# Raoult's Law Calculator

Free Raoult's Law calculator. Calculate vapor pressure of solutions, mole fractions, and vapor pressure lowering using P = x × P°.

## What this calculates

Calculate the vapor pressure of a solution using Raoult's Law: P = x_solvent × P°. Adding a non-volatile solute lowers the vapor pressure proportional to its mole fraction.

## Inputs

- **Solve For** — options: Solution Vapor Pressure (P), Mole Fraction of Solvent (x), Pure Solvent Vapor Pressure (P°), Vapor Pressure Lowering (ΔP) — Select what to calculate.
- **Pure Solvent Vapor Pressure (P°)** (mmHg) — min 0 — Vapor pressure of the pure solvent (e.g., water at 25°C = 23.8 mmHg).
- **Mole Fraction of Solvent (x)** — min 0, max 1 — Mole fraction of the solvent in the solution.
- **Solution Vapor Pressure (P)** (mmHg) — min 0 — Vapor pressure of the solution.
- **Moles of Solvent (optional)** (mol) — min 0 — Enter to calculate mole fraction automatically.
- **Moles of Solute (optional)** (mol) — min 0 — Enter to calculate mole fraction automatically.

## Outputs

- **Result** — The calculated value.
- **Unit** — formatted as text — Unit of the result.
- **Vapor Pressure Lowering (ΔP)** (mmHg) — How much the vapor pressure is lowered by the solute.
- **Formula** — formatted as text — Step-by-step calculation.

## Frequently Asked Questions

**Q: What is Raoult's Law?**

A: Raoult's Law states that the vapor pressure of a solution equals the mole fraction of the solvent times the vapor pressure of the pure solvent: P = x × P°. Adding solute always lowers the vapor pressure.

**Q: What is an ideal solution?**

A: An ideal solution perfectly obeys Raoult's Law. The solute-solvent interactions are the same strength as solvent-solvent and solute-solute interactions. Most real solutions show deviations.

**Q: What is vapor pressure lowering?**

A: ΔP = x_solute × P°. The decrease in vapor pressure is proportional to the mole fraction of the solute. This is a colligative property -- it depends only on the number of solute particles, not their identity.

**Q: Does Raoult's Law work for electrolytes?**

A: For electrolytes, you must account for dissociation using the van't Hoff factor (i). NaCl produces 2 particles, so its effective mole fraction of solute particles is doubled.

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Source: https://vastcalc.com/calculators/chemistry/raoults-law
Category: Chemistry
Last updated: 2026-04-21