A-level Chemistry/OCR (Salters)/Weak acids
Calculating the pH of a weak acid solution
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