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Two liquids, A and B, form an ideal mixture. At 30°C, a solution with 1 mole of A and 2 moles of B has a total vapor pressure of 250 mmHg. When 1 additional mole of A is added to this mixture, the vapor pressure rises to 300 mmHg. What are the vapor pressures of pure A and pure B at 30°C?

1. 150 mmHg and 450 mmHg
2. 125 mmHg and 150 mmHg
3. 450 mmHg and 150 mmHg
4. 250 mmHg and 300 mmHg

Correct answer: 450 mmHg and 150 mmHg

Solution

First mix (xA=1/3, xB=2/3): PA0/3 + 2PB0/3 = 250 => PA0 + 2PB0 = 750. After adding 1 mol A (xA=xB=1/2): (PA0+PB0)/2 = 300 => PA0+PB0 = 600. Solving gives PB0 = 150 and PA0 = 450 mmHg, which is option (c).

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