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Henry's Law

    William Henry, an English chemist, showed that at 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. This is known as Henry's Law.

    1. 1. Introduction
    2. 2. Things to Note
    3. 3. References
    4. 4. Outside Links
    5. 5. Contributors

    Introduction

    To explain this law, he used 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)

    Example  

    Step 1: Find Henry's Law Constant (k)

    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  

    Step 2: Compute the molar solubility in water that is saturated with air

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

    Contributors

    •  Krystianne Yamuni (UCD)

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