Question

What is the effect on the amount of CaHPO₄ that dissolves and the concentrations of Ca²⁺ and [HPO₄²⁻] when each of the following are added to a mixture of solid CaHPO₄ and water at equilibrium?

a) Addition of more solid CaHPO₄.

b) Addition of HCl (a strong acid).

c) Addition of NaOH (a strong base).

d) Addition of a common ion, Ca²⁺.

Answer 1

When more solid CaHPO₄ is added, it will increase the amount of CaHPO₄ that dissolves and therefore increase the concentrations of Ca²⁺ and HPO₄²⁻ in the solution. Addition of HCl (a strong acid) will decrease the amount of CaHPO₄ that dissolves and decrease the concentrations of Ca²⁺ and HPO₄²⁻. Addition of NaOH (a strong base) will increase the amount of CaHPO₄ that dissolves and increase the concentrations of Ca²⁺ and HPO₄²⁻.

Step-by-step explanation:

The dissolution equilibrium for CaHPO₄ is represented as:

CaHPO₄(s) ⇌ Ca²⁺(aq) + HPO₄²⁻(aq)

a) When more solid CaHPO₄ is added, it will increase the amount of CaHPO₄ that dissolves and therefore increase the concentrations of Ca²⁺ and HPO₄²⁻ in the solution.

b) Addition of HCl (a strong acid) will shift the equilibrium to the left, decreasing the amount of CaHPO₄ that dissolves and decreasing the concentrations of Ca²⁺ and HPO₄²⁻.

c) Addition of NaOH (a strong base) will shift the equilibrium to the right, increasing the amount of CaHPO₄ that dissolves and increasing the concentrations of Ca²⁺ and HPO₄²⁻.

d) Addition of a common ion, Ca²⁺, will cause the equilibrium to shift to the left, decreasing the amount of CaHPO₄ that dissolves and decreasing the concentrations of Ca²⁺ and HPO₄²⁻.