A-level Chemistry/OCR (Salters)/Buffer solutions

Calculating the pH of a buffer solution Derivation For any equilibrium the equilibrium constant, K, is defined as Therefore, for the dissociation equilibrium of any acid the acid dissociation constant, Ka, is defined as This equation can be rearranged to make [H+(aq)] the subject: Two assumptions are required: 1 Every A− ion comes from the salt Although this is not quite true, it is a close enough that the pH value we get from the final equation is very close to that found experimentally. It allows us to assume that 2 Every HA molecule remains undissociated Again, despite being slightly inaccurate, this assumption creates the following useful equation The equations in assumptions 1 and 2 allow us to replace [A−(aq)] with [salt] and [HA(aq)] with [acid] as follows. The effect of assumption 1 is that becomes The effect of assumption 2 is that becomes By definition, so