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MCAT Chemistry Question — Solutions

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

 

A. MgCl2 precipitates

B. Mg(OH)2 precipitates

C. Ksp for Mg(OH)2 increases

D. [H2O] increases

Click for Explanation

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, Ksp, is a constant that varies only with temperature making choice C incorrect.

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