Question

I have a 1L solution of 0.02M sodium acetate and 0.02M acetic acid. What is the pH after the addition of 0.05 moles of hydrochloric acid?

I know this is a buffer question but I have no idea how I am supposed to go about getting the answer... I know Ka values for acetic acid, but I still can't get anywhere.

Answer 1

What happens in a buffered solution when HCl is added? The acetate ion combines with H of the strong acid to form acetic acid, thus making a weak acid out of a strong acid. If we let Ac^- stand for acetate and HAc acetic acid, the equation you have is
Ac^- + H^+ ==> HAc
So look at the moles H^+ added from HCl. The moles HAc will be increased by the moles H^+ added (from the HCl) and the moles of Ac^- will be decreased by that same amount. Then plug in these new moles for base and acid in the Henderson-Hasselbalch equation and you have it.