Determine whether Cd2+ can be separated from Zn2+ by bubbling H2S through a 0.3 M HCl solution that contains 0.005 M Cd2+ and 0.005 M Zn2+. Ksp for CdS is 8x10^-7 and Ksp for ZnS is 3x10^-2.

Answer 1

K for H2S is k1k2 = (H^+)^2(S^-2)/(H2S)
These problems usually say that the solution is saturated with respect to H2S and that is 0.1 M.
So plug in H^+ from HCl and 0.1M for H2S and calculate (S^-2).
Then calculate Qsp for ZnS and CdS.
Qsp = (Zn^+2)(S^-2)
Qsp = (Cd^+2)(S^-2)
Finally, compare Qsp with Ksp of each.
I should point out that neither Ksp you list is correct. CdS is closer to 10^-28 and ZnS closer to 10^-24

Answer 2

My experience in 0.3M HCl is that both CdS and ZnS ppt.