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Topic: Vapour pressure and Raoult's law· Solutions

5 moles of liquid X and 10 moles of liquid Y make a solution having a vapour pressure of 70 torr. The vapour pressures of pure X and Y are 63 torr and 78 torr respectively. Which of the following is true regarding the described solution?

1. The solution shows positive deviation.

2. The solution shows negative deviation.

3. The solution is ideal.

4. The solution has volume greater than the sum of individual volumes.

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Answer: 2. The solution shows negative deviation.

Key idea: compare the observed vapour pressure with the Raoult's law value.

  1. \(x_X=\frac{5}{15}=\frac13\), \(x_Y=\frac23\).
  2. Ideal \(p=\frac13(63)+\frac23(78)=21+52=73\) torr.
  3. Observed 70 torr < 73 torr.

Lower than ideal means negative deviation (stronger X-Y attractions), with \(\Delta V_{mix}<0\). Option (2).

Doubtly AI · 30 Oct 2026 AI-generated · not yet verified Report
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