Solubility Product 2

2010-04-04 7:54 am

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?

回答 (1)

胡雪8°

2010-04-04 10:03 am

✔ 最佳答案

(a)
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.

(b)
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

(c)
[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%

參考: 胡雪八

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