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 dissociates in water:
C6H5COOH ⇌ C6H5COO^- + H^+
The pKa of this reaction is 4.2. In a 0.65 M solution of benzoic acid, what percentage of the molecules are ionized?
Express the percentage numerically using two significant figures.