The molar solubility of ZnS means that this much ZnS dissociates into Zn(2+) and S(2-) ions. Since 1 mole of ZnS produces on mole each of Zn(2+) and (S2-), the concentration of each ion in the solution is also 1.6 * 10^-12 M. The Ksp expression for ZnS is simply: Ksp = [Zn(2+)][S(2- )]. So, Ksp = (1.6*10^-12)^2 = 2.56 * 10^-24 M^2. Solution The molar solubility of ZnS means that this much ZnS dissociates into Zn(2+) and S(2-) ions. Since 1 mole of ZnS produces on mole each of Zn(2+) and (S2-), the concentration of each ion in the solution is also 1.6 * 10^-12 M. The Ksp expression for ZnS is simply: Ksp = [Zn(2+)][S(2- )]. So, Ksp = (1.6*10^-12)^2 = 2.56 * 10^-24 M^2..