Chemistry · Solutions · NEET
Henry's Law says that at a fixed temperature, the amount of a gas that dissolves in a liquid is directly proportional to the pressure of that gas above the liquid. Push harder (more pressure) and more gas dissolves. The equation is p = KH · x, where p is the partial pressure of the gas over the solution and x is the mole fraction of the gas dissolved in the liquid. This is why a soda bottle stays fizzy under pressure but goes flat once you open it and the pressure drops.
Higher KH means LESS solubility. This confuses many students. Rearrange the formula: x = p / KH. Because KH is in the denominator, a big KH gives a small x (small solubility). So a gas with a large KH is hard to dissolve. This exact idea is tested in NEET 2024: among gases with KH = 145, 2×10⁻⁵ and 35 kbar, the one with the smallest KH (2×10⁻⁵) is the MOST soluble.
KH has units of pressure, usually kbar (kilobar) or bar or torr, because in p = KH · x, mole fraction x has no units, so KH must carry the same units as pressure p. When you compare KH values of different gases, always compare at the SAME temperature, because KH changes with temperature.
Henry's Law is for a GAS dissolving in a liquid: p = KH · x. Raoult's Law is for the vapour of a liquid component: p = p°·x, where p° is the pure component's vapour pressure. They look similar, and in fact Henry's Law is a special case of Raoult's Law where the constant KH replaces p°. Use Henry's Law when the question is about a gas (like O₂, CO₂, N₂) dissolving in water; use Raoult's Law for volatile liquid mixtures.
When you increase the pressure of a gas above a liquid, more gas molecules are pushed into the liquid surface, so more of them dissolve. Since p = KH · x, if p goes up and KH stays fixed (same temperature), then x (solubility) must go up too. This is used in making soft drinks fizzy: CO₂ is sealed under high pressure so a lot of it dissolves.
Rearrange the formula to x = p / KH. Take the partial pressure of the gas (in the same units as KH), divide by KH, and you get the mole fraction of the gas dissolved in the liquid. For example, if a gas has partial pressure 0.5 bar and KH = 100 bar, then x = 0.5 / 100 = 0.005.
The Henry's law constant (KH) values of three gases (A, B, C) in water are 145, 2×10⁻⁵ and 35 kbar, respectively. The solubility of these gases in water follows the order:
In water-saturated air the mole fraction of water vapour is 0.02. If the total pressure of the saturated air is 1.2 atm, the partial pressure of dry air is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
p = KH · x, where p is the partial pressure of the gas above the solution, x is the mole fraction (solubility) of the gas in the liquid, and KH is the Henry's Law constant, which has units of pressure.
Lower solubility. Because x = p / KH, a larger KH gives a smaller mole fraction x, so the gas dissolves less. This is directly tested in NEET 2024.
KH increases as temperature rises. A higher KH means lower solubility, so gases become LESS soluble in warmer liquids. That is why warm soda goes flat faster.
Yes. Henry's Law is a special case of Raoult's Law. In Raoult's Law p = p°·x, and in Henry's Law the constant KH simply replaces p° for a dissolved gas.
Inside, CO₂ is sealed under high pressure, so a lot of it dissolves (high p means high x). When you open it, the pressure drops sharply, so x must drop too — the extra CO₂ escapes as fizz.
