JEE Mains Chemistry · Solutions

Henry's Law and Solubility of Gases

The partial pressure of a gas above a solution equals its Henry constant times its mole fraction in the solution, p = KH·x, and the constant depends on the gas, the solvent and the temperature.

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Why this matters

Eight PYQs, six of them multiple choice, and three from 2026. Four apply p = KH·x to find a mole fraction, a number of millimoles or a mass of dissolved gas; four ask what the Henry constant depends on and why warm water holds less gas. The arithmetic is short, so the marks go on the partial pressure and the units.

Concept 1 of 2: Henry's law, p = KH·x

A gas dissolves until the pressure it exerts from inside the liquid matches its partial pressure outside. Double the partial pressure and twice as much dissolves. The Henry constant

K_H

is the pressure needed per unit mole fraction, so a LARGE

K_H

means a gas that dissolves poorly.

Definition

$p = K_H\,x$ : $p$ is the PARTIAL pressure of the gas, $x$ its mole fraction in the solution.
Partial pressure from air: $p = (\text{mole fraction in air}) \times P_{\text{total}}$ .
Units of $K_H$ are those of pressure; convert $p$ to the same unit first ( $1\ \text{atm} = 760\ \text{mmHg}$ , $1\ \text{kbar} = 1000\ \text{bar}$ ).
Moles dissolved in 1 L of water: water is $1000/18 = 55.56$ mol, and $x$ is tiny, so $n \approx 55.56\,x$ .
Log form: $\log p = \log K_H + \log x$ , a straight line of slope 1 and intercept $\log K_H$ .

Henry's law

p = K_H\,x \qquad n_{\text{gas}} \approx x \times 55.56 \text{ mol per litre of water}

Worked example

Air at a total pressure of 2 atm contains 20 mol%

\mathrm{O_2}

. For

\mathrm{O_2}

in water,

K_H = 4.0 \times 10^{4}

atm. Find the mole fraction of

\mathrm{O_2}

in the water and the millimoles dissolved in 1 L.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2026 · 24 Jan 2026 Shift 2 · Q43Moderate

Example 1 · Solutions · Henry's Law and Solubility of Gases

At 298 K , the mole percentage of

N_{2}(\text{ }g)

in air is

80\%

. Water is in equilibrium with air at a pressure of 10 atm . What is the mole fraction of

N_{2}(\text{ }g)

in water at 298 K ? (

K_{H}

for

N_{2}

6.5 \times10^{7}\text{ }mmHg

)

Use the partial pressure, not the total pressure

Henry's law uses the gas's own partial pressure. For a gas that is 20% of air at 5 atm,

p = 1

atm. Putting 5 atm into

p = K_H x

gives a mole fraction five times too large.

Match the pressure unit to KH

K_H

is in mmHg, convert the partial pressure from atm to mmHg (

\times 760

) before dividing. Dividing atm by mmHg gives an answer 760 times too small, and that wrong value is usually an option.

Drill 3 more on henry's law, p = kh·x

Concept 2 of 2: What the Henry constant depends on

K_H

belongs to a gas-solvent pair at a given temperature. It does not change with concentration while the solution stays dilute. Dissolving a gas gives out heat, so warming the liquid pushes gas out:

K_H

rises with temperature and solubility falls.

Definition

Constant with concentration over the ideally dilute range.
Different for the same gas in different solvents.
Rises with temperature for most gases in water, so solubility falls as the water warms.
Larger $K_H$ at the same partial pressure means a smaller mole fraction dissolved.
Cold water holds more dissolved oxygen than hot water; boiling drives dissolved gases out.

Gas	Temperature	Henry constant in water (kbar)	What it shows
He	293 K	144.97	The largest constant here, so the least soluble gas
$\mathrm{H_2}$	293 K	69.16	About half of helium's constant, so about twice as soluble
$\mathrm{N_2}$	293 K	76.48	Less soluble than oxygen at the same temperature
$\mathrm{N_2}$	303 K	88.84	The constant rises on warming by 10 K, so solubility falls
$\mathrm{O_2}$	293 K	34.86	About 2.2 times as soluble as nitrogen at 293 K
$\mathrm{O_2}$	303 K	46.82	Warmer water holds less oxygen The same gas at two temperatures: the constant is not fixed for a gas.
$\mathrm{CO_2}$	298 K	1.67	A small constant: very soluble, which is why soda water holds so much

A larger constant means a less soluble gas; every gas listed at two temperatures has the larger constant at the higher one.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2026 · 22 Jan 2026 Shift 1 · Q45Moderate

Example 2 · Solutions · Henry's Law and Solubility of Gases

Given below are two statements: Statement I : The Henry's law constant

K_{H}

is constant with respect to variations in solution's concentration over the range for which the solutions is ideally dilute. Statement II:

K_{H}

does not differ for the same solute in different solvents. In the light of the above statements, choose the correct answer from the options.

KH is not a property of the gas alone

The statement 'KH does not differ for the same gas in different solvents' is false. The constant measures how readily the gas dissolves in that particular liquid, so it changes with the solvent and with temperature.

Warm water holds less gas

Dissolving a gas is exothermic, so raising the temperature drives gas out. The Henry constant rises with temperature and the solubility falls; water near 4 °C holds more oxygen than boiling water.

Drill 3 more on what the henry constant depends on

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

Henry's law, p = KH·x
Henry's law
$p = K_H\,x \qquad n_{\text{gas}} \approx x \times 55.56 \text{ mol per litre of water}$

Reference tables (1)

What the Henry constant depends on7 rows

Gas	Temperature	Henry constant in water (kbar)	What it shows
He	293 K	144.97	The largest constant here, so the least soluble gas
$\mathrm{H_2}$	293 K	69.16	About half of helium's constant, so about twice as soluble
$\mathrm{N_2}$	293 K	76.48	Less soluble than oxygen at the same temperature
$\mathrm{N_2}$	303 K	88.84	The constant rises on warming by 10 K, so solubility falls
$\mathrm{O_2}$	293 K	34.86	About 2.2 times as soluble as nitrogen at 293 K
$\mathrm{O_2}$	303 K	46.82	Warmer water holds less oxygen The same gas at two temperatures: the constant is not fixed for a gas.
$\mathrm{CO_2}$	298 K	1.67	A small constant: very soluble, which is why soda water holds so much

A larger constant means a less soluble gas; every gas listed at two temperatures has the larger constant at the higher one.

Watch out for (4)

Use the partial pressure, not the total pressure
→ Henry's law, p = KH·x
Match the pressure unit to KH
→ Henry's law, p = KH·x
KH is not a property of the gas alone
→ What the Henry constant depends on
Warm water holds less gas
→ What the Henry constant depends on

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