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Consider a binary solution of volatile liquids A and B. When the mole fraction of A is X_A = 0.4, the observed vapour pressure of the solution is 580 torr. Given p_A⁰ = 300 torr and p_B⁰ = 800 torr, the solution shows positive deviation from Raoult's law. Which of the following pairs of liquids could form such a mixture?

1. CHCl3 + CH3COCH3
2. C6H5Cl + C6H5Br
3. C6H6 + C6H5CH3
4. H2O + Ethanol

Correct answer: H2O + Ethanol

Solution

P_ideal = 0.4*300 + 0.6*800 = 120 + 480 = 600 torr. Observed P = 580 torr < 600 torr, so this is negative deviation from Raoult's law. Negative deviation occurs when A-B interactions are stronger than A-A or B-B interactions. CHCl3 + acetone shows strong hydrogen bonding (negative deviation). The question asks which mixture could be prepared with these vapour pressures showing negative deviation.

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