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A 0.2 mol kg−1 aqueous solution of X has a higher boiling point than an equimolal aqueous solution of Y. In this situation, which statement must be correct?

1. The molar mass of X is greater than the molar mass of Y.
2. The molar mass of X is less than the molar mass of Y.
3. Y dissociates in water, whereas X remains unchanged.
4. X dissociates in water.

Correct answer: X dissociates in water.

Solution

Boiling-point elevation is colligative: it depends on the number of dissolved particles, not on molar mass. If equimolal X gives a higher BP than Y, X must produce more particles, i.e. X dissociates in water. So the correct option is 'X dissociates in water.'

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