A 0.175 M weak acid solution has a pH of 3.25. Find Ka for the acid.?

Let HA be the weak acidHA ==> H^+ + A^-

Ka = [H+][A-]/[HA]

Since pH = 3.25, this means [H+] = 1x10^-3.5 = 3.16x10^-4 = [A-] also.

Ka = (3.16x10^-4)(3.16x10^-4)/0.175

Ka = 5.71x10^-7