Question

Which metal sulfides can be precipitated from a solution that is 0.01 M in Mn2+, Zn2+, Pb2+ and Cu2+ and 0.10 M in H2S at a pH of 1.0? Explain.

Answer 1

We have,

MnS      Ksp = 3 x 10^-11

ZnS        Ksp = 2 x 10^-25

PbS        Ksp = 3 x 10^-28

CuS        Ksp = 8 x 10^-37

Precipitation of the ionic compound will occur if the initial ion product is greater than the Ksp

H₂S                     2H⁺ + S^2-     Ka = Ka₁ x Ka₂ = 1.25 x 10^-21

(0.1-x) ........................2x ......x

Ka = [H⁺]² [S^2-] / [H₂S]

1.25 x 10^-21 = 0.1² (x) / 0.10

x = 1.25 x 10^-20

[S^2-] = 1.25 x 10^-20

Now,

Q = [M²⁺] [S^2-] = [0.01][ 1.25 x 10^-20] = 1.25 x 10^-22

Thus Ksp for MnS > Q and will not precipitate. PbS, ZnS and CuS will precipitate