Explain why there is only a small change in pH when a small amount of acid is added to this buffer solution.

Answer 1

What buffer system? If it is the ethanoic acid/ethanoate buffer system (or any buffer system for that matter), the added acid reacts with the conjugate base (salt) to produce more of the WEAK acid. Since the acid being formed is a WEAK acid, it essentially acts to remove the added H+. This is seen in the following equation:

Addition of H+ to buffer:

A^- + H⁺ ==> HA (a weak acid)

For the ethanoic acid/ethanoate buffer, it would be the following:

CH3COO^- + H⁺ ==> CH3COOH (a weak acid)