# FEC: Henry's Law

### Henry's Law

Henry's Law is a special case of the general vapor-liquid equilibrium expression(s):

$x_i \gamma_i f_i^o = y_i \hat \phi_i P_{tot}$

##### Definition

Henry's Law is based on the assumptions that the vapor phase behaves as an ideal gas, while the component of interest in the liquid phase behaves as a (Henry) ideal solution. This would be a valid assumption if that component was very dilute or if it had much stronger interactions with the other mixture components than it does with itself.

From the combination of these equations and assumptions, we can replace these terms to yield the following:

$x_i\mathcal{H}_1 = y_iP_{tot}$

##### Note

The left hand side of this "law" is a particularly good assumption for components that are above their critical temperature. Examples would include "gases" (as opposed to vapors) like nitrogen, oxygen, etc. at room temperature.

##### Outcome

Use Henry's Law to calculate VLE for gases dissolved in liquids