# Raoult's Law Calculator

p_i = x_i·p_i* for ideal solutions, with total pressure and vapour-phase composition for binary mixtures.

> Canonical page: https://elysiatools.com/en/tools/raoult-law-calculator

- **Category:** Science & Education

- **Keywords:** raoult law, partial pressure, vapour pressure, ideal solution, mole fraction, distillation

## Overview

The Raoult's Law Calculator computes partial vapour pressures, total mixture vapour pressure, and vapour-phase mole fractions for ideal solutions. Whether evaluating vapour pressure lowering from non-volatile solutes or analyzing binary liquid mixtures for distillation separation, it delivers exact thermodynamic calculations instantly.

## Inputs

- **Mode** (select)
- **Mole fraction of component A, x_A** (number): e.g. 0.5 for an equimolar mix
- **Vapour pressure of pure A, p_A*** (number): e.g. 95.1 torr (benzene, 25 °C)
- **Vapour pressure of pure B, p_B*** (number): e.g. 28.4 torr (toluene, 25 °C)
- **Mole fraction x of the solvent** (number): e.g. 0.85 (single-component mode)
- **Vapour pressure of the pure liquid p*** (number): e.g. 23.76 torr (water, 25 °C)

## When to use

- Calculating total vapour pressure and vapour-liquid equilibrium compositions for binary ideal mixtures.
- Evaluating vapour-pressure lowering caused by non-volatile solutes dissolved in a pure solvent.
- Estimating top-vapour component enrichment when designing or modeling distillation columns.

## How it works

- Select your calculation mode: single component for solvent vapour lowering, or binary mixture for two-component equilibrium.
- Input the liquid-phase mole fraction (x or x_A) and pure-liquid vapour pressures (p* or p_A* and p_B*) at your operating temperature.
- The tool applies Raoult's law (p_i = x_i · p_i*) and Dalton's law to calculate partial pressures, total pressure, and vapour-phase mole fractions.

## Use cases

- Chemical engineering students solving physical chemistry equilibrium and distillation problem sets.
- Process technicians modeling binary solvent separation across fractional distillation stages.
- Laboratory researchers predicting solvent evaporation rates and colligative properties in solute solutions.

## Frequently asked questions

### What is Raoult's law?

Raoult's law states that the partial vapour pressure of a component in an ideal solution is equal to its liquid mole fraction multiplied by its pure-component vapour pressure.

### How is vapour-phase composition calculated?

Using Dalton's law alongside Raoult's law, the vapour mole fraction of component A is computed as y_A = p_A / p_total.

### What pressure units can I use?

You can use any pressure unit (such as torr, mmHg, kPa, or bar) as long as you input all pure-component vapour pressures in the same unit.

### What does single component mode calculate?

It calculates the lowered vapour pressure of a solvent containing a non-volatile solute and reports the total vapour-pressure drop (Δp = (1 - x) · p*).

### When does a liquid mixture follow Raoult's law?

A mixture follows Raoult's law when intermolecular forces between dissimilar molecules are similar to those between like molecules, behaving as an ideal solution.

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