Write the solubility product constant for calcium fluoride.

**Solution:**

We write the equation for the equilibrium between salt and its ions:

$CaF_2(s)\rightleftharpoons Ca^{2+}(ac)+2F^-(ac)$

The solubility product constant is given by:

\begin{equation} K_{ps}=[Ca^{2+}][F^-]^2 \end{equation}

The solid does not participate in the constant and the concentrations must be raised to the stoichiometric coefficient.