Gas Laws for JEE Main 2026: Complete Guide

Gas laws are the quantitative backbone of physical chemistry, linking pressure, volume, temperature, and the amount of gas. JEE Main tests them through numerical problems and conceptual MCQs that reward a clean understanding of each law's conditions and limitations. This guide covers every gas law tested, in the sequence that builds the clearest conceptual map.

The Three Classical Gas Laws

Boyle's law (constant T and n): PV = constant, so P₁V₁ = P₂V₂. Charles's law (constant P and n): V/T = constant, so V₁/T₁ = V₂/T₂ (T in Kelvin always). Gay-Lussac's law (constant V and n): P/T = constant, so P₁/T₁ = P₂/T₂. Combining all three gives the combined gas law: P₁V₁/T₁ = P₂V₂/T₂ — the most versatile formula for JEE numericals. Use this combined form whenever two variables change simultaneously; you only need to identify which quantity is fixed. For the broader physical chemistry context see our physical chemistry formulas guide.

The Ideal Gas Equation

PV = nRT, where P is in Pa, V in m³, n in moles, R = 8.314 J/(mol·K), T in Kelvin. Equivalently PV = (m/M)RT where m is mass in grams and M is molar mass. In CGS or atm-L units: R = 0.0821 L·atm/(mol·K). Density of an ideal gas: d = PM/(RT). JEE regularly asks for molar mass using this density form — rearrange to M = dRT/P. Dalton's law of partial pressures: P_total = ΣP_i, where P_i = n_i RT/V is the partial pressure of each component.

Molar Volume and STP/NTP Conditions

At STP (0°C, 1 atm = 101.325 kPa): one mole of ideal gas occupies 22.4 L. At NTP (20°C, 1 atm): one mole occupies 24.04 L. IUPAC STP (0°C, 1 bar = 100 kPa): one mole occupies 22.7 L. JEE problems that specify STP usually mean 0°C and 1 atm unless stated otherwise. Confirm which definition the question uses when calculating volumes from moles or molar mass from density at STP.

Kinetic Theory Link and Real Gas Deviations

The ideal gas equation assumes zero intermolecular forces and negligible molecular volume. Real gases deviate at high pressure (volume effect dominates — molecules take up space) and low temperature (intermolecular attraction dominates — molecules cluster). The compressibility factor Z = PV/(nRT) equals 1 for ideal gas; Z < 1 at moderate pressure (attraction dominates) and Z > 1 at high pressure (repulsion/volume dominates). CO₂ and NH₃ show significant deviation; He and H₂ approximate ideal behaviour well.

van der Waals Equation

The van der Waals equation for real gases: (P + an²/V²)(V − nb) = nRT. The term an²/V² corrects for intermolecular attraction (reduces effective pressure); nb corrects for molecular volume (reduces effective volume). 'a' is high for easily liquefied gases (strong attraction); 'b' is proportional to molecular size. JEE asks for qualitative reasoning about 'a' and 'b' trends — stronger attraction = larger 'a', bigger molecule = larger 'b'. After working through all gas law numerical types, take a free mock test on physical chemistry.