Raoult’s Law Calculator

Compute partial vapor pressures, total vapor pressure, and vapor-phase composition of ideal liquid solutions using Raoult’s law. Supports binary and multi‑component mixtures with up to 5 components.

Raoult’s Law – Interactive Calculator

Use the same unit for all pure-component vapor pressures.

You can model binary or multi‑component ideal solutions.

Component Label Liquid mole fraction xi Pure vapor pressure Pisat

How to use this Raoult’s law calculator

  1. Choose the pressure unit (kPa, bar, atm, mmHg, or torr). All vapor pressures must use the same unit.
  2. Select the number of components in your liquid solution (1–5).
  3. For each component, enter:
    • a label (e.g., “benzene”, “toluene”);
    • its liquid mole fraction \(x_i\);
    • its pure-component vapor pressure \(P_i^{\text{sat}}\) at the working temperature.
  4. Click Calculate to obtain:
    • partial vapor pressures \(P_i\);
    • total vapor pressure \(P_{\text{total}}\);
    • vapor-phase mole fractions \(y_i\).
  5. Use Reset to clear inputs and restore the default binary example.

Raoult’s law – formula

Raoult’s law for component i in an ideal solution:

\[ P_i = x_i \, P_i^{\text{sat}} \]

  • \(P_i\) = partial vapor pressure of component i
  • \(x_i\) = liquid mole fraction of component i
  • \(P_i^{\text{sat}}\) = vapor pressure of pure component i at the same temperature

Total vapor pressure:

\[ P_{\text{total}} = \sum_i P_i = \sum_i x_i \, P_i^{\text{sat}} \]

Vapor-phase mole fraction:

\[ y_i = \frac{P_i}{P_{\text{total}}} \]

Worked example

Consider an ideal binary solution of benzene (1) and toluene (2) at 25 °C:

  • \(x_1 = 0.40\), \(P_1^{\text{sat}} = 100\ \text{kPa}\)
  • \(x_2 = 0.60\), \(P_2^{\text{sat}} = 30\ \text{kPa}\)

Partial pressures:

\[ P_1 = 0.40 \times 100 = 40\ \text{kPa} \] \[ P_2 = 0.60 \times 30 = 18\ \text{kPa} \]

Total vapor pressure:

\[ P_{\text{total}} = 40 + 18 = 58\ \text{kPa} \]

Vapor composition:

\[ y_1 = \frac{40}{58} \approx 0.69,\quad y_2 = \frac{18}{58} \approx 0.31 \]

The vapor is richer in benzene than the liquid because benzene is more volatile (higher \(P^{\text{sat}}\)).

Assumptions and limitations

  • Ideal solution: intermolecular interactions between unlike molecules are similar to those between like molecules.
  • Same temperature: all \(P_i^{\text{sat}}\) values correspond to the same temperature.
  • Low to moderate pressure: gas phase behaves nearly ideally.
  • No activity coefficients: non‑ideal behavior (γi ≠ 1) is not included. For strongly non‑ideal systems, use a γ–φ model instead of pure Raoult’s law.

Frequently asked questions

What does this Raoult’s law calculator output?

For each component, it returns the normalized liquid mole fraction \(x_i\), pure vapor pressure \(P_i^{\text{sat}}\), partial pressure \(P_i\), and vapor-phase mole fraction \(y_i\), plus the total vapor pressure of the mixture.

Do my mole fractions have to sum to 1?

Ideally yes, but the calculator can normalize them for you. If the “Normalize liquid mole fractions” option is checked, the entered \(x_i\) values are scaled so that \(\sum_i x_i = 1\).

Can I mix different pressure units?

No. Raoult’s law is linear in pressure, so all \(P_i^{\text{sat}}\) must be in the same unit. The calculator assumes a single consistent unit and reports results in that unit.