Industrial wastewater contains 0.015 M Pb(NO3)2 and 0.015 M Cd(NO3)2. To remove lead(II) ions selectively, engineers add aqueous Na2S, introducing sulfide ion into the solution. The solubility‐product constants at 25 °C are Ksp(PbS)=8.0×10−28 and Ksp(CdS)=1.0×10−27. Assume the added volume of Na2S solution is negligible.
(a) Derive an expression for the minimum sulfide ion concentration required to reduce the equilibrium concentration of Pb2+ to 1.0×10−6 M.
(b) Calculate this minimum [S2−].
(c) Using the [S2−] obtained in part (b), determine the ionic product Q for CdS and predict whether CdS will precipitate under these conditions. Show your reasoning.
(d) Discuss the effectiveness of sulfide addition as a selective precipitation method for separating Pb2+ from Cd2+ in this wastewater. Include the balanced net ionic equation for any precipitation that occurs.