A chemist prepares 200.0 mL of an aqueous solution that is initially 0.020 M in Ca2+. The chemist then adds solid NaF(s) in three equal portions, allowing each portion to dissolve completely before adding the next. Each portion contains 0.0030 mol of NaF.
Ksp[CaF2] = 1.5 × 10−10 at 25 °C. Assume solution volume remains effectively constant at 200.0 mL throughout the experiment.
(a) Calculate the concentration of Ca2+ in the original solution before any NaF is added.
(b) After the first portion of NaF dissolves, calculate the reaction quotient Q1 for the potential formation of CaF2 and state whether precipitation occurs. Show all work.
(c) The chemist records whether a solid appears after each addition. Repeat the calculation of the reaction quotient for the second (Q2) and third (Q3) additions, and predict after which additions (if any) CaF2(s) precipitates. Justify each prediction quantitatively.
(d) For the first addition that results in precipitation, determine the concentration of F− that remains in solution at equilibrium after precipitation is complete. Clearly outline any assumptions made.