Henry's Law
Henry's Law states that the amount of a gas dissolved in a liquid at constant temperature is directly proportional to the partial pressure of that gas above the liquid. It applies to dilute solutions of gases that do not react chemically with the solvent. Henry's Law explains why carbonated beverages fizz when opened (pressure release), and is critical in understanding oxygen transport in blood, gas exchange in lungs, and environmental fate of volatile compounds.
Key Formula
C = k_H × p
LaTeX: C = k_H \cdot p
| Symbol | Meaning | Unit |
|---|---|---|
| C | Concentration of dissolved gas | mol/L or mol/m³ |
| k_H | Henry's law constant (solubility coefficient, temperature-dependent) | mol/(L·atm) |
| p | Partial pressure of the gas above the solution | atm or Pa |
Worked Example
Problem
The Henry's law constant for CO₂ in water at 25 °C is 3.4 × 10⁻² mol/(L·atm). What is the concentration of dissolved CO₂ when the partial pressure of CO₂ above the solution is 0.0004 atm (atmospheric CO₂)?
Solution
Step 1: Use Henry's Law: C = k_H × p. Step 2: Substitute values: C = (3.4 × 10⁻² mol/L·atm) × (0.0004 atm). Step 3: Calculate: C = 1.36 × 10⁻⁵ mol/L.
Answer
Dissolved CO₂ concentration ≈ 1.36 × 10⁻⁵ mol/L
Henry's Law Constants for Common Gases in Water at 25 °C
| Gas | Formula | k_H (mol/L·atm) | Application |
|---|---|---|---|
| Oxygen | O₂ | 1.3 × 10⁻³ | Blood oxygenation, aquatic life |
| Carbon dioxide | CO₂ | 3.4 × 10⁻² | Carbonated drinks, ocean acidification |
| Nitrogen | N₂ | 6.5 × 10⁻⁴ | Decompression sickness (diving) |
| Hydrogen | H₂ | 8.1 × 10⁻⁴ | Hydrogen fuel cell research |
| Carbon monoxide | CO | 9.5 × 10⁻⁴ | Toxicology, environmental monitoring |
Interactive Tools
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Related Terms
Solubility
Solubility is the maximum amount of a substance (solute) that can dissolve in a given quantity of solvent at a specified temperature and pressure to form a homogeneous solution. Beyond this limit, the solution is saturated and excess solute remains undissolved. Solubility depends on the chemical nature of solute and solvent, temperature (usually increases for solids in liquids, decreases for gases), and pressure (significant only for gases, governed by Henry's Law).
Mole Fraction
Mole fraction (χ) is a dimensionless concentration unit expressing the ratio of the moles of one component to the total moles of all components in a mixture. The sum of all mole fractions in a mixture always equals exactly 1. Mole fraction is used in Raoult's law, Dalton's law of partial pressures, and chemical thermodynamics because it is independent of temperature and pressure.
Molarity
Molarity (M) is the most common measure of solution concentration, defined as the number of moles of solute dissolved per litre of solution. It is temperature-dependent because liquid volumes change with temperature. Molarity is widely used in titrations, reaction stoichiometry involving solutions, and the preparation of standard laboratory solutions.
Named after English chemist William Henry (1774–1836), who first quantified the relationship between gas pressure and dissolution in a liquid in his 1803 paper published in the Philosophical Transactions of the Royal Society.