Question

A solution contains 0.0170 M Pb^2+ (aq) and 0.0170 M Sr^2+ (aq). If we add SO4^2- (aq), what will be the concentration of Pb^2+ (aq) when SrSO4(s) begins to precipitate?

[Pb^2+] =

Answer 1

Ksp of PbSO4 = 2.53×10^–8

Ksp of SrSO4 = 3.44×10^–7

SrSO4 <----------------> Sr+2   + SO42-

                0.0170   S

Ksp = [Sr+2][SO42-]

3.44×10^–7 = 0.0170 x [SO42-]

[SO42-] = 2.02 x 10^-5 M

PbSO4 <----------------> Pb+2 + SO42-

                                         S   2.02 x 10^-5

Ksp = [Pb+2][SO42-]

2.53×10^–8 =[Pb+2] x [2.02 x 10^-5]

[Pb+2] = 1.25 x 10^-3 M