The acid dissociation constant or Ka is a value used to measure the strength of a specific acid in solution. For a general dissociation of an acid solution,
HA = H+ + A-
we express Ka as follows:
Ka = [H+] [A-] / [HA]
Where the terms represents the concentrations of the acid and the ions. Assuming that the weak acid in the problem is HA, we first calculate for the concentration of H+ from the pH.
pH = - log [H+]
3.25 = - log [H+]
[H+] = 0.0005623 M
By the ICE table, we can calculate the equilibrium concentrations,
HA = H+ + A-
I 0.175 0 0
C -x +x +x
--------------------------------------------------
E .174438 0.0005623 0.0005623
Ka = (0.0005623) (0.0005623) / .174438
Ka = 1.81x10^-6