Given that
the K_{sp} of CaF_{2} is 4.0 × 10^{-11}, what is the
concentration of fluoride ion if a 1.2 × 10^{-2} M CaCO_{3}
solution is saturated with CaF_{2}?

Let x = molar solubility of calcium fluoride

Because the
K_{sp} of CaF_{2} is so small compared to the concentration of
Ca^{2+} ions 1.2 × 10^{-2} + x 1.2 × 10^{-2}