EasyChemistryHigh School (11–12)Solutions › Solubility, Vapour Pressure and Raoult's LawNumericalAP

Liquids A and B form an ideal solution at 298 K…

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Liquids A and B form an ideal solution at $298\ \text{K}$ . Pure A has a vapour pressure of $48.0\ \text{kPa}$ and pure B has $21.0\ \text{kPa}$ . A solution has a mole fraction of A of $0.30$ . Find the partial pressure of each liquid, the total pressure and the mole fraction of A in the vapour.

Need a reminder? The key ideaShow

For two volatile liquids, Raoult's law gives each partial pressure as $p = x\,p^\circ$ , and the total pressure is the sum, $P = p_A + p_B$ . The vapour has $y_A = \dfrac{p_A}{P}$ . For a non-volatile solute, $p = x_{solvent}\,p^\circ$ and $\dfrac{p^\circ - p}{p} = \dfrac{n_B}{n_A}$ . Mole fractions use moles, so divide each mass by its molar mass first.

The worked solution

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