Henry's law
Summary
Henry's law says that at constant temperature, the concentration of a gas dissolved in a liquid is proportional to that gas's partial pressure above the liquid. Doubling the partial pressure doubles the dissolved amount, in dilute solutions where the gas does not react much with the solvent.
Detail
Meaning: At equilibrium, the rate of gas molecules entering a liquid equals the rate leaving. Raising the gas's partial pressure increases the entry rate, so more gas dissolves. Each gas in a mixture follows its own partial pressure. The law works best at constant temperature, for dilute solutions, and for gases that do not react extensively with the solvent.
Equation: C = kH × P, where C is the dissolved gas concentration (mol/m³), kH is the Henry's law constant for that gas, solvent and temperature (mol/(m³·Pa)), and P is the partial pressure of the gas above the liquid (Pa). At fixed temperature, C₁/P₁ = C₂/P₂.
On the MCAT: Typical setups are a carbonated drink (opening the bottle lowers the CO₂ pressure above the liquid, so gas leaves and bubbles form), O₂ or CO₂ dissolving in blood plasma, or a diver at depth. You predict the direction of change or use the ratio form: triple the partial pressure and the dissolved concentration triples. Temperature questions are common too, since warm soda loses its fizz faster.
Common confusion: Students use total pressure instead of the partial pressure of the specific gas. Also, kH is not a universal constant. It changes with temperature, and for most gases in water, solubility decreases as temperature rises, unlike many solid solutes. Do not mix it up with Raoult's law, which concerns the vapor pressure of a solvent.
Sources
- Chemistry: The Central Science
- Chemistry: Atoms First
- General Chemistry: Principles and Modern Applications
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.