Question

1. Which of the following slightly soluble substances are more soluble in acidic solution: AgCl AgCN CaF2? Explain why.

2. 50.0 ml of a solution containing 0.010M Pb(NO3)2 is mixed with 50.0 ml of a solution containing 0.10 M NaCl. What will the concentrations of Pb++ and of Cl– be after the mixing? PbCl2 has a Ksp = 1.7 x 10-5 . What is the value of Q for the lead and chloride ions? Will a precipitate form in the mixture? Why or why not.

Answer 1

Consider the equilibrium between solid calcium fluoride and its ions in aqueous solution.

CaF₂(s) → Ca⁺²(aq)+ 2 F⁻¹(aq)

Because the fluoride ion is the conjugate bas of a weak acid (HF), you would expect it to react with any H+ion that is added:

F⁻¹(aq)+ H⁺(aq) → HF(aq)

According to LeChatelier's Principle, as F−1 ions are removed by the reaction with H+ ion, more calcium fluoride can dissolve to replenish the fluoride ions.

Therefore you expect calcium fluoride to be more soluble in acidic solution.

Looking at the two dissolution equations

AgCl(s)→Ag⁺(aq)+ Cl^-1(aq)

AgCN(s)→Ag⁺(aq)+ CN^-1(aq)

we can assume that the chloride ion will not undergo hydrolysis since it is the conjugate base of the strong acid, HCl. However, the cyanide ion is the conjugate base of the weak acid, HCN, and consequently will react with any H+ ions present to reform HCN, thus removing cyanide ions from the solution.

CN⁻¹(aq)+ H⁺(aq)→HCN(aq)

Based on this, one would assume that the solubility of the silver cyanide would be more affected by changes in pH.

Solubility of AgCl,AgCN and CaF₂ in acidic solutions is as follows

CaF₂ > AgCN > AgCl