Raoult's Law Calculator

How it is calculated

Raoult's law says that adding a non-volatile solute to a solvent lowers the solvent's vapour pressure in proportion to the solvent's share of the mixture: P = x × P0. Here P⁰ is the vapour pressure of the pure solvent at the temperature of interest, and x is the mole fraction of the solvent, a number from 0 to 1, found as moles of solvent / (moles of solvent + moles of solute). The tool also gives the vapour pressure lowering, ΔP = P0 − P. Results are in the same pressure unit as P⁰ (mmHg, kPa, atm) and rounded to two decimals. x must lie between 0 and 1. The law is exact only for ideal solutions; strong interactions between solute and solvent cause deviations.

Common mistakes

• Using the solute's mole fraction as x instead of the solvent's, which gives the lowering rather than the vapour pressure.
• Entering a mass or mole percentage, such as 90, instead of a fraction (0.9), or using mass fraction where a mole fraction is required.
• Using the vapour pressure of the pure solvent at the wrong temperature; vapour pressure changes strongly with temperature.
• Applying the law to a solute that is itself volatile, where each component's partial pressure must be added together.
• Expecting accurate results from strongly non-ideal mixtures, such as ethanol and water, where observed pressure differs from the straight-line prediction.