MCAT Organic Chemistry Question 174: Answer and Explanation

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Question: 174

9. A mixture of sand, benzoic acid, and naphthalene in ether is best separated by:

  • A. filtration, followed by acidic extraction, followed by recrystallization.
  • B. filtration, followed by basic extraction, followed by evaporation.
  • C. extraction, followed by distillation, followed by gas chromatography.
  • D. filtration, followed by size-exclusion column chromatography, followed by extraction.

Correct Answer: B

Explanation:

In this question, three substances must be separated using a combination of techniques. The first step should be the most obvious: remove the sand by filtration. The remaining compounds—benzoic acid and naphthalene—are still dissolved in ether. If the solution is extracted with aqueous base, the benzoate anion is formed and becomes dissolved in the aqueous layer, while naphthalene, a nonpolar compound, remains in the ether. Finally, evaporation of the ether will yield purified naphthalene.

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