A saturated solution of calcium fluoride, CaF2, is prepared at 25 °C.
At this temperature Ksp for CaF2 is 1.5×10−10.
A second solution is prepared by placing excess solid CaF2 in 250mL of 0.050M NaF, also at 25 °C, and allowing the system to reach equilibrium.
Assume temperature remains constant and the volume change upon dissolving is negligible.
(a) Calculate the molar solubility of CaF2 in pure water at 25 °C.
(b) Calculate the molar solubility of CaF2 in the 0.050M NaF solution.
(c) Determine the percentage by which the solubility decreases when the common ion is present, relative to the solubility in pure water.
(d) Explain, in terms of equilibrium shifts, why the presence of fluoride ions leads to the numerical result obtained in part (c).