Raoult's Law Calculator (Vapour Pressure of a Mixture)
Vapour pressure of an ideal mixture of two liquids and the composition of the vapour above it, from the mole fraction and the pure vapour pressures.
Results
Total vapour pressure of the mixture
7.360
kPa
Partial pressure of A
5.080
kPa
Partial pressure of B
2.280
kPa
Share of A in the vapour
69.02
%
The dashed line marks the value you entered.
What this tool does
In an ideal mixture each liquid contributes its pure vapour pressure scaled by its share of the molecules. The vapour is always richer in the more volatile liquid than the liquid itself — with the defaults (roughly benzene and toluene at 25 °C), 40 % of A in the liquid becomes about 69 % in the vapour — and that difference is what distillation exploits, step after step. Real mixtures such as ethanol and water deviate strongly from Raoult's law, which is why they form azeotropes.
Formula
p_A = x_A p*_A · p_B = x_B p*_B · P = p_A + p_B · y_A = p_A ÷ P
Variables
| Symbol | Meaning | Unit |
|---|---|---|
xa | Mole fraction of liquid A | — |
pa | Vapour pressure of pure A | kPa |
pb | Vapour pressure of pure B | kPa |
P | Total vapour pressure of the mixture | kPa |
PA | Partial pressure of A | kPa |
PB | Partial pressure of B | kPa |
YA | Share of A in the vapour | % |
Worked example
- Mole fraction of liquid A0.4
- Vapour pressure of pure A12.7 kPa
- Vapour pressure of pure B3.8 kPa
- Total vapour pressure of the mixture7.360 kPa
- Partial pressure of A5.080 kPa
- Partial pressure of B2.280 kPa
- Share of A in the vapour69.02 %
Limitations
- The formula assumes ideal conditions: no friction losses, no air resistance and no efficiency losses unless you enter them.
- The default values are typical reference figures, not measurements of your situation. Replace them with your own data whenever you have it.