ethanoic acid is a weak acid it dissociates, very SMALL of it ionises though... CH3COOH <----->CH3COO- + H+
Sodium ethanoate is an ionic salt so in solution it will be in the form of ions CH3COONa <------> CH3COO- + Na+
So the buffer solution contains ethanoate ions which could be assumed to be all from the ionic salt since ethanoic acid is a weak acid and very little of it dissociates, a lot of undissociated ethanoic acid and enough hydrogen ions to make the buffer solution acid.
When a little acid is added, it will react with the ethanoate ions to form ethanoic acid:
CH3COO- + H+ -----> CH3COOH
When a little alkali is added, it will react with the undissociated ehanoic acid to form ethanoate ions and water:
CH3COOH + OH- ----> CH3COOH + H20
when you calculate the pH of the buffer solution (using Ka of ethanoic acid and the above stated assumptions) prior to and after adding acid or alkali it will be the same or vary with a very small amount. Hope this helps...
