Adding which of the following substances will decrease the solubility of calcium chloride in water: cacl2(s) ca2+(aq) + 2 cl–(aq)?

a. sodium chloride
b. sodium fluoride
c. no right choice.

Option b. Sodium chloride.

Explanation:

Consider the dissociation equilibrium of in water:

.

By the Le Chatelier's Principle, increasing the concentration of either product will shift the equilibrium to the left. Some that was initially dissolved will precipitate out of the solution. This effect is called the common-ion effect.

For this solution, the products of dissociation are

Adding either to the solution will trigger the common-ion effect and reduce the solubility of .

In the two choices,

contains ions. It is capable of triggering the common-ion effect. However contains neither ions nor ions. It will not trigger the common-ion effect.