Help Finding Dissociation Constant.?

Question

A 0.500M solution of a weak acid, HX, is only partially ionized. The [H+] was found to be 4.02 x 10^-3M. Find the dissociation constant for this acid.

Answer 1

HX <----> H+ + X-

[H+] = [X-]

[HX] = 0.500 - 4.02 x 10^-3 = 0.496 M

Ka = ( 4.02 x 10^-3)^2 / 0.496 =3.26 x 10^-5