Raoult’s Law Calculator

01 Set up your calculation

1. Enter liquid-phase amounts of the two components in moles.
2. Enter their pure-component vapor pressures at the same temperature.
3. Set the second vapor pressure to zero when that solute is effectively nonvolatile.

02 The relationship

For the assumptions behind these results, read the explanation below. You can also explore Mole Fraction and Molar Mass.

03 Composition lowers a component’s vapor contribution

In an ideal liquid mixture, a component’s vapor partial pressure is proportional to its liquid mole fraction. A nonvolatile solute therefore lowers solvent vapor pressure by reducing the fraction of solvent molecules in the liquid.

When both components are volatile, each contributes to the total pressure. The vapor composition usually differs from the liquid composition because the two pure substances generally have different vapor pressures. The inputs here describe the liquid phase.

04 Ideal behavior is an assumption

Raoult’s law is most useful for mixtures whose intermolecular interactions are similar enough to approximate ideal behavior. Strong attractions or repulsions between unlike molecules can produce deviations, and azeotropes require more detailed treatment.

Pure-component vapor pressures must refer to the same temperature as the mixture. This tool does not estimate those pressures from temperature, account for electrolyte dissociation, or enforce a phase-stability calculation. An entered zero pressure is an approximation of nonvolatility over the conditions of interest.