Question

Which salt would you predict to have a solubility in water that increases as the pH decreases?

A) mercury(I) bromide

B) silver iodide

C) calcium fluoride

D) lead(II) chloride

E) silver chloride

Answer 1

Solubility of calcium fluoride in water will increase with decrease in pH

CaF2  $\leftrightharpoons$ Ca2+ + 2F-

On adding H+  , pH of the solution decreases and solubility increases because fluoride ion reacts with hydronium ion to form HF

Since the fluoride ions are reacting their concentration decreases in the solution and the equilibrium is disturbed

According to Le chatelier's principle reaction will shift in forward direction to attain the equilibrium and produce fluoride ions and hence the solubility of CaF2 increases

Reaction of fluoride ions with hydronium ions

2H3O+ + 2F- $\leftrightharpoons$ 2HF + 2H2O

Solubility of other salts do not increase with decrease in pH because the anions (bromide, chloride and iodide) are weaker base and does not react with hydronium ions. Hence, the equilibrium does not shift and solubility remains almost constant.