A solution is 0.0100 M in each of the metal ions in the following table:

Question

Metal sulfide, Kspa

FeS, Kspa = 6 x 10^2

NiS, Kspa = 8 x 10^(-1)

PbS, Kspa = 3 x 10^(-7)

CuS, Kspa = 6 x 10^(-16)

HCl is added to the solution so that the H3O concentration is 0.300 M. H2S gas is bubbled through the solution to attain a H2S concentration of 0.100 M. Predict which of the sulfides precipitate under the given conditions

Answer 1

In saturated solution the ion concentrations satisfy the equilibrium

Ksp = [X²⁺] ∙[S²⁻]

with X = Fe, Ni, Pb or Cu

The sulfide is set to 0.1M. So the maximum metal ion concentration, for which no sulfide precipitates  is:

[X²⁺] = Ksp/[S²⁻]

If this concentration is lower than the 0.01M, which are present in solution, the metal sulfide precipitates.

[Fe²⁺] = 6×10²/0.1 = 6×10³M  > 0.01M => no precipitate

[Ni²⁺] = 8×10⁻¹ / 0.1 = 8M   > 0.01M => no precipitate

[Pb²⁺] = 6×10⁻⁷/0.1 = 6×10⁻⁶M < PbS preciptates

[Cu²⁺] = 6×10⁻¹⁶/0.1 = 6×10⁻¹⁵M < 0.01M => CuS precipitates