Question 17.SE.10: Write the expression for the solubility-product constant for...

Analyze We are asked to write an equilibrium expression for the process by which CaF$_2$ dissolves in water.

Plan We apply the general rules for writing an equilibrium-constant expression, excluding the solid reactant from the expression. We assume that the compound dissociates completely into its component ions:

$CaF _2(s) \rightleftharpoons Ca ^{2+}(a q)+2 F ^{-}(a q)$

Solve The expression for K$_{sp}$ is

$K_{s p}=\left[ Ca ^{2+}\right]\left[ F ^{-}\right]^2$

Appendix D gives 3.9 × 10$^{-11}$ for this K$_{sp}$.