MCAT General Chemistry Question 36: Answer and Explanation

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Question: 36

6. Which of the following has the largest dipole moment?

  • A. HCN
  • B. H2O
  • C. CCl4
  • D. SO2

Correct Answer: A

Explanation:

The best way to approach this problem is to draw the structure of each of these molecules, then consider the electro-negativity of each bond as it might contribute to an overall dipole moment. HCN is the correct answer because of the large differences in electronegativity aligned in a linear fashion. There is a strong dipole moment in the direction of nitrogen, without any other moments canceling it out. Water, choice (B), has two dipole moments, one from each hydrogen pointing in the direction of oxygen. The molecule is bent, and the dipole moments partially cancel out. There is a molecular dipole, but it is not as strong as in HCN. Sulfur dioxide, choice (C), has a similar bent configuration, and its dipole will again be smaller than that of HCN. Further, oxygen and sulfur do not have as large a difference in electro-negativity, so even the individual bond dipoles are smaller than those in the other molecules. CCl4, choice (D), has tetrahedral geometry. Although each of the individual C-Cl bonds is highly polar, the orientation of these bonds causes the dipoles to cancel each other out fully, yielding no overall dipole moment.

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