Raoult's Law Calculator — Vapour Pressure of Binary Solutions
Enter the moles and pure vapour pressures of two components to get the total vapour pressure, partial pressures, and vapour-phase composition using Raoult's Law for ideal binary solutions.
mol
mol
mmHg
mmHg
P_total = x_A × P°_A + x_B × P°_B
- 1
Total moles
3 + 7 = 10 - 2
Mole fraction A (x_A)
3 ÷ 10 = 0.3 - 3
Mole fraction B (x_B)
7 ÷ 10 = 0.7 - 4
Partial pressure A
0.3 × 40 = 12 mmHg - 5
Partial pressure B
0.7 × 80 = 56 mmHg - 6
Total vapour pressure
12 + 56 = 68 mmHgRaoult's Law: P_total = x_A × P°_A + x_B × P°_B
How does this calculator work?
Raoult's Law: P_total = x_A × P°_A + x_B × P°_B, where x_i = n_i / (n_A + n_B). Enter moles and pure vapour pressures to get total pressure, partial pressures, and vapour-phase mole fractions. Valid for ideal binary solutions only; real mixtures deviate. Pure vapour pressures must be at the same temperature.
Formula
How this is calculated
Raoult's Law states that the partial vapour pressure of each component in an ideal mixture equals the vapour pressure of the pure component multiplied by its mole fraction in the liquid. For a binary mixture of A and B: P_total = x_A × P°_A + x_B × P°_B, where x_A = n_A/(n_A + n_B) and x_B = n_B/(n_A + n_B) are the liquid-phase mole fractions, and P°_A, P°_B are the pure-component vapour pressures measured at the same temperature.
The partial pressure of each component is p_i = x_i × P°_i, and the total pressure is their sum. The vapour-phase composition (y_A, y_B) follows from Dalton's law: y_i = p_i / P_total. Because the more volatile component (higher P°) contributes proportionally more to the vapour, the vapour is always richer in that component than the liquid — the principle behind fractional distillation.
Raoult's Law holds strictly for ideal solutions where A–B interactions equal A–A and B–B interactions (e.g., benzene–toluene, or any two chemically similar liquids). Mixtures with stronger or weaker unlike-pair interactions deviate positively or negatively from ideality and can form azeotropes; this calculator does not model those deviations. Pure-component vapour pressures depend on temperature (Antoine equation) and must be looked up at the operating temperature.
Frequently asked questions
Raoult's Law states that the partial vapour pressure of a component in an ideal solution equals its mole fraction times its pure-component vapour pressure (P_i = x_i × P°_i). The total vapour pressure is the sum of all partial pressures.
The vapour-phase mole fraction y_i = x_i × P°_i / P_total. The component with the higher P° contributes a larger partial pressure, so it makes up a larger share of the vapour than of the liquid. This enrichment is the basis of distillation.
It fails for non-ideal solutions where unlike-pair interactions differ from like-pair interactions. Positive deviations (e.g., ethanol–water) give higher total pressures than predicted; negative deviations give lower pressures. Extreme deviations produce azeotropes that cannot be separated by simple distillation.
Also known as
TG we-Calculate Editorial Team. (2026). Raoult's Law Calculator — Vapour Pressure of Binary Solutions [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/raoults-law-calculator
TG we-Calculate Editorial Team. "Raoult's Law Calculator — Vapour Pressure of Binary Solutions." TG we-Calculate. 2026. https://we-calculate.com/calculator/raoults-law-calculator.
TG we-Calculate Editorial Team, "Raoult's Law Calculator — Vapour Pressure of Binary Solutions," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/raoults-law-calculator
@misc{wecalculate_raoults_law_calculator, title = {Raoult's Law Calculator — Vapour Pressure of Binary Solutions}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/raoults-law-calculator}}, year = {2026}, note = {TG we-Calculate} }
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