You are an analytical chemist and are working out a procedure for precipitating lead from a solution of lead nitrate. You know that lead(II) iodide is insoluble, so you decide to use potassium iodide solution. Will the lead precipitate from the solutions that you have available? Will lead(II) iodide precipitate if you mix equal volumes of 0.2 m Pb(NO₃)₂(aq) and KI(aq) at 25°C?

ANTICIPATE Because the concentrations of the Pb²⁺ and I^– ions are high and the solubility product of PbI₂ is low, you should expect precipitation to occur.

PLAN First calculate the new molar concentrations of the ions in the mixed solution, before reaction occurs. Then compare the value of the reaction quotient, Q_sp to K_sp for PbI₂. Remember that the final volume of the solution is the total volume of the mixture, so the molar concentrations to use in Q_sp must be adjusted accordingly.

What should you assume? Assume that the lead(II) iodide is fully dissociated (in the sense that its ions have separated) in aqueous solution and that activities can be replaced by molar concentrations.