An unknown weak acid with a concentration of 0.093 M has a pH of 1.80. What is the Ka of the weak acid?

Answer 1

Let HA represent the weak acid.

HA ==> H⁺ + A^-

Ka = [H^+][A^-] / [HA]

From the pH, we can determine [H⁺] and [A^-]...

pH = 1.80

[H⁺] = [A^-] = 1x10^-1.80 = 1.58x10^-2 M

Ka = (1.58x10^-2)(1.58x10^-2) / 0.093 - 1.58x10^-2

Ka = 2.5x10^-4 / 0.0914

Ka = 2.74x10^-3