Question

Determine the molar solubility of CuS in 0.25M HNO3. The answer is about 3.0 x 10^-6 M.

Answer 1

CuS(s) <==> Cu2+ + S^2-           Ksp = 1 x 10^-36

S^2- + 2H+ <==> H2S                 1/Ka = 1/9 x 10^-8

adding the two equations,

                 CuS(s) + 2H+ <==> Cu2+ + H2S               K = Ksp/Ka = 1.0 x 10^-29

I                   -          0.25           -            -  

C                  -          -2x              +x         +x

                    -        0.25-2x           x          x

So,

1 x 10^-17 = x^2/(0.25-2x)^2  

6.25 x 10^-19 - 1 x 10^-17x + 4 x 10^-17x^2 = x^2

x^2 + 1 x 10^-17x - 6.25 x 10^-19 = 0

molar solubility = x = 1.58 x 10^-9 M