Question

The cations Hg2 2+, Ag+, and Pb2+ can be precipitated as insoluble chlorides: Hg2Cl2 (Ksp=1.2x10^-18), AgCl (Ksp=1.8x10^-10), PbCl2 (Ksp=1.7x10^-5). If you have a solution containing these three cations, each with a concentration of 0.10 Mol/L, what is the order in which they precipitate as hydroxides?

Answer 1

find solubilities of each; since we cant compare directly Ksp

Ksp Hg2Cl2 = [Hg2+2][Cl-]^2

Assume S to be solubility

then

Ksp Hg2Cl2 = [Hg2+2][Cl-]^2 = (S)(2S)^2 = 4S^3

S = ((Ksp)/4)^(1/3) = ((1.2*10^-18)/4)^(1/3) = 6.694*10^-7 M

Ksp PbCl2 = [Pb+2][Cl-]^2

Ksp PbCl2 = [Pb+2][Cl-]^2 = (S)(2S)^2 = 4S^3

S = ((Ksp)/4)^(1/3) = S = ((1.7*10^-5))/4)^(1/3) = 0.016198059 M = 1.6*10^-2

Ksp = AgCl = [Ag+][Cl-]

S*S = Ksp

S = Sqrt(Ksp = Sqrt(1.8*10^-10) =0.0000134164 = 1.34*10^-5 M

Hg2Cl2 first

AgCl second

PbCl2 third