Henry's Law

Introduction to Henry's Law

Henry's Law describes the relationship between the solubility of a gas in a liquid and the partial pressure of that gas above the liquid. This law is fundamental in understanding how gases dissolve in solutions, which is important in various chemical and biological processes.

• Solubility refers to the concentration of a dissolved gas in a liquid.

• Partial pressure is the pressure exerted by a single gas in a mixture of gases.

• Henry's Law Constant (kH) is a proportionality constant specific to each gas-solvent pair and temperature.

Henry's Law Formula

The basic form of Henry's Law is:

• Formula:

• Sgas: Solubility of the gas in molarity (mol/L)

• kH: Henry's Law Constant in mol/(L·atm)

• Pgas: Partial pressure of the gas in atm

Example

Calculate the solubility of carbon dioxide gas, CO2, when its Henry's Law Constant is M/atm at 3.29 atm.

• Solution: M

Henry's Law Formula (Two Point Form)

When comparing the solubility of a gas at two different pressures, the two-point form of Henry's Law is used. This is especially useful when the pressure or solubility changes and you need to find the new value.

• Formula:

• S1: Initial solubility of the gas

• P1: Initial partial pressure of the gas

• S2: Final solubility of the gas

• P2: Final partial pressure of the gas

Example

At a pressure of 2.88 atm the solubility of dichloromethane, CH2Cl2, is 0.384 mg/L. If the solubility decreases to 0.225 mg/L, what is the new pressure?

• Solution: atm

Practice Problems

• Problem 1: Henry's Law Constant for nitrogen in water is M·atm-1. If a closed canister contains 0.103 M nitrogen, what would its pressure be in atm? Solution: atm

• Problem 2: At 0°C and 1.00 atm, as much as 0.84 g of O2 can dissolve in 1.0 L of water. At 0°C and 4.00 atm, how many grams of O2 dissolve in 1.0 L of water? Solution: g

• Problem 3: The atmospheric pressure in a lab is calculated as 1.3 atm. If oxygen gas contributes 62% of this atmospheric pressure, determine its mass (g) dissolved at room temperature in 2.5 L of water. The Henry's Law Constant for oxygen in water at this temperature is M/atm. Solution:

  • Partial pressure of O2: atm

  • Solubility: M

  • Moles in 2.5 L: mol

  • Mass: g

Summary Table: Henry's Law Variables

Additional info: Henry's Law is widely used in environmental chemistry (e.g., gas exchange in water bodies), medicine (e.g., oxygen transport in blood), and industrial processes (e.g., carbonation of beverages).