At 63.5 °C, the vapor pressure of H₂O is 175 torr, and that of ethanol (C₂H₅OH) is 400 torr. A solution is made by mixing equal masses of H₂O and C₂H₅OH. (a) What is the composition in mole fraction of a solution that has a vapor pressure of 35 torr at 20 °C?

Key Concepts

Vapor Pressure

Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid or solid phase at a given temperature. It reflects the tendency of particles to escape from the liquid phase into the vapor phase. The higher the vapor pressure, the more volatile the substance. Understanding vapor pressure is crucial for predicting how mixtures behave when components are combined.

Raoult's Law

Raoult's Law states that the vapor pressure of a solvent in a solution is directly proportional to the mole fraction of the solvent present. This law is fundamental in determining the vapor pressures of components in a mixture and is essential for calculating the overall vapor pressure of a solution. It helps in understanding how the addition of a solute affects the vapor pressure of the solvent.

Mole Fraction

Mole fraction is a way of expressing the concentration of a component in a mixture, defined as the number of moles of that component divided by the total number of moles of all components in the mixture. It is a dimensionless quantity that provides insight into the composition of solutions. In the context of vapor pressure, mole fractions are used to apply Raoult's Law to find the vapor pressures of individual components.