MCAT General Chemistry Question 252: Answer and Explanation

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Question: 252

1. Which of the following molecules has nine normal modes of vibration?

A. NI₃
B. CH₄
C. PF₅
D. SCl₆

Correct Answer: B

Explanation:

B None of these molecules are linear. The passage states that the number of normal modes of vibration is 3N – 6 for non-linear molecules. If there are nine normal modes of vibration, 3N – 6 = 9, and N = 5. Only methane, choice B, has 5 atoms.

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