Henry's Law

Overview
  • Henry's Law describes the relationship between the partial pressure of a gas within a liquid and its concentration within that liquid (See: Gases in Liquids) for a discussion of gas partial pressures within liquids). Henry's Law states that the concentration of a gas dissolved within a liquid, that is moles of the gas per unit liquid volume, is dependent the partial pressure of the gas within the liquid and the unique chemical properties of the gas and liquid.
Derivation
  • Henry's Law: Cgas = k * Pgas
  • Cgas = Concentration of gas in the liquid (mol/L)
  • k = Empirically-derived solubility constant dependent on the unique chemical nature of the gas and liquid
  • Pgas = Gas partial pressure of gas in the liquid


Comparison of Two Liquids With Different Solubility Coefficients (k)
Henrys law states that the concentration of a gas in a liquid is proportional to its partial pressure in the traditional gas phase surrounding the liquid and the solubility of the liquid in the gas. This last variable is empirically-derived and makes up the solubility coefficient (k) in Henrys Law. As seen above, these two gases have equivalent partial pressures, but radically different concentrations in the same liquid because of their different solubility coefficients.

Significance
  • Henry's Law states that the concentration of a gas within a liquid is proportional to its partial pressure within that liquid. However, Henry's Law also allows us to appreciate that the unique chemical properties of the gas and liquid exert significant influence on how much of the gas actually dissolves into the liquid. This is captured by the "k" term which can be large for some gas-liquid pairs and very small for others. Consequently, the liquid partial pressure of two gases cannot be compared to provide an indication of their relative concentrations within the liquid. For example, two gases may display equivalent partial pressures within a liquid but display highly different liquid concentrations if their solubility constants for the liquid (k) are different.