Which one of the following has higher vapour pressure?
Correct answer: C. Isopentane
- A. Acetone
- B. Acetaldehyde
- C. Isopentane
- D. Benzene
Explanation
This is the option marked correct in the official UHS answer key. Isopentane is a branched non polar hydrocarbon whose only intermolecular attractions are weak dispersion forces, and branching reduces the contact area further, so it evaporates readily. Note that acetaldehyde, boiling at about 21 degrees Celsius, is in fact the more volatile of the two in standard data, so this is a question where the official key is worth knowing alongside the underlying principle that weak intermolecular forces mean high vapour pressure.
This question appeared on the UHS MDCAT 2025 paper, which you can sit online with every answer explained.
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About Evaporation, Boiling Point and Vapour Pressure
Evaporation is the escape of higher-energy molecules from a liquid surface, while vapour pressure is the pressure exerted by vapour in equilibrium with that liquid. The topic distinguishes evaporation from boiling and relates both to temperature, intermolecular forces, surface area, atmospheric pressure and the boiling point.
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