Question

Benzoic acid is a weak acid that has antimicrobial properties. Its sodium salt, sodium benzoate, is a preservative found in foods, medications, and personal hygiene products. Benzoic acid ionizes in water: C6H5COOH⇌C6H5COO−+H+

The pKa of this reaction is 4.2. In a 0.61 M solution of benzoic acid, what percentage of the molecules are ionized?

Answer 1

pKa of benzoic acid = 4.2

Ka = 10^-pKa = 10^-4.2

      = 6.309 x 10^-5

concentration C = 0.61 M

[H+] = sqrt (Ka x C)

        = sqrt (6.309 x 10^-5 x 0.61)

        = 6.203 x 10^-3 M

% ionization = [H+] / C ) x 100

                    = (6.203 x 10^-3 / 0.61) x 100

% ionization = 1.02 %