Henry's Law

Henry's Law: At constant temperature, the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas above the liquid.

$p = K_H \cdot x$

where:

• $p$ = partial pressure of the gas above the solution
• $x$ = mole fraction of the dissolved gas in the solution
• $K_H$ = Henry's law constant (units: pressure, e.g. bar or Pa)

Important: $K_H$ has a large value for gases that are less soluble (like $\ce{N2}$, $\ce{O2}$). A small $K_H$ means the gas dissolves easily at low pressure (like $\ce{CO2}$).

Henry's law holds strictly only under these conditions:

1. Low pressure — the gas must not interact strongly with the solvent
2. Low concentration of dissolved gas
3. The gas must not react with the solvent — $\ce{HCl}$ ionises in water ($\ce{HCl + H2O -> H3O+ + Cl-}$), so it does not obey Henry's law. $\ce{CO2}$ partially reacts ($\ce{CO2 + H2O -> H2CO3}$) so it shows deviations at higher concentrations.
4. The gas must not associate in solution