Chemistry: Ka Calculate from pH

Question

In the laboratory, a general chemistry student measured the pH of a 0.337 M aqueous solution of benzoic acid, C₆H₅COOH to be 2.324.

Use the information she obtained to determine the K_a for this acid.

K_a(experiment) = 

Answer 1

C6H5COOH ==> C6H5COO^- + H⁺

Ka = [ C6H5COO^-][H⁺] / [C6H5COOH]

From the pH, we can find the [H+] and [C6H5COO^- ] since they are the same

[H⁺] =1x10^-2.324 = 5.83x10^-3 M (this is less than 2% of the 0.337 M C6H5COOH so we don't have to subtract it from the original concentration.

Ka = (5.83x10^-3 )(5.83x10^-3 ) / 0.337

Ka = 1.0x10^-4