Raoult's Law
Definition and meaning of Raoult's Law in chemistry.
Raoult's Law states that the partial vapor pressure of a component in an ideal solution equals the vapor pressure of that pure component multiplied by its mole fraction in the solution. Mathematically, this is expressed as P_i = x_i P_i°, where P_i is the partial vapor pressure, x_i is the mole fraction, and P_i° is the vapor pressure of the pure component.
In more detail
Raoult's Law describes how the presence of a solute reduces the vapor pressure of a solvent in ideal solutions. In ideal solutions, each volatile component behaves independently, contributing to the total vapor pressure according to its mole fraction, consistent with Dalton's Law. This law is fundamental to understanding colligative properties, including boiling point elevation, freezing point depression, and osmotic pressure. Real solutions typically deviate from Raoult's Law because intermolecular forces between different molecular species differ from forces between identical molecules.
Key facts
| Mathematical formula | P_i = x_i P_i° |
|---|---|
| Applies to | Ideal solutions; volatile components |
| Field | Physical Chemistry |
| Named after | François-Marie Raoult (1887) |
In a solution of benzene and toluene at 25°C, if pure benzene has a vapor pressure of 95 mmHg and pure toluene has 28 mmHg, then a 60% benzene solution (by mole fraction) would have a partial pressure of benzene of 57 mmHg (0.60 × 95) and a partial pressure of toluene of 11.2 mmHg (0.40 × 28).
Frequently asked questions
Does Raoult's Law apply to nonvolatile solutes?
Technically yes, but the effect is immeasurable since nonvolatile solutes have negligible vapor pressure.
When do real solutions deviate from Raoult's Law?
Deviations occur when intermolecular forces between unlike molecules differ significantly from forces between like molecules, creating non-ideal behavior.