Raoult's Law Calculator
Vapour pressure of an ideal two-component solution — partial pressures, total pressure, vapour-phase composition and relative volatility from mole fractions and pure-component vapour pressures.
How it works
P_A = x_A·P_A°, P_B = x_B·P_B°; P_total = P_A + P_B; y_A = P_A ÷ P_total; α = P_A°/P_B°
In an ideal solution each component contributes vapour in proportion to its mole fraction and its pure vapour pressure. The more volatile component is always enriched in the vapour, which is what distillation exploits: condense the vapour, boil again, and each stage climbs the composition ladder by the relative volatility. With a non-volatile solute the law reduces to vapour-pressure lowering, the root of boiling-point elevation and freezing-point depression. Real mixtures deviate — positively (ethanol–water) or negatively (acetone–chloroform) — and can form azeotropes.
Worked example
Benzene (x = 0.4, P° = 94.6 kPa) with toluene (P° = 38.8 kPa): P = 37.8 + 23.3 = 61.1 kPa; the vapour is 62% benzene against 40% in the liquid, relative volatility 2.44.
Frequently asked questions
Which solutions are close to ideal?
Chemically similar pairs — benzene/toluene, hexane/heptane, isotopologues. Hydrogen-bonding mixtures deviate strongly.
How does this relate to Henry’s law?
Raoult’s law holds for the solvent as x → 1; Henry’s law describes the dilute solute with its own constant instead of P°.
Related calculators
For study and planning — check units and conditions against your data source before using a result in the lab.