# Go over Henry's law in detail?

Mar 4, 2017

You mean the concepts about it? The equation is pretty easy to remember; it's the concepts that could be hard to get.

There are different versions of Henry's law, so if this one is not familiar, tell me.

$s = {k}_{H} P$

This version, which is taught at my university, has:

• $s$ defined as solubility of the gas in $\text{mol/L}$ (molar concentration), which is basically the quantity of gas that can be dissolved per unit volume.
• ${k}_{H}$ is the Henry's law constant in $\text{mol/L"cdot"atm}$ for that gas dissolved in a particular solvent (it doesn't have to be water, but it usually is).
• $P$ is the vapor pressure of the gas above the solution. That means it's the pressure the solution vapor exerts on the walls of the container.

Essentially what this equation says is that the vapor pressure of a gas above the solution increases with increasing solubility. So some of the trends that come from this relationship are:

• Gases have lower solubilities at higher temperatures, because they move faster at higher temperature, escaping the solution more easily. That reduces their solubility because they are less able to stay in solution.
• Gases have higher solubilities at higher pressures, because the surrounding air particles above the solution are moving around with more force, pushing the gas above the solution down into the solution more. That increases its solubility.

Do note that other versions (mole fraction version, molality version) of Henry's law are out there, where ${k}_{H}$ is defined with different units (sometimes even reciprocal to the one I gave).