Benzoic acid is a weak acid and it dissociates proton and benzoate ions after its reaction with water.

benzoic acid dissociate equilibrium
dissociate equilibrium

Ka value
Ka value

benzoic acid pka and ph

relationship between pka and ph

Above is the dissociation equation of benzoic acid and the relationship between PKa and PH, HA stands for benzoic acid, A- stands for benzoate ion.

Step 1: Suppose the concentration of total HA is 0.001 mol/L.

There can be more benzoic acid soluble than 0.001 mol/L if at the temperature 25°C. To be exactly, the mol concentration would be n=solubility/molecular formula, that’s say n=3.44/122.123 = 0.028mol/L. Here using 0.001 mol/L just to make it easy.

Step 2: Write the ionization equilibrium equation as follows:


HA(aq)+H2O(l)⇌A−(aq)+H3O+(aq)

[H3O+]  is the Hydronium concentration
[A−] is the conjugate base concentration
[HA] is the benzoic acid concentration
The concentration of [A−] and [H3O+] is the same. And we suppose the concentration is X. And the final concentration of HA in solution is 0.001 – X.

Step 3: Then the equation can be written X*X/0.001-X = Ka.

As 0.001-X can be similar to 0.001, so we can get the X value 10-3.6, so the PH value is 3.6.