Question

At what molar [H3O+] will PbS just start to precipitate from a solution consisting of 1.592e-6 M Pb(NO3)2 in 0.100 M H2S (satd)? H2S is bubbled through the solution to keep its concentration constant.

PbS(s) ↔ Pb2+ (aq) + S2- (aq) Ksp = 2.51e-27

H2S(aq) ↔ 2H+ (aq) + S2- (aq) K = 1.1x10-20

Answer 1

when ionic product just cross the solubility product then precipitation start. here concentration of Pb ²⁺ 1.592e-6 M, optium S2- concentration require = Ksp / [Pb 2+]

                                                 =  2.51e-27 / 1.592e-6 M

                                                  = 1.1 * 10^-9M

now dissocation os H₂S

                H2S(aq) ↔ 2H+ (aq) + S2- (aq) K = 1.1x10-20

K = [H+ (aq)]² [S2- (aq)] / [H2S(aq)]

[H+ (aq)]2 = ( 1.1x10-20 *0 .1 ) / 1.1 * 10^-9 = 10^-12

[H3O+] = 10^-6 M