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

Calculating the pH of a buffer solution edit


Derivation edit

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




The effect of assumption 2 is that




By definition,