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Liquids A and B have vapour pressures of 35.0 kPa and 17.0 kPa when pure, at one temperature…

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Liquids A and B have vapour pressures of $35.0\ \text{kPa}$ and $17.0\ \text{kPa}$ when pure, at one temperature. A solution with a mole fraction of A of $0.25$ has a measured total pressure of $24.1\ \text{kPa}$ . Find the pressure from Raoult's law and the percentage deviation. Say which kind of deviation it is.

Need a reminder? The key ideaShow

An ideal solution obeys Raoult's law at every composition, with $\Delta_{mix}H = 0$ and $\Delta_{mix}V = 0$ . A positive deviation has a pressure above the Raoult line, a weaker A–B attraction, $\Delta_{mix}H > 0$ and $\Delta_{mix}V > 0$ . A negative deviation is the opposite. The deviation is $\dfrac{P_{measured} - P_{Raoult}}{P_{Raoult}} \times 100\ \%$ .

The worked solution

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