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Two liquids P and Q create an ideal solution. At 300 K, the vapor pressure of a solution with 1 mole of P and 3 moles of Q is 550 mmHg. If 1 mole of Q is added to this mixture at the same temperature, the vapor pressure rises by 10 mmHg. What are the vapor pressures of pure P and Q (in mmHg)?

1. 300 and 400
2. 400 and 600
3. 500 and 600
4. 200 and 300

Correct answer: 400 and 600

Solution

With 1 P + 3 Q: 0.25*P0p + 0.75*P0q = 550. Adding 1 Q (1 P + 4 Q): 0.20*P0p + 0.80*P0q = 560. Solving: P0q = 600 and P0p = 400 mmHg, so the pure vapor pressures are 400 and 600 mmHg.

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