Problem: Determine the percent ionization of a 0.250M solution of benzoic acid.

We’re being asked to calculate the percent ionization of a 0.250 M solution of benzoic acid.

Recall that the percent ionization is given by:

$\boxed{\mathbf{\%}\mathbf{ }\mathbf{ionization}\mathbf{=}\frac{\mathbf{\left[}{\mathbf{H}}^{\mathbf{+}}\mathbf{\right]}}{\mathbf{[}\mathbf{HA}{\mathbf{]}}_{\mathbf{initial}}}\mathbf{\times }\mathbf{100}}$

We know the initial concentration of benzoic acid, 0.250 M. We just need to find the concentration of H⁺ at equilibrium.

Since benzoic acid has a low K_a value, it’s a weak acid. 

Remember that weak acids partially dissociate in water and that acids donate H⁺ to the base (water in this case). 

The dissociation of benzoic acid is as follows:

C₆H₅COOH(aq) + H₂O(l) ⇌ H₃O⁺(aq) + C₆H₅COO^–(aq); K_a = 6.3 x 10^-5