Chapter 15, Problem 109E

The following question is taken from a Chemistry Advanced Placement Examination and is used with the permission of the Educational Testing Service.

Solve the following problem:

${\text{MgF}}_2\left(s\right)\rightleftharpoons {\text{Mg}}^{2+}\left(aq\right)+2{\text{F}}^{-}\left(aq\right)$

In a saturated solution of MgF₂ at 18 °C, the concentration of Mg²+ is $\text{1}.\text{21}\times \text{1}{0}^{-}{}^3$M . The equilibrium is represented by the preceding equation.

(a) Write the expression for the solubility-product constant, K_sp, and calculate its value at 18 °C.

(b) Calculate the equilibrium concentration of Mg²+ in 1.000 L of saturated MgF₂ solution at 18 °C to which 0.100 mol of solid KF has been added. The KF dissolves completely. Assume the volume change is negligible.

(c) Predict whether a precipitate of MgF₂ will form when 100.0 mL of a $\text{3}.00\times \text{1}{0}^{-\text{3}}$ -M solution of Mg(NO₃)₂ is mixed with 200.0 mL of a . $2.00\times {10}^{-3}$ -M solution of NaF at 18 °C. Show the calculations to support your prediction..

(d) At 27 °C the concentration of Mg²+ in a saturated solution of MgF₂ is $\text{1}.\text{17}\times \text{1}{0}^{-\text{3}}$M . Is the dissolving of MgF₂ in water all endothermic or an exothermic process? Give an explanation to support your conclusion.