Enough of a monoprotic weak acid is dissolved in water to produce a 0.0125 M solution. The pH of the resulting solution is 2.35 . Calculate the Ka for the acid.

Answer 1

Let the monoprotic weak acid be represented by HA

HA <==> H⁺ + A^-

Ka = [H⁺][A^-] / [HA]

[H⁺] = [A^-] = 1x10^-2.35 = 2.24x10^-3

Ka = (2.24x10^-3)(2.24x10^-3) / 0.0125 - 2.24x10^-3

Ka = 5.02x10^-6 / 0.01026

Ka = 4.89x10^-4