A sparingly soluble salt, MX₂, dissociates according to the equation MX2(s)⇌M2+(aq)+2X−(aq). The dissolution of MX₂ is endothermic. The solubility-product constants are Ksp=1.0×10−11 at 25 °C and Ksp=3.2×10−11 at 35 °C.
Two separate experiments are performed with 1.00 L portions of water that are initially at 25 °C and contain no dissolved ions.
• Experiment A: The temperature of the solution is raised from 25 °C to 35 °C.
• Experiment B: The temperature is kept at 25 °C, and 0.010 mol of a highly soluble salt that contains the anion X⁻ is added; the volume change is negligible.
(a) For each experiment, predict qualitatively whether the molar solubility of MX₂ increases, decreases, or remains unchanged relative to its value in pure water at 25 °C. Justify each prediction.
(b) Using the given Ksp values, calculate the molar solubility of MX₂ in pure water at both 25 °C and 35 °C.
(c) For Experiment B, calculate the equilibrium molar concentration of M²⁺ after the 0.010 mol of the soluble X⁻ salt has been added and equilibrium is re-established at 25 °C.
(d) Compare quantitatively the change in molar solubility produced in Experiment A with that produced in Experiment B. Which experimental change has the greater effect on the solubility of MX₂? Support your answer with numerical evidence and chemical reasoning.