Buffers on a System

Question

What are the effects on pH by adding
a. 0.0060 mol of HCl
b. 0.0060 mol of NaOH to 0.300L of a buffer system that is 0.250M acetic acid and 0.560M sodium acetate?
First find the pH of the buffer system only.

Answer 1

To answer this you should be familiar with the Henderson Hasselbalch equation. pH = pKa + log [conj.base]/[acid]

So, first of all you’ll need the pKa of acetic acid. Look it up. Then plug the values in to the HH equation to find the pH of the buffer system prior to addition of any acid or base. 

 

To find pH after addition of HCl or NaOH, refer to the following:

HCl + Ac^- ==> HAc + Cl^-

NaOH + HAc ==> NaAc + H2O

From adding 0.006 mol HCl or NaOH will use up 0.006 mol of Ac^- or of HAc, respectively. Use that info, and the concentration of remaining acid or conjugate base and the HH equation to find resulting pH.