + OH-

With this with you would the Kb .

So my question is it possible to use the second equation for the ice table as well and if so why?

Answer 1

YES. But maybe not for the reason you think. If you are only interested in the pH of the buffer before any additions of acid or base, then either way will work fine. After all KaKb = 1x10-14 so using Ka or Kb will be fine and you'll arrive at the same answer. It's similar to using two different forms of the Henderson Hasselbalch equation.

For acid buffers you can use pH = pKa + log [salt] / [acid]

For basic buffers you can use pOH = pKb + log [salt] / [base]

Again, this is because of the relationship between Ka and Kb and pKa and pKb and pH and pOH.

Now, if you are interested in the pH AFTER the addition of acid or base, then it is a lot easier to use the acid dissociation equation in the ICE table if you are adding a base, and the conjugate base dissociation in the ICE table if you are adding an acid.

Does this make sense?