Which law describes the dependence of gas solubility on the partial pressure of the gas above the liquid?

• A. Raoult's Law
• B. Mole fraction
• C. Henry's Law
• D. Boiling Point Elevation

Answer: (C)

Gas solubility in a liquid at a fixed temperature depends on how much partial pressure of that gas exists above the liquid. Henry's Law states that the amount of gas dissolved is proportional to this partial pressure: S ∝ P_g, so S = k_H P_g, where S is the dissolved concentration and k_H is Henry’s constant for the particular gas–solvent pair. This means that increasing the partial pressure of the gas above the liquid pushes the equilibrium toward more gas dissolving, while lowering the pressure reduces solubility. The proportionality constant depends on temperature, and the law applies best to dilute solutions where gas molecules interact minimally with each other.

This concept is different from Raoult’s Law, which relates to vapor pressures in liquid mixtures, and from boiling point elevation, a colligative property. It’s also not a simple mole fraction statement, but a specific relationship tying solubility to the external gas pressure.