Question

CaF2(s)⇄Ca2+(aq)+2F−(aq) Ksp=3.9×10−11

HF(aq)⇄H+(aq)+F−(aq) Kc=6.8×10−4

The dissolution of calcium fluoride is represented by the equilibrium system above at 25°C. The F− ion is produced when the weak acid HF dissociates. If solid calcium fluoride is added to equal volumes of the following solutions at 25°C, in which solution will the most calcium fluoride dissolve.

a.Pure distilled water

b. 1MHNO3(aq)

c. 1 M NaOH(aq)

d. A saturated aqueous CaF2 solution

Answer 1

According to Le Chatelier's principle, if a system at equilibrium is subjected to a change (change in concentration/pressure of reactants/products, temperature etc.) the system moves in a direction so as to counteract the change to restore the equilibrium.

HNO₃ is a strong acid which will dissociate completely to give H⁺ and NO₃^- in solution. This increased concentration of H⁺ will drive the second equilibrium reaction (given in question) in backward direction thereby producing more of HF. The resulting decrease in concentration of F^- causes the dissolution equilibrium reaction of CaF₂ to move in forward direction thereby dissolving more of solid CaF₂ to give Ca²⁺ and F^- i.e. increasing the solubility of CaF₂. So option b. is correct.

NaOH is a strong base which will dissociate completely to give Na⁺ and OH^- in solution. This OH^- will consume the H⁺ and drive the second equilibrium reaction (given in question) in forward direction thereby producing more of H⁺ and F^-. The increased concentration of F^- causes the dissolution equilibrium reaction of CaF₂ to move in backward direction thereby producing more of undissolved CaF₂ i.e. reducing the solubility of CaF₂. So option c. is incorrect.

A saturated aqueous CaF₂ solution contains the maximum amount of CaF₂ that it can dissolve. So no more of the added CaF₂ will dissolve in it. So option d. is incorrect.

Pure distilled water has equal concentration of H⁺ and OH^-. So it has no impact on solubility of CaF₂ unlike NaOH and HNO_{​​​​​​3}. So option a. is incorrect.

CaF₂ will be most soluble in HNO₃(aq), then in pure distilled water, then in NaOH(aq) and least soluble in saturated aqueous solution of CaF₂.