MCAT General Chemistry Question 34: Answer and Explanation
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Question: 34
4. Which of the following correctly ranks the compounds below by ascending boiling point?
I. Acetone
II. KCl
III. Kr
IV. Isopropyl alcohol
- A. I < II < IV < III
- B. III < IV < I < II
- C. II < IV < I < III
- D. III < I < IV < II
Correct Answer: D
Explanation:
The key to answering this question is to understand the types of intermolecular forces that exist in each of these molecules because larger intermolecular forces correspond to higher boiling points. Kr is a noble gas with a full octet, so the only intermolecular forces present are London dispersion forces, the weakest type of intermolecular forces. Acetone and isopropyl alcohol are both polar, so both have dipole-dipole interactions, which are stronger than dispersion forces. However, isopropyl alcohol can also hydrogen bond, increasing its boiling point. Finally, the strongest interactions are ionic bonds, which exist in potassium chloride.