Raoult's Law Mixture Calculator
Calculate vapor pressure of ideal liquid mixtures using Raoult's law.
What Raoult’s Law Assumes
Raoult’s law states that each component of a liquid mixture contributes vapour pressure in proportion to its mole fraction. Its underlying assumption is strong and worth stating plainly: a molecule of A is surrounded by neighbours it finds indistinguishable from its own kind.
That holds only when A–A, B–B and A–B intermolecular forces are essentially equal. Benzene and toluene satisfy this almost perfectly — both are non-polar aromatics of similar size — which is why they are the standard textbook example of an ideal mixture.
| Term | Meaning | Note |
|---|---|---|
| x1 | Mole fraction in the liquid | Not mass fraction, and not the vapour composition |
| P1* | Vapour pressure of pure component 1 | Strongly temperature dependent |
| Ptotal | Total vapour pressure above the mixture | Sum of partial pressures |
Deviations and What They Mean
| Behaviour | A–B forces | Observed P | Example | ΔHmix |
|---|---|---|---|---|
| Ideal | Equal to A–A and B–B | Matches Raoult | Benzene / toluene | ~0 |
| Positive deviation | Weaker than pure | Higher than predicted | Ethanol / water | Endothermic |
| Negative deviation | Stronger than pure | Lower than predicted | Acetone / chloroform | Exothermic |
The logic is direct. If A and B attract each other less strongly than they attract their own kind, molecules escape into the vapour more readily than expected — pressure comes out high. Ethanol and water behave this way because mixing disrupts ethanol’s hydrogen-bonded network. Acetone and chloroform do the opposite: they form a new hydrogen bond between the chloroform proton and the acetone carbonyl, holding both more tightly in the liquid.
Large deviations produce azeotropes — compositions where liquid and vapour have identical composition, so distillation cannot separate them further. The ethanol–water azeotrope at 95.6% ethanol is why absolute alcohol cannot be produced by simple distillation.
Why the Vapour Is Richer in the Volatile Component
The vapour composition is not the same as the liquid. Each component’s share of the vapour is its partial pressure divided by the total, which favours whichever component has the higher pure vapour pressure. That enrichment is the entire basis of fractional distillation — each theoretical plate repeats the enrichment step.
Worked Examples
Common Mistakes
Raoult’s law is written in mole fractions. Converting from mass requires dividing by molar masses first, and the two differ substantially when the components have different molecular weights.
The vapour is always richer in the more volatile component. Confusing the two makes distillation impossible to understand and gives wrong answers on any vapour-phase question.
Raoult’s law is an idealisation. Ethanol/water deviates so strongly that it forms an azeotrope, and predictions from the ideal equation are simply wrong there.
Pure vapour pressures rise steeply with temperature via the Clausius–Clapeyron relation. Using values from the wrong temperature invalidates the whole calculation.
Frequently Asked Questions
Formula Explorer connections
Interpretation: This relationship connects pressure, volume, temperature, amount or phase composition for gases and volatile mixtures. Assumption: Use absolute temperature and compatible pressure-volume units. Ideal behavior weakens at high pressure, low temperature, strong intermolecular attraction or near phase change.