Henry was the first to give a quantitative relation between pressure and solubility of a gas in a solvent. Choose the correct statement given by Henry.
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → The partial pressure of the gas in vapour phase is proportional to the mole fraction of the gas in the solution.
Henry’s law relates the pressure of a gas to its solubility. At a constant temperature, the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas above it. Taking the mole fraction (x) of the gas in the solution as the measure of its solubility, the law is written as:
\( \displaystyle p = K_H\,x \)
where \( K_H \) is the Henry’s law constant, which depends on the nature of the gas and the temperature. A graph of p against x is a straight line through the origin with slope \( K_H \). So the statement given by Henry is that the partial pressure of the gas in the vapour phase is proportional to its mole fraction in the solution.
Why the other options are incorrect
Option (1) The partial pressure of the gas in vapour phase is inversely proportional to the mole fraction of the gas in the solution. — Incorrect. An inverse relation would mean that less gas dissolves at higher pressure; in fact the solubility rises with pressure, as when soda water is bottled under high \( \ce{CO2} \) pressure.
Option (3) The total pressure of the gases in vapour phase is proportional to the multiplication of the mole fraction of all the gases present in the solution. — Incorrect. Henry’s law deals with the partial pressure of one gas, not the total pressure, and no gas law multiplies mole fractions together.
Option (4) The total pressure of the gases in vapour phase is proportional to the sum of the mole fraction of all the gases present in the solution. — Incorrect. The mole fractions of all the components add up to 1, so a total pressure proportional to their sum would be a constant; it is the total pressure that equals the sum of the partial pressures (Dalton’s law).
NCERT (2026-27 edition):
Solubility of a Gas in a Liquid – …
