This question would be a 5/6 mark question- marks are displayed throughout
You always start by defining what a buffer solution is: A buffer solution is a system that minimises a change in pH on the addition of a small amount of acid or a base. (1) Then you would write out the equilibrium of the buffer solution, for example, CH3COOH (=) CH3COO- + H+ (2). The conjugate base in this solution is CH3COO- and the weak acid is the CH3COOH.
Then you would explain the effect of adding a small amount of acid to the buffer solution:
The conjugate base (CH3COO-) removes the added acid. When an acid is added the [H+] increases and these H+ react with the conjugate base (3). The equilibrium shifts to the left (4), removing most of the H+ ions.
The weak acid (CH3COOH) removes the added alkali. When an alkali is added the [OH-] increases. The small amount of H+ present reacts with the OH- to form H2O (5). The weak acid dissociates, causing the equilibrium to shift to the right (6) to restore most of the H+ ions.
I would the attach an example exam question which they can complete.