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An equimolar homogeneous solution of two volatile liquids A and B has a vapour pressure of 310 torr. The vapour pressures of pure A and pure B are P_A* = 200 torr and P_B* = 500 torr respectively. Identify the incorrect option(s) for this binary solution.

1. The enthalpy of mixing for this solution is negative.
2. Intermolecular interactions between A and B are weaker than those between A-A and B-B.
3. The boiling point of this solution is lower than the ideal boiling point at the same composition.
4. The volume of the solution formed is less than the sum of volumes of pure A and B.

Correct answer: Intermolecular interactions between A and B are weaker than those between A-A and B-B.

Solution

Ideal VP = 0.5*200 + 0.5*500 = 350 torr. Observed = 310 torr < 350 torr. This is a negative deviation from Raoult's law. Negative deviation implies: (a) A-B interactions are STRONGER than A-A and B-B (not weaker), so option B is INCORRECT. (b) Mixing is exothermic → enthalpy of mixing is negative (option A is correct). (c) Boiling point is HIGHER than ideal (not lower) because vapour pressure is lower, so option C is also INCORRECT. (d) Volume contracts on mixing (negative volume of mixing), so option D is correct. The question asks for incorrect options; both B and C are incorrect. However, if only one answer is expected, B is the most clearly incorrect statement (commonly tested). C is also incorrect (BP is higher, not lower).

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