How do I solve this AP Chemistry problem on CaF2?

Question

If a constant  [F-]=0.003M is maintained in a solution  in which the initial [Ca2+]=0.038M, what percentage of Ca2+ will remain in  solution after CaF2 precipitates? What  [F-] should be maintained to ensure that only 3.4% of the Ca2+ remains unprecipitated?

Answer 1

Hi John F.,

This is a fairly routine problem. Set up the equilibrium constant expression for the dissolution of CaF₂ , then solve for the unknown(s) successively for the two parts. You'll have to convert the initial Ca level to a final level for part (b) by multiplying the initial by the decimal form of the desired end percentage.

So:

[CaF2] = [Ca+2][F-]² and so on.