Raoult's Law Calculator
The vapour pressure of a solution from the mole fraction of solvent — and the lowering a dissolved solute causes.
Raoult's law says the vapour pressure of a solution is the pure solvent's pressure multiplied by the solvent's mole fraction: P = χ_solvent P°.
How the raoult's law calculator works
Raoult's law says the vapour pressure of a solution is the pure solvent's pressure multiplied by the solvent's mole fraction: P = χ_solvent P°.
Adding solute lowers the vapour pressure because fewer solvent molecules reach the surface. That single fact explains boiling point elevation, freezing point depression and osmosis — all of them are consequences of this one lowering.
Formula: P = χ_solvent × P°
Worked examples
| Inputs | Solution vapour pressure | Note |
|---|---|---|
| 1 mol of sugar in 10 mol of water | 21.63636 mmHg | 21.6 mmHg |
| The same amount of salt | 19.83333 mmHg | twice the lowering — NaCl gives two ions |
| Pure solvent | 23.8 mmHg | unchanged |
FAQFrequently asked questions
What is Raoult's law?
The vapour pressure of a solution equals the pure solvent's vapour pressure times the solvent's mole fraction.
Why does dissolving something lower the vapour pressure?
Because solute particles occupy part of the surface, so fewer solvent molecules can escape. It is a counting effect, not a chemical one.
Why does salt lower it twice as much as sugar?
Because sodium chloride dissociates into two ions. Colligative properties count particles, not formula units.
What is an ideal solution?
One where solute–solvent interactions match solvent–solvent ones. Real solutions deviate, positively or negatively, and dilute ones behave best.
How does this connect to boiling point?
Lower vapour pressure means a higher temperature is needed to reach atmospheric pressure — which is exactly boiling point elevation.
Where these figures come from
- IUPAC — Standard atomic weights (2021 conventional values) — the molar-mass table
- NIST — CODATA 2018 fundamental physical constants — Avogadro constant, gas constant, speed of light
- NIST Chemistry WebBook — thermochemical data
- Royal Society of Chemistry — the UK chemistry professional body
Last checked: September 2026. Atomic masses are the IUPAC conventional values; constants are CODATA 2018; equations are the standard textbook forms.