MCAT General Chemistry Question 129: Answer and Explanation
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Question: 129
9. Which of the following combinations of liquids would be expected to have a vapor pressure higher than the vapor pressure that would be predicted by Raoult's law?
- A. Ethanol and hexane
- B. Acetone and water
- C. Isopropanol and methanol
- D. Nitric acid and water
Correct Answer: A
Explanation:
Mixtures that have a higher vapor pressure than predicted by Raoult's law have stronger solvent-solvent and solute-solute interactions than solvent-solute interactions. Therefore, particles do not want to stay in solution and more readily evaporate, creating a higher vapor pressure than an ideal solution. Two liquids that have different properties, like hexane (hydrophobic) and ethanol (hydrophilic, small) in Choice (A), would not have many interactions with each other and would cause positive deviation. Choices (B) and (C) are composed of liquids that are similar to one another and would not show significant deviation from Raoult's law. Choice (D) contains two liquids that would interact very well with each other, which would actually cause a negative deviation from Raoult's law—when attracted to one other, liquids prefer to stay in liquid form and have a lower vapor pressure than predicted by Raoult's law.