# Solubility of Gases and Henry's Law > Henry's law and gas solubility in JEE Chemistry: p = KH x, NCERT KH values, temperature effect, scuba and soda applications and worked numericals. - Canonical URL: https://prepelephant.com/topics/jee/chemistry/solubility-henry-law - Exam / course: JEE · Subject: Chemistry - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Solubility of Gases and Henry's Law", PrepElephant, https://prepelephant.com/topics/jee/chemistry/solubility-henry-law ## Direct answer The mass of a gas dissolving in a given volume of solvent at constant temperature is directly proportional to the pressure of that gas above the solvent — Henry's law, written p = K_H × x, where x is the mole fraction of the dissolved gas. A large K_H means low solubility, which is the point most students invert under exam pressure. K_H itself rises sharply with temperature, so gases flee from warm water — which is why aquatic life concentrates in cold, oxygen-rich water and why a warm soda goes flat. The law holds for dilute solutions at low pressure and only when the gas does not react chemically with the solvent, which is why ammonia, with its proton-hungry lone pair, is a poor candidate. ## What you must remember - **The equation:** p = K_H x; rearranged, x = p/K_H, so solubility is inversely proportional to K_H — ranking questions live on this inversion. - **NCERT values at 293 K (kbar):** helium 144.97, nitrogen 76.48, oxygen 34.86 — helium is the least soluble and oxygen the most soluble of the three, numbers worth memorising outright. - **Temperature dependence:** K_H increases with temperature, solubility falls; thermal pollution of rivers is the standard application JEE Main quotes. - **Applications:** carbonated drinks bottled under high CO2 pressure; scuba cylinders diluted with helium (low solubility plus no nitrogen narcosis) to avoid the bends; deep water anoxia. - **Validity limits:** low pressure, dilute solution, no chemical reaction with the solvent — CO2, NH3 and SO2 deviate because they react with water. - **Mole-fraction bridge to molarity:** in 1 L of water (about 55.5 mol), dissolved moles of gas = x × 55.5, letting you convert Henry's law answers into grams per litre. ## One clean worked numerical Water sits in equilibrium with air at 1 bar total pressure, so the partial pressure of nitrogen is about 0.8 bar. With K_H = 76.48 kbar = 76,480 bar for nitrogen at 293 K, the mole fraction dissolved is x = 0.8/76,480 = 1.05 × 10^-5. In one litre of water, roughly 55.5 mol, the dissolved nitrogen amounts to 1.05 × 10^-5 × 55.5 = 5.8 × 10^-4 mol, which is about 16 mg — the tiny solubility that keeps fish alive but barely. Notice the method's skeleton, because JEE reuses it with different gases: write x = p/K_H, convert to moles through the 55.5 mol of water in a litre, and only then attach a mass. If the same water is warmed, K_H climbs, x shrinks, and the bubble you see leave the glass is the calculation running backwards. ## Where students slip The reversal trap tops the list: K_H for helium is the largest in the NCERT table, and helium is the least soluble — candidates see a big constant, think "big solubility", and lose the question. The second slip is the temperature direction; remember that a hot cup of tea releases dissolved air as it heats, so K_H must rise with T. The third is scope: Henry's law speaks of mole fraction of the dissolved gas, not volume of gas collected, and it presumes no reaction — asked which of HCl, O2 and NH3 obeys the law best in water, the answer is O2 because HCl ionises completely and ammonia hydrogen-bonds and reacts. JEE Advanced has dressed this up with the bends: nitrogen dissolving at depth (higher p) and bubbling out of tissues during ascent, which is precisely p = K_H x operating in both directions. ## Frequently asked questions ### What does a high Henry's law constant mean? High K_H means low solubility — helium's 144.97 kbar at 293 K against oxygen's 34.86 kbar marks helium as far less soluble in water. ### Why does gas solubility decrease on heating? K_H increases with temperature, so x = p/K_H falls; dissolved gas escapes, which is why boiled water tastes flat and warm lakes hold less oxygen. ### Why is helium mixed into scuba-diving cylinders? Helium is much less soluble in blood than nitrogen (higher K_H), so less gas dissolves at depth and fewer bubbles form during ascent; it also avoids nitrogen narcosis. ### Which gases do not obey Henry's law in water? Gases that react with water — ammonia, hydrogen chloride, sulphur dioxide — because their dissolution is chemical, not just physical dissolution. ### How do you convert Henry's law mole fraction into grams per litre? Multiply x by about 55.5 mol of water per litre to get moles of dissolved gas, then by the molar mass — the final step of nearly every numerical on this law.