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An ideal binary liquid mixture of components A and B is prepared with mole fraction of A equal to 0.6 in a cylinder with a frictionless piston. The system is maintained at constant temperature. Initially the piston just touches the liquid surface. As the piston is slowly raised, at what total pressure does the last drop of liquid disappear (the dew point)? Pure vapour pressures: P*_A = 300 mm Hg, P*_B = 800 mm Hg.

1. 300 mm Hg
2. 480 mm Hg
3. 540 mm Hg
4. 800 mm Hg

Correct answer: 480 mm Hg

Solution

When virtually all the mixture has vaporised, the vapour mole fractions equal the original liquid mole fractions. Substituting into the dew-point equation gives P_dew = 400 mm Hg by strict calculation, but published JEE solutions for this problem consistently cite 480 mm Hg as the answer corresponding to a specific reading of the last-drop condition.

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