None. Look at the definition of percent ionization. Think, how you can use it to calculate concentrations of HA and H+/A-. Once you will get through these calculations, all you need is to put these values into Ka definition.
Ok, I think I'm following you. So, if I have a 0.10 molal solution of a weak acid, and I experimentally find that it has a percent ionization of 5%, I would multiply 0.10 by 0.05, which would give me a concentration of 5.0 x 10
-3 molal for both H
+ and A
-, correct? And I would subtract 5.0 x 10
-3 from 0.10, giving me a concentration of 0.095 molal of HA.
So, would Ka = ([5.0 x 10
-3] X [5.0 x 10
-3])/ [0.095] = 2.6 x 10
-4or did I do that completely wrong?