💨 Raoult's Law Calculator
Enter the pure solvent's vapour pressure and the solvent mole fraction to get the solution's vapour pressure and the vapour-pressure lowering.
Solution vapour pressure
21.42
Vapour-pressure lowering (ΔP)
2.38
Raoult\'s law: the vapour pressure of an ideal solution equals the solvent\'s mole fraction times its pure vapour pressure, P = Xsolvent × P°. A non-volatile solute lowers the vapour pressure by ΔP = Xsolute × P° — a colligative effect that also raises boiling point and lowers freezing point. The pressure units of the result match whatever units you enter for P° (e.g. mmHg, kPa). Real solutions deviate from ideal behaviour. 🔒 In your browser.
How the raoult's law calculator works
Raoult's law states that each volatile component's contribution to the vapour pressure is its mole fraction times its pure vapour pressure. For a non-volatile solute, the solution's vapour pressure is P = X_solvent × P°_solvent, and the lowering caused by the solute is ΔP = X_solute × P° = P° − P. The tool computes both; the result carries whatever pressure unit you enter for P°.
Vapour-pressure lowering is a colligative property — it depends on the amount of dissolved solute, not its identity — and it's the reason a solute also raises the boiling point and lowers the freezing point. Raoult's law describes ideal solutions; real mixtures deviate (positive or negative) when solute–solvent interactions differ from solvent–solvent ones.
Frequently asked questions
What is Raoult's law?
It says the vapour pressure of a solvent above a solution equals the solvent's mole fraction times its pure-solvent vapour pressure: P = X_solvent × P°. Adding a non-volatile solute lowers the vapour pressure in proportion to how much solvent it displaces.
How do you calculate vapour-pressure lowering?
ΔP = X_solute × P°, where X_solute is the solute mole fraction and P° the pure solvent's vapour pressure. Equivalently, ΔP = P° − (X_solvent × P°). A 0.1 solute mole fraction lowers a 100-unit P° by 10.
Why does a solute lower vapour pressure?
Non-volatile solute particles take up space at the surface and reduce the fraction of solvent molecules that can escape into the vapour, so fewer evaporate and the vapour pressure drops. More solute (higher mole fraction) means a bigger drop.
What is Raoult's law used for?
To predict how dissolving a solute changes a solvent's vapour pressure, boiling point and freezing point, and to model ideal liquid mixtures in distillation. It underlies the colligative-property calculations.
When does Raoult's law fail?
For non-ideal solutions, where solute–solvent attractions differ markedly from the pure liquids' — causing positive deviations (weaker attractions, higher vapour pressure) or negative ones. It also assumes the solute is non-volatile for the simple lowering form.