Question

It asks which will increase the molar solubility of CuCl (s)

A adding more CuCl(s)

B adding CuSO4(s)

C adding NaCl(s)

D adding AgNO3(s)

Answer is D.

Explanation given is that adding AgNO3 solid will form precipitate and shift equilibrium to the right increasing solubility.

My question is:

First how do I know if something forms precipitate?

Second can this be explained in terms of Qsp and balanced Chemical equation?

Does Qsp decrease or increase.

Answer 1

CuCl + AgNO3 -------> AgCl + CuNo3

Ksp of CuCl = 1.2 * 10^-6

Ksp of AgCl = 1.8 * 10^-10

Ksp of CuCl is greater than Ksp of AgCl

Since Ksp of CuCl is more then Solubility of CuCl is also more.So more number of CU+ and Cl- ions are present in solution.Simirarly AgNo3 is good electrolyte and it is easily dissociate as Ag+ and NO3- ions.

The produced Ag+ and Cl- ions are combined and Form AgCl precipitate. this has low Ksp value hence its dissolution is very weak.

ionic equation is

Cu+ + Cl- + Ag+ + NO3- -----> AgCl (s) + Cu+ + No3-

Net ionic equation is

Ag+ + Cl- -----> AgCl

Due to decrease in concentration of Cl- and Ag+ ions on left side the reaction moves forward direction and more amount of precipite will be formed.

Here the Ksp > Qsp so more amount of precipitate will be formed.Hence Qsp of the reaction is decreased.