Calculate Ka of an Acid

Question

A 0.660 M solution of a weak acid (HA) is made. The solution has a pH of 4.81. Calculate the Ka of this acid. Report your answer in scientific notation with 3 sig figs.

How do I calculate the Ka value given pH and M of a solution? I'm getting this question wrong and I don't know why.

Answer 1

HA <==> H⁺ + A^-

Ka = [H⁺][A^-] / [HA]

We know the [HA] since it is given as 0.660 M

We can find the [H⁺] and the [A^-] from the pH: pH = -log [H⁺] and so [H⁺] = 1x10^-4.81 = 1.55x10^-5 M

Ka = (1.55x10^-5)(1.55x10^-5) / 0.660

Ka = 3.64x10^-10