The vapor pressure of a solvent in solution equals its mole fraction times its pure vapor pressure.
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Raoult's law states that the partial vapour pressure of a component in an ideal solution equals its mole fraction multiplied by the vapour pressure of the pure component, P_A = x_A P_A*. For a solution containing a nonvolatile solute, the vapour pressure of the solvent is reduced in proportion to the mole fraction of the solvent. This reduction in vapour pressure is the origin of the colligative effects of boiling point elevation and freezing point depression. Raoult's law is strictly obeyed by ideal solutions, in which the attractions between unlike molecules are similar to those between like molecules. Real solutions deviate from it to varying degrees.
Key ideas: Formulas: P_solution = x_solvent P0 for a nonvolatile solute, so lowering is ΔP = x_solute P0. For two volatile liquids, P_total = x_A P_A0 + x_B P_B0, and vapour composition is y_A = P_A / P_total, richer in the more volatile liquid. A plot of P versus x_A is linear for ideal mixtures.
Common mistakes and connections: Typical errors are using mass fractions or molarity in place of mole fraction, and using the solute fraction instead of the solvent fraction. Another is applying it to electrolytes without multiplying by i. Real solutions deviate, especially for polar mixtures. Links to colligative properties, ideal solutions, azeotropes and fractional distillation.
Raoult's law: in an ideal solution the vapour pressure of each component is proportional to its mole fraction, giving straight lines. The interactive lesson shows this as a live animation or chart.
For a solution in which the mole fraction of water is 0.90, the vapour pressure of water is 90 percent of that of pure water.
The vapor pressure of a solvent in solution equals its mole fraction times its pure vapor pressure.
Raoult's Law is a intermediate topic in States of Matter & Solutions. It builds on the vocabulary of beginner topics in this area.
Study these earlier topics in States of Matter & Solutions first: Dilution, Henry's Law, Temperature and Solubility.
The next topics in States of Matter & Solutions are Ideal and Non-ideal Solutions and Azeotropes.
Yes. The interactive lesson includes a self-check question on Raoult's Law, and the States of Matter & Solutions quiz draws on every topic in this area.
Raoult's Law is topic 84 of 109 in States of Matter & Solutions.