In Henry's law, the partial pressure of gas in the vapor phase is pro…

The correct answer is Option (1) → Mole fraction of gas in solution

Henry’s law states that at a constant temperature, the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas above the liquid. Taking the mole fraction of the gas in the solution as the measure of its solubility, the most commonly used form of the law is: the partial pressure of the gas in the vapour phase (p) is proportional to its mole fraction (x) in the solution:

    \( \displaystyle p = K_{H}\,x \)

Here \( K_H \) is the Henry’s law constant, which depends on the nature of the gas and on temperature. The higher the value of \( K_H \) at a given pressure, the lower the solubility of the gas.

Why the other options are incorrect

• Option (2) Normality of gas in solution — Incorrect. Normality is used for acids, bases and redox reagents in titrations; it is not used to express the solubility of a gas in Henry’s law.

• Option (3) Volume of gas in solution — Incorrect. It is the amount of gas (moles or mass) dissolved that rises with pressure. The volume of the dissolved gas, measured at the pressure under which it dissolved, stays about the same, because the gas is compressed as much as the extra amount dissolves.

• Option (4) Molality of gas in solution — Incorrect. For a very dilute solution the molality is roughly proportional to the mole fraction, but the law in its standard form, \( p = K_H\,x \), is written with the mole fraction.

NCERT (2026-27 edition):
Solubility of a Gas in a Liquid – “… If we use the mole fraction of a gas in the solution as a measure of its solubility, then it can be said that the mole fraction of gas in the solution is proportional to the partial pressure of the gas over the solution. The most commonly used form of Henry’s law states that ‘the partial pressure of the gas in vapour phase (p) is proportional to the mole fraction of the gas (x) in the solution’ and is expressed as: p = K_H x”