Calculate Ka for the acid

Question

Enough of a monoprotic acid is dissolved in water to produce a 1.821.82 M solution. The pH of the resulting solution is 2.972.97. Calculate the K_a for the acid.

Answer 1

Let the weak, monoprotic acid be represented by HA.

HA <==> H⁺ + A^-

Ka = [H⁺][A^-] / [HA]

From the pH of 2.97, we can calculate the [H+], which is also the [A-]

[H⁺] = 1x10^-2.97 = 1.08x10^-3 M

Ka = (1.08x10^-3)(1.08x10^-3) / 1.82

Ka = 6.41x10^-7