In chemistry, Henry's law is one of the gas laws, formulated by William Henry. It states that, at a constant temperature, the amount of a given gas dissolved in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid.

A formula for Henry's Law is:

$\backslash \; e^P\; =\; e^\{kC\}$

where:

• P = the partial pressure of the gaseous solute above the solution
• C = the concentration of the dissolved gas in mol/L
• k = the Henry's Law constant, which has units such as L·atm/mol, atm/(mol fraction) or Pa·m³/mol.

Taking the natural logarithm of the formula, gives us the more commonly used formula:University of Delware physical chemistry lecture

(1)     $P\; =\; kC$

This version is used to showcase the effectiveness of the law for dilute solutions of gases that don't react with the solvent. Some values for k include:

• O₂ : 769.2 L·atm/mol       which converts to 4.27×10⁴ atm/(mol fraction)
• CO₂ : 29.4 L·atm/mol      which converts to 1.63×10⁴ atm/(mol fraction)
• H₂ : 1282.1 L·atm/mol     which converts to 7.12×10⁴ atm/(mol fraction)

when these gases are dissolved in water at 299 kelvins. Note that the Henry's Law constant, k, varies with the solvent and the temperature.

Henry's Law is sometimes written as:University of Arizona chemistry class notesAn extensive list of Henry's Law constants

(2)     $C\; =\; kP$

As can be seen by comparing equations (1) and (2) above, the Henry's Law constant $k$ in equation (2) is simply the inverse of the constant in equation (1). Since both may be referred to as the Henry's Law constant, readers of the technical literature must be quite careful to note which version of the Henry's Law equation is being used.

It should also be noted the Henry's Law is an approximation that only applies for dilute, ideal solutions and for solutions where the liquid solvent does not react chemically with the gas being dissolved.

Henry's law in geophysics

A version of Henry's law applies to the solubility of a noble gas in contact with silicate melt. One equation used is

$$

\rho_m/\rho_g=e^{-\beta(\mu_{{\rm ex},m}-\mu_{{\rm ex},g})}

where:

• subscripts m = melt
• subscript g = gas phase
• $\backslash rho$ = the number densities of the solute gas in the melt and gas phase
• $\backslash beta=1/k\_BT$ an inverse temperature scale
  • $k\_B$ = the Boltzmann constant
• $\backslash mu\_\{\{\backslash rm\; ex\},m\}$ and $\backslash mu\_\{\{\backslash rm\; ex\},g\}$ = the excess chemical potential of the solute in the two phases.

See also

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• Raoult's law
• Dalton's law
• Partial pressure
• Like-A-Fish - Israeli company applying Henry's law to extract air from water

References

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External links

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• www.henrys-law.org - Large compilation of Henry's law constants

• New 'no air tanks' diving system, based on Henry's law - An article with flash presentation

Physical chemistry | Thermodynamics | Chemical engineering | Gas laws

Henry-Gesetz | Loi de Henry | Legge di Henry | ヘンリーの法則 | Wet van Henry | Prawo Henry'ego | Henryn laki