Question

1. A solution contains 1.48×10-2 M potassium sulfide and 5.38×10-3 M potassium hydroxide. Solid copper(II) nitrate is added slowly to this mixture.

What is the concentration of sulfide ion when hydroxide ion begins to precipitate? [sulfide] = ? M

2. A solution contains 1.21×10^-2 M potassium sulfide and 1.08×10^-2 M potassium carbonate.

Solid nickel(II) nitrate is added slowly to this mixture.

What is the concentration of sulfide ion when carbonate ion begins to precipitate?
[sulfide] = ? M

Answer 1

1)

Ksp of Cu(OH)2 = 1.6 x 10^-19

Ksp of CuS = 7.9 x 10^-37

Cu(OH)2   ---------------> Cu2+   +   2 OH-

                                             S          5.38 x 10^-3

Ksp = [Cu2+][OH-]^2

1.6 x 10^-19 = [Cu2+] x (5.38 x 10^-3)^2

[Cu2+] = 5.53 x 10^-15 M

CuS   ----------------> Cu2+ + S2-

Ksp = [Cu2+] [S2-]

7.9 x 10^-37 = ( 5.53 x 10^-15 ) [S2-]

[S2-] = 1.43 x 10^-22 M

[sulfide] = 1.43 x 10^-22 M

2)

NiCO3 ----------->   Ni2+ +   CO32-

Ksp = [Ni2+] [CO32-]

6.6 x 10^-9 = [Ni2+] [1.08 x 10^-2]

[Ni2+] = 6.11 x 10^-7 M

NiS ------------> Ni2+ +   S2-

Ksp = [Ni2+][S2-]

3.0 x 10^-21 = ( 6.11 x 10^-7) [S2-]

[S2-] = 4.91 x 10^-15 M

[sulfide] = 4.91 x 10^-15 M