For a solution of a non-volatile solute, Raoult law implies that the vapour pressure of the pure solvent
A{'text': 'Rises as solute is added', 'label': 'A'}
B{'text': 'Falls linearly with solute mole fraction', 'label': 'B'}
C{'text': 'Is unchanged', 'label': 'C'}
D{'text': 'Falls exponentially with solute mole fraction', 'label': 'D'}
Answer & Solution
Correct answer: B. {'text': 'Falls linearly with solute mole fraction', 'label': 'B'}
1. Raoult law: p_A = x_A × p_A° for volatile solvent A.
2. If a non-volatile solute is added, the solvent mole fraction x_A drops below 1.
3. Therefore the vapour pressure of the solvent falls linearly as x_solute increases.
4. This lowering is the basis for colligative properties like boiling point elevation.
_Source: NCERT Class 12 Chemistry, Unit 1 "Solutions", §1.5_
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