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 0.75 M solution of benzoic acid, what percentage of the molecules are ionized? In what unit?