The Henry’s law constant (KH) for CO2 in water at 298 K is 1.67 kbar. What is the mole fraction of CO2 in water when the partial pressure of CO2 above the water is 2.5 bar at 298 K?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 1.5 × 10−3
Henry’s law: the partial pressure of a gas above a solution is proportional to its mole fraction in the solution, p = KH x. Both pressures must be in the same unit, so KH is first changed from kbar to bar.
\( \displaystyle x = \frac{p}{K_H} \)
\( \displaystyle K_H = 1.67\ \mathrm{kbar} = 1670\ \mathrm{bar} \)
\( \displaystyle x = \frac{2.5\ \mathrm{bar}}{1670\ \mathrm{bar}} = 1.5 \times 10^{-3} \)
A mole fraction is a ratio, so it has no unit and must be less than 1.
NCERT (2026-27 edition):
1.3.2 Solubility of a Gas in a Liquid – “…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 = KH x … Here KH is the Henry’s law constant.”