Raoults Law Vapor Pressure Calculator

Raoult's law states that the vapor pressure of a solvent above an ideal solution equals the mole fraction of the solvent times the vapor pressure of the pure solvent. Dissolving a nonvolatile solute lowers the solvent's vapor pressure in proportion to how much of the liquid is now solute particles.

For a nonvolatile solute, P_solution = x_solvent * P_pure, where x_solvent is the mole fraction of the solvent and P_pure is the vapor pressure of the pure solvent. The vapor-pressure lowering is Delta P = P_pure - P_solution = x_solute * P_pure, since the mole fractions sum to one.

Solute particles occupy part of the liquid surface and dilute the solvent, so fewer solvent molecules escape into the vapor at any instant. Because vapor pressure depends on the fraction of surface that is solvent, it scales with the solvent mole fraction, which is the heart of Raoult's law.

Yes. Raoult's law holds exactly only for ideal solutions, where solute-solvent interactions resemble solvent-solvent interactions. Real solutions show positive or negative deviations, and the law works best for dilute solutions of a nonvolatile, non-dissociating solute. For dissociating solutes, use the effective solute mole fraction.

The solvent mole fraction is dimensionless between 0 and 1. The pure solvent vapor pressure can be in any pressure unit you choose (for example mmHg, kPa, or atmospheres); the solution vapor pressure and the lowering are returned in the same unit you enter, since the law multiplies a dimensionless fraction by a pressure.