What is the mathematical expression for Henry's law?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → p = KH.x
In its most used form, Henry’s law is written with the partial pressure p of the gas on the left and its mole fraction x in the solution on the right:
\( \displaystyle p=K_H\,x \)
KH is Henry’s law constant, which has the unit of pressure and depends on the nature of the gas. The larger KH at a given pressure, the smaller x, so the lower the solubility of the gas.
Why the other options are incorrect
Option (1) m = KH.x — Incorrect. Henry’s law relates the pressure of the gas to its mole fraction; m (molality) does not appear in it.
Option (3) x = KH. p — Incorrect. x is proportional to p, but with KH defined as in p = KHx, the correct rearrangement is x = p/KH, not KHp.
Option (4) p = KH/m — Incorrect. This makes pressure inversely proportional to the amount dissolved, which is the opposite of Henry’s law.
NCERT (2026-27 edition):
Solubility of a Gas in a Liquid – …
