Solubility Product 2
Concentrated NaF (aq) is slowly added to a solution that is 0.010 M CaCl2 and 0.010M MgCl2. Ksp of CaF2 and MgF2 are 5.3*10^-9 and 3.7*10^-8.
(a) What is the first precipitate to form?
(b) What [F-] is needed to start precipitating the second precipitate?
(c) Find the percentage of Ca2+ remaining in the solution when the second precipitate just starts precipitation?
- 胡雪8°Lv 71 decade agoFavorite Answer
Both CaF2 and MgF2 precipitates are in the same form of MX2, and CaF2 has a smaller Ksp.
Hence, CaF2 is the first to precipitate from solution.
Assume that the change in volume is negligible when NaF is added, because NaF is much concentrated than Ca^2+ and Mg^2+.
[F^-] when MgF2 begins to precipitate
= √[(3.7 x 10^-8) / 0.01]
= 1.92 x 10^-3 M
[Ca^2+] when MgF2 begins to precipitate
= (5.3 x 10^-9) / (1.92 x 10^‑3)^2
= 1.44 x 10^-3 M
% of Ca^2+ remaining in the solution when MgF2 begins to precipitate
= [(1.44 x 10^-3) / 0.01] x 100%
= 14.4%Source(s): 胡雪八