Finding pH of a weak acid solution

Question

A 0.171M weak acid solution has a pH of 4.27. Find the Ka for this acid.

Answer 1

Let the weak acid be HA

HA <==> H⁺ + A^-

Ka = [H⁺][A^-] / [HA]

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

pH = -log [H⁺]

[H⁺] = 1x10^-4.27

[H⁺] = 5.37x10^-5 = [A^-]

Ka = (5.37x10^-5)(5.37x10^-5) / 0.171

Ka = 2.88x10^-9 / 0.171

Ka = 1.69x10^-8