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MCAT Chemistry Question — Solutions

  • by John
  • Feb 07, 2017
  • MCAT Question of the Day

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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