Question

A solution of 0.049 M NiBr2 is saturated with H2S. ([H2S] = 0.087 M. Assume saturation is maintained by bubbling H2S through the solution for the duration of the experiment.) NiS(s) equilibrium reaction arrow Ni2+(aq) + S2−(aq)   Ksp = 3.0 ✕ 10−19 H2S(aq) + 2 H2O(l) equilibrium reaction arrow 2 H3O+(aq) + S2−(aq)   K = 8.9 ✕ 10−27 What is the minimum pH at which NiS begins to precipitate?

Answer 1

for the reaction

NiS(s) <------------> Ni2+(aq) + S2−(aq)  

Ksp = 3.0 ✕ 10−19

and [NiBr2] = [Ni2+] = 0.049 M

therefore

Ksp = [Ni2+] [S2-] = 3.0 ✕ 10−19

or

[S2-] = 3.0 ✕ 10−19 / 0.049

= 6.12 x 10^-18 M

and for the reaction

H2S(aq) + 2 H2O(l) <------------> 2 H3O+(aq) + S2−(aq)  

K = 8.9 ✕ 10−27 = [H3O+]^2 [S2-] / [H2S]

or

[H3O+]^2 =  (8.9 ✕ 10−27 x 0.087M )/ 6.12 x 10^-18 M

= 1.27 x 10^-10

or

[H3O+] = 1.12 x 10^-5 M

and

pH = - log( 1.12 x 10^-5 M )

= 4.95