Question

An aqueous solution containing 0.0287 M Zn2+ and 0.0287 M Ag+ is saturated with H2S. Above what concentration of S2− will ZnS precipitate? The 𝐾sp of ZnS is 2.0×10−25.

Answer 1

Ksp = 2.0E-25 = (Zn^2+)(S^2-)

(S^2-) = 2.0E-25/0.0287 = about 7 x 10^-24 M
I don't know why the (Ag^+) is given in the problem but you should note that Ag2S will ppt far quicker than ZnS and almost all of the Ag2S in solution will ppt before (S^2-) becomes large enough to ppt ZnS.