A saturated aqueous solution of sparingly soluble calcium fluoride, CaF2, is at equilibrium according to the dissolution equation CaF2(s) ⇌ Ca2+(aq) + 2 F−(aq). The overall fluoride-ion concentration is then increased by dissolving a small amount of soluble sodium fluoride, NaF. Assume that temperature remains constant so that the solubility-product constant, Ksp, is unchanged.
Ksp[CaF2] = 1.5 × 10−10
(a) Describe qualitatively how the addition of NaF affects the equilibrium position of the dissolution reaction and the molar solubility of CaF2. Explain your reasoning in terms of Le Châtelier’s principle.
(b) Write the mathematical expression for the reaction quotient, Q, after fluoride ions from NaF have dissolved, and identify the species whose concentration changes most significantly.
(c) Explain why a precipitate of CaF2 may form even though no calcium ions were added to the system. In your explanation refer explicitly to the relative magnitudes of Q and Ksp. No numerical calculation is required.
(d) Suppose enough NaF is added so that the fluoride-ion concentration immediately after dissolution doubles. Without performing numerical calculations, predict whether CaF2 precipitates under these conditions and justify your prediction.