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.
Want more MCAT practice?
We’ve got options for every schedule and learning style!
From the best online MCAT course created by top instructors with 524+ MCAT scores to the most representative full-length practice exams and private tutoring, we can custom tailor your MCAT prep to your goals!
Not sure which option is right for you? Schedule a free MCAT consultation with an MCAT expert using the form below. No obligation, just expert advice.
MCAT is a registered trademark of the Association of American Medical Colleges (AAMC), which is not affiliated with Blueprint.