Metal sulfides are prepared by bubbling hydrogen sulfide gas, H₂S, through an aqueous solution containing the metal ion and then adjusting the pH. When the solution is saturated with H₂S, which occurs when [H₂S] = 0.100 M, the following equilibrium is established.

H₂S+2H₂O → S²⁻ +2H₃O⁺ Ksp=1.1 x 10⁻²⁰

An aqueous solution containing 0.0437 M Sn₂ and 0.0437 M Ag is saturated with H₂S. Above what concentration of S²⁻ will SnS precipitate? The Ksp of SnS is 3.2 × 10⁻²⁸. [S⁻ =?]