What is the Ka of the weak acid?

Question

An unknown weak acid with a concentration of 0.260 M has a pH of 5.600. What is the Ka of the weak acid?

Answer 1

Let the unknown weak acid be HA

HA <==> H⁺ + A^-

Ka = [H⁺][A^-]/[HA]

From the pH, we can find [H⁺] which is also the [A^-]

[H⁺] = 1x10^-5.600 = 2.5x10^-6 M

Ka = (2.5x10^-6)(2.5x10^-6) / 0.260

Ka = 6.25x10^-12/0.260

Ka = 2.4x10^-11