Which law states that the partial pressure of a gas in vapour phase varies…

Concept

Henry’s law describes how a gas dissolves in a liquid: for a dilute solution, the partial pressure of the gas in the vapour phase above the solution is directly proportional to the mole fraction of that gas dissolved in the solution, p = K_H × x, where K_H (the Henry’s law constant) depends on the gas-solvent pair and the temperature.

Application

The stem describes exactly this relationship — partial pressure of a gas in the vapour phase varying directly with its mole fraction in the solution. That direct proportionality between a dissolved gas’s vapour-phase partial pressure and its solution-phase mole fraction is the defining statement of Henry’s law.

Cross-check / Contrast

Checking this against the other named gas/solution laws confirms the match and rules out each alternative by what it actually governs:

• Boyle’s law relates the pressure and volume of a fixed amount of gas at constant temperature (P ∝ 1/V); it says nothing about a solution or mole fraction.

• Charles’s law relates the volume and temperature of a fixed amount of gas at constant pressure (V ∝ T); it also has no solution or mole-fraction term.

• Raoult’s law relates the vapour pressure of a volatile component of a solution to its own mole fraction using that pure component’s vapour pressure, p_i = x_i × p_i° — it uses the component’s pure vapour pressure as the proportionality factor, not a solute-specific Henry constant, and is applied to volatile liquid components of a solution rather than a dissolved gas.

• Only the gas-dissolved-in-liquid, proportionality-constant (K_H) formulation in the stem matches Henry’s law.