Chemistry · General chemistry II · Worked example
Vapor pressure and vapor composition of a benzene–toluene mixture
A liquid mixture contains 1.00 mol benzene and 2.00 mol toluene at 25 °C, where the pure vapor pressures are 95.1 torr (benzene) and 28.4 torr (toluene). Treating the mixture as ideal, find the total vapor pressure and the mole fraction of benzene in the vapor.
Liquid mole fractions
Both components are volatile, so each keeps its own Raoult’s-law term.
Partial pressures
Multiply each mole fraction by that component’s pure vapor pressure.
Total pressure and vapor composition
Dalton’s law adds the partial pressures. The benzene mole fraction in the vapor is its share of the total.
Result
The total vapor pressure is 50.6 torr. The vapor is 62.6% benzene although the liquid is only 33.3% benzene: the vapor is enriched in the more volatile component.
Your turn
If the vapor above were condensed and allowed to evaporate again, would the new vapor contain more or less than 62.6% benzene?
Show the answer and explanation
More.
The condensed liquid is 62.6% benzene, so benzene’s partial pressure share rises again. Each repeat enriches the more volatile component; fractional distillation does this continuously.
Keep exploring
Write the Raoult’s-law sum in a Math box and change the mole fractions to see how the vapor composition moves.
Return to the concept →Sources and scope
Authored study material. Tool results depend on the stated inputs and model assumptions.
- Tro, Chemistry: A Molecular Approach, 4th ed., §13.5 Expressing Solution Concentration, pp. 585–592 (molality, p. 588)
- Tro, Chemistry: A Molecular Approach, 4th ed., §13.6 Colligative Properties: Vapor Pressure Lowering, Freezing Point Depression, Boiling Point Elevation, and Osmotic Pressure, pp. 593–605 (volatile solutes, p. 597; osmotic pressure, pp. 603–604)
- Tro, Chemistry: A Molecular Approach, 4th ed., §13.7 Colligative Properties of Strong Electrolyte Solutions, pp. 605–607
- OpenStax Chemistry 2e — Colligative properties