20.4 The pH of weak acids

For monobasic acid, HA in aqueous solution

• a strong acid HA completely dissociates

• a weak acid HA partially dissociates

[H+(aq)] depends on

• the concentration of the acid [HA(aq)]

• the acid dissociation constant Ka.

When HA molecules dissociate, H+(aq) and A-(aq) ions are formed in equal quantities.

Approximation 1

• HA dissociates to product equilibrium concentrations of H+(aq) and A-(aq) that are equal. there will also be a very small concentration of H+ (aq) from the dissociation of water but this will be extremely small and can be neglected compared with the H+ concentration from the acid.

Approximation 2

The equilibrium concentration of HA is smaller than the undissociated concentration.

As the dissociation of weak acids is small, you can assume that [HA(aq)] start » [H+(aq)] and you can neglect any decrease in the concentration of HA from dissociation.

[HA(aq) eqm = [HA(aq) start

Simplifying the Ka expression

By applying the two approximations, the Ka expression can be greatly simplified.

Provided that two of [H+(aq)] (or pH), Ka and [HA(aq] are known, you can always calculate the third quantity.

Calculating pH

To calculate pH using the Ka expression above, you first need to make [H+(aq)] the subject of the equation.

Determination of Ka

Experimentally the Ka for a weak acid can be determined by

• preparing a standard solution of the weak acid of known concentration

• measuring the pH of the standard solution using a pH meter.

Approximations in calculations involving weak acids

Approximation 1

The first approximation assumes that the dissociation of water is negligible.

Approximation 2

The second approximation assumes that the concentration of acid is much greater than the H+ concentration at equilibrium.