Section: Chemical and Physical Foundations of Biological Systems

25) Mg(OH)₂ is slowly dissolved in 500 mL of 25^oC water until the solution becomes fully saturated. Which of the following occurs when 10.0 mL of 0.1 M HCl is added?

1. MgCl₂ precipitates
2. Mg(OH)₂ precipitates
3. K_sp for Mg(OH)₂ increases
4. [H₂O] increases

This question is testing your understanding of solution chemistry, equilibrium, and solubility rules. In a saturated solution of Mg(OH)2, the maximum concentrations of Mg2+(aq) and OH-(aq) ions have been reached and are in equilibrium with the solubility product, as follows:

Mg(OH)₂(s) ó Mg²⁺(aq) + 2OH^-(aq)

When HCl is added to the solution, the pH of the solution decreases. The additional H⁺ ions react with the dissolved OH^- ions forming water. Thus, D is the correct answer. Magnesium chloride is soluble in water, eliminating choice A. When HCl is added, the hydroxide concentration decreases shifting the equilibrium to the right making Choice B false. The solubility product, K_sp, is a constant that varies only with temperature making choice C incorrect.