If a solute dimerises in solution, its observed molar mass compared with the calculated value will be
A{'text': 'Twice the calculated value', 'label': 'A'}
B{'text': 'Equal to the calculated value', 'label': 'B'}
C{'text': 'Half the calculated value', 'label': 'C'}
D{'text': 'Four times the calculated value', 'label': 'D'}
Answer & Solution
Correct answer: A. {'text': 'Twice the calculated value', 'label': 'A'}
1. Colligative properties measure the number of solute particles.
2. Dimerisation reduces the number of particles by half (2 molecules → 1 dimer).
3. The measured colligative effect is halved, so the apparent (observed) molar mass appears doubled.
4. Van't Hoff factor i = 0.5 for complete dimerisation.
_Source: NCERT Class 12 Chemistry, Unit 1 "Solutions", §1.7_
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