Henry's Law - Comparison To Raoult's Law

Comparison To Raoult's Law

For a dilute solution, the concentration of the solute is approximately proportional to its mole fraction x, and Henry's law can be written as:

This can be compared with Raoult's law:

where p* is the vapor pressure of the pure component.

At first sight, Raoult's law appears to be a special case of Henry's law where k_H = p*. This is true for pairs of closely related substances, such as benzene and toluene, which obey Raoult's law over the entire composition range: such mixtures are called "ideal mixtures".

The general case is that both laws are limit laws, and they apply at opposite ends of the composition range. The vapor pressure of the component in large excess, such as the solvent for a dilute solution, is proportional to its mole fraction, and the constant of proportionality is the vapor pressure of the pure substance (Raoult's law). The vapor pressure of the solute is also proportional to the solute's mole fraction, but the constant of proportionality is different and must be determined experimentally (Henry's law). In mathematical terms:

Raoult's law:

Henry's law:

Raoult's law can also be related to non-gas solutes.