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The total vapour pressure of a binary solution is expressed as P = 80*X_A + 240*X_B mmHg, where X_A and X_B are the mole fractions of components A and B respectively. Which of the following statements are correct?

1. The vapour pressure of the solution is always less than that of pure component B.
2. The vapour pressure of the solution is always greater than that of pure component A.
3. The vapour pressure of pure A is 80 mmHg and that of pure B is 240 mmHg.
4. The vapour pressure of pure A is 240 mmHg and that of pure B is 80 mmHg.

Correct answer: The vapour pressure of pure A is 80 mmHg and that of pure B is 240 mmHg.

Solution

By Raoult's law, setting X_A=1 gives P=80 (pure A vapour pressure) and X_B=1 gives P=240 (pure B vapour pressure). The solution pressure is always between 80 and 240, meaning it is less than pure B and more than pure A.

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