What is the pH of a 0.40 mol L^-1 H2Se solution that has the stepwise dissociation constants

Question

Ka1 = 1.3 × 10^-4 and Ka2 = 1.0 x 10^-11 ?

Thanks in advance

Answer 1

First, since the k2 is so small, ignore it. The pH will be determined by the dissociation of k2 only.

..........H2Se ==> H^+ + HSe^-
I.........0.4......0......0
C..........-x......x......x
E......0.4-x.......x......x

Substitue the E line into k1 expression and solve for x = (H^+), then convert to pH.