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

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Calculating the pH of a buffer solution[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,