Last updated: 2026-06-15

Understanding Raoult's law

Raoult's law describes how the vapor pressure of an ideal solution depends on composition. When a non-volatile solute dissolves in a liquid, the solvent's vapor pressure drops in proportion to how much solute is present. The law connects this pressure to the mole fraction of the solvent:

where $P$ is the vapor pressure of the solution, $x_{\text{solvent}}$ is the mole fraction of the solvent, and $P^{\circ}$ is the vapor pressure of the pure solvent at the same temperature.

Vapor-pressure lowering

Because $x_{\text{solvent}} < 1$ whenever any solute is present, the vapor pressure of the solution is always less than that of the pure solvent. The drop is called vapor-pressure lowering:

Since $x_{\text{solvent}} + x_{\text{solute}} = 1$, the lowering equals the mole fraction of solute times the pure vapor pressure. This is a colligative property — it depends only on the number of solute particles, not on their chemical identity.

Worked example

A solution is prepared by dissolving glucose ($M = 180\ \text{g/mol}$) in water ($M = 18\ \text{g/mol}$). The mixture contains 18 g of glucose and 90 g of water at 25 °C, where the vapor pressure of pure water is 23.8 mmHg. Find the vapor pressure of the solution and the vapor-pressure lowering.

Step 1 — find moles:

Step 2 — find mole fraction of solvent:

Step 3 — apply Raoult's law:

Step 4 — find the lowering:

Raoult's law and other colligative properties

Vapor-pressure lowering is the starting point for three other colligative effects:

• Boiling-point elevation: a lower vapor pressure means the solution must be heated to a higher temperature before its vapor pressure reaches atmospheric pressure.
• Freezing-point depression: the reduced vapor pressure shifts the solid–liquid equilibrium, lowering the freezing point.
• Osmotic pressure: when a solution is separated from pure solvent by a semipermeable membrane, the difference in vapor pressure drives solvent across the membrane until osmotic pressure builds up to compensate.

All four effects depend on $x_{\text{solute}}$ (or equivalently, on molality), not on the nature of the solute.

When Raoult's law holds — and when it does not

Raoult's law is exact for ideal solutions, in which solute–solvent interactions are the same strength as solvent–solvent interactions. In practice, the law works well for:

• Dilute solutions of non-electrolytes (sugars, urea, alcohols in a compatible solvent)
• Mixtures of structurally similar liquids (benzene and toluene)

It breaks down for concentrated solutions, for electrolytes (which release multiple ions per formula unit), and for solutes that associate or react with the solvent. For electrolytes, the effective solute particle count is increased by the van't Hoff factor $i$, so $x_{\text{solute,eff}} = i \cdot x_{\text{solute,nominal}}$.

Vapor pressures of common solvents at 25 °C

These values let you compute the vapor pressure of a dilute solution directly: enter the mole fraction and the pure-solvent vapor pressure from the table above.