# Henry's Law

Henry's law is one of the gas laws formulated by William Henry in 1803. It states: "At a constant temperature, the amount of a given gas that dissolves in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid." An equivalent way of stating the law is that the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas above the liquid.

To explain this law, Henry derived the equation:

$C =k P_{gas}$

where

• C is the solubility of a gas at a fixed temperature in a particular solvent (in units of M or mL gas/L)
• k is Henry's law constant (often in units of M/atm)
• Pgas is the partial pressure of the gas (often in units of Atm)

Figure 1: Positive non-ideal behavior of the vapor pressure of a solution follows Henry's Law at low concentrations an Raoult's Law at high concentrations (pure).

Example 1

To solve for the constant, we use our givens: CNe=23.5 mL/L solution and STP volume (22,414 mL/mole gas) and pressure (1 atm).

• Now we can rearrange our equation from above to solve for the constant: k= C/PNe.
• To use C we must convert 23.5 mL/L solution to Molarity. Since Ne is a gas, we can use our standard molar volume. Thus giving us: (23.5 mL/L soln) (1 mole Ne/22,414 mL)= 0.00105M.
• Now we have solved for the solubility of Ne in the solution. C= 0.00105M and we know the pressure at STP is 1 atm, so we can now use our rearranged equation: k= C/PNe.
• Where C= 0.00105M, PNe = 1 atm, thus giving us k=0.00105 M/atm
Example 2

Compute the molar solubility in water that is saturated with air.

Solution

To solve the final part of this question, we must use Henry's law again, with different givens.

This time, we need to use constant (k) that we just calculated and our PNe in air.

$C =k P_{gas}$

C=(0.00105 M/atm)(0.0341 atm)

C=3.58 x 10-5 M

### Things to Note

• Henry's law only works if the molecules are at equilibrium and the same molecules are present throughout the solution. As seen above, the only gas accounted for is Neon. It is not possible to mix gases and still use Henry's law.
• Henry's law does not work for gases at high pressures (e.g., N2(g) at high pressure becomes very soluble and harmful when in the blood supply).
• Henry's law does not work if there is a chemical reaction between the solute and solvent (e.g., HCl(g) reacts with water in the dissociation reaction and affects the solubility).

### References

1. Zumdahl, Steven and Susan. Chemistry. Fifth Edition. Boston, MA: Houghton Mifflin Company, 2000.
2. Petrucci, Ralph, and William Harwood. F. Geoffrey Herring. Jeffry Madura. General Chemistry: Principles and Modern Applications. 9th ed. Upper Saddle River, NJ: Pearson, 2007.

### Contributors

•  Krystianne Yamuni (UCD)

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http://www.chemengr.ucsb.edu/~ceweb/courses/che128/pdf/090733%20Avoid%20Common%20Pitfalls.pdf Edited 20:35, 21 Oct 2013
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