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 0.57 M
solution of benzoic acid, what percentage of the molecules are
ionized?
Express your answer to two significant figures and include the
appropriate units.