Question

What is the effect on the amount of solid Mg(OH)₂ that dissolves and the concentrations of Mg²⁺ and OH⁻ when each of the following are added to a mixture of solid Mg(OH)₂ and water at equilibrium?

a) Increase Mg²⁺ concentration; decrease OH⁻ concentration
b) Decrease Mg²⁺ concentration; increase OH⁻ concentration
c) Increase both Mg²⁺ and OH⁻ concentrations
d) Decrease both Mg²⁺ and OH⁻ concentrations

Answer 1

The concentrations of Mg²⁺ and OH⁻ ions change when certain compounds are added to a mixture of solid Mg(OH)₂ and water at equilibrium. Adding MgCl₂ or KOH will increase the concentration of either Mg²⁺ or OH⁻, respectively, resulting in a decrease in the other ion and an increase in the amount of undissolved Mg(OH)₂.

Step-by-step explanation:

When MgCl₂ is added to a mixture of solid Mg(OH)₂ and water at equilibrium, the concentration of Mg²⁺ increases. This increase in Mg²⁺ concentration will shift the solubility equilibrium to the left, decreasing the concentration of OH⁻ and increasing the amount of undissolved Mg(OH)₂.

Similarly, when KOH is added, the concentration of OH⁻ increases. This will also shift the solubility equilibrium to the left, resulting in a decrease in the concentration of Mg²⁺ and an increase in the amount of undissolved Mg(OH)₂.

On the other hand, adding more solid Mg(OH)₂ will increase the amount of undissolved compound in the mixture, but since the solution is already saturated, the concentrations of dissolved Mg²⁺ and OH⁻ ions will remain the same.