A-level Chemistry/OCR (Salters)/Weak acids

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Calculating the pH of a weak acid solution[edit]

The pH of a weak acid solution can be calculated approximately using the following formula:


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

Two assumptions are required:

1 The concentrations of H+(aq) and A(aq) are equal, or in symbols:

The reason this is an approximation is that a very slightly higher concentration of H+(aq) exists in reality, due to the autodissociation of water, H2O(l) H+(aq) + A(aq). We neglect this effect since water produces a far lower concentration of H+(aq) than most weak acids. If you were studying an exceptionally weak acid (you won't at A-level), this assumption might begin to cause big problems.

2 The amount of HA at equilibrium is the same as the amount originally added to the solution.

This cannot be quite true, otherwise HA wouldn't be an acid. It is, however, a close numerical approximation to experimental observations of the concentration of HA in most cases.

The effect of assumption 1 is that


The effect of assumption 2 is that


which can be rearranged to give

and therefore

By definition,