MCAT General Chemistry Question 284: Answer and Explanation

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Question: 284

3. What is the approximate concentration of CO3-2 when enough CaCO3(s) is dissolved in water to form a saturated solution?

  • A. 2.5 × 10-9 M
  • B. 5.0 × 10-9 M
  • C. 6.0 × 10-5 M
  • D. 1.0 × 10-4 M

Correct Answer: C

Explanation:

C Calcium carbonate dissolves in water as follows:

CaCO3(s) Ca2+ + CO32-

The Ksp defines the maximum solubility of a saturated solution. Solve the solubility expression for the maximum theoretical concentration of CO32-:

Ksp = [Ca2+][CO2-3]
3.4 × 10-9 = [x][x]
x2 = 34 × 10-10
x ≈ 6 × 10-5M

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