JEE · Chemistry
Select ideal, mixture, kinetic, diffusion, or real-gas models and use each equation only under its stated assumptions.
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In short
Identify whether the gas can be treated as ideal, whether a mixture is non-reacting, whether a speed or diffusion result comes from kinetic theory, and whether pressure and temperature make molecular volume or attraction important.
Convert temperature to kelvin and keep units compatible before using any equation.
The official JEE documents define content scope. They do not publish chapter weightage, so none is asserted here.
Verified against the current JEE Main 2026 and JEE Advanced 2026 syllabus documents on 8 September 2026.
Sources: JEE Main 2026 syllabus and JEE Advanced 2026 syllabus, both linked in the sources section below.
Prerequisite
Mole Concept
Moles, molar mass, and proportional reasoning used in every gas equation.
Parent
States of Matter
Advanced umbrella and cross-state model selector this focused page sits within.
Parent
JEE Chemistry
Subject hub for the Chemistry chapter set.
Parent
JEE syllabus
Chemistry-level official scope.
This is a readiness check, not a weightage or scoring-priority list.
P, V, n, and T describe the macroscopic state under the selected model.
Every gas problem begins by identifying pressure, volume, amount, and temperature, and confirming which of these is fixed, changing, or requested.
Molecules have negligible volume and no intermolecular force except during elastic collisions.
The ideal-gas model assumes point-like, non-interacting molecules. Real gases only approach this behaviour under suitable pressure and temperature conditions.
Temperature links to the distribution of molecular kinetic energies, not one identical molecular speed.
Kinetic theory describes a distribution of molecular speeds at a given temperature; most probable, mean, and rms speeds are three distinct summary values of that distribution.
In an ideal non-reacting gas mixture, composition determines partial pressure.
Dalton's relation applies to a non-reacting ideal mixture where mole fraction determines each component's partial pressure of the total.
Attraction and excluded volume modify pressure-volume behaviour.
The van der Waals equation corrects the ideal equation with an attraction term and an excluded-volume term, and should approach ideal behaviour as these corrections become negligible.
Match the described prompt to a model and its required first check.
Pressure times volume equals moles times the gas constant times absolute temperature.
Ideal-gas state equation.
Use when — The ideal-gas model or an acceptable limit applies, with SI inputs.
Common trap — Mixing R with incompatible pressure-volume units.
Total pressure equals the sum of partial pressures; each partial pressure equals mole fraction times total pressure.
Dalton mixture relation for partial pressure.
Use when — An ideal, non-reacting mixture is at a common temperature and volume.
Common trap — Using mole fraction after a reaction changes composition.
Most probable speed equals the square root of two R T over M.
Most probable molecular speed.
Use when — An ideal-gas Maxwell speed distribution applies.
Common trap — Inserting molar mass in g/mol with SI R.
Mean speed equals the square root of eight R T over pi M.
Mean molecular speed.
Use when — Same ideal distribution as the most probable speed.
Common trap — Calling it rms speed.
Root-mean-square speed equals the square root of three R T over M.
Root-mean-square molecular speed.
Use when — Same ideal distribution.
Common trap — Assuming every molecule moves at this speed.
Pressure plus a n squared over V squared, times volume minus n b, equals n R T.
Van der Waals correction for attraction and excluded volume.
Use when — A model for a specified real gas is required; constants are gas-specific.
Common trap — Swapping the physical roles of a and b.
Compressibility factor equals pressure times volume divided by moles times R times temperature.
Compressibility factor.
Use when — Defined from a measured state and chosen amount.
Common trap — Saying Z equals 1 proves a gas is ideal at all states.
The ratio of rate one to rate two approximately equals the square root of molar mass two over molar mass one.
Graham-type rate comparison for diffusion or effusion.
Use when — Comparable temperature, pressure gradient, aperture, and ideal-like conditions apply.
Common trap — Using it for arbitrary bulk flow.
Answer: p(He) = 2.0 bar, p(Ne) = 1.0 bar. The result relies on the stated ideal, non-reacting mixture and is not a claim that all gas mixtures obey ideality.
Total amount is 3.0 mol.
Mole fraction of helium is 2/3 and of neon is 1/3.
Partial pressure of helium = (2/3)(3.0) = 2.0 bar. Partial pressure of neon = (1/3)(3.0) = 1.0 bar.
Check: 2.0 + 1.0 = 3.0 bar, matching the stated total.
Calling Gaseous State current Main Chemistry scope.
Needs reviewWhy it happens
Gaseous State is not a named Chemistry unit in the current JEE Main 2026 syllabus.
How it is corrected
Check the official syllabus mapping table on this page before assuming Main relevance.
Using Celsius directly in a gas or speed formula.
Execution errorWhy it happens
Every ideal-gas and kinetic-theory formula requires absolute (kelvin) temperature.
How it is corrected
Convert Celsius to kelvin before substitution.
Mixing SI R with litre-atmosphere data.
Execution errorWhy it happens
R has different numerical values depending on the pressure-volume unit system used.
How it is corrected
Choose one consistent unit system and matching R value throughout.
Inserting g/mol into SI kinetic-speed formulas.
Execution errorWhy it happens
SI kinetic-speed formulas require molar mass in kg/mol to produce a speed in m/s.
How it is corrected
Convert molar mass to kg/mol before substituting into a speed formula.
Treating rms speed as the speed of every molecule.
Knowledge gapWhy it happens
Rms speed is a statistical summary of a speed distribution, not the actual speed of every molecule.
How it is corrected
Remember the speed ordering: most probable speed less than mean speed less than rms speed.
Applying Dalton's law across a reacting mixture without updating composition.
Decision / selection errorWhy it happens
Dalton's relation assumes a non-reacting mixture; a reaction changes the mole fractions used in the formula.
How it is corrected
Recompute mole fractions after any stated reaction before applying Dalton's relation.
Interchanging the attraction correction a and volume correction b in the van der Waals equation.
Recall gapWhy it happens
a corrects for intermolecular attraction and b corrects for molecular excluded volume; they play different physical roles.
How it is corrected
Confirm which constant is which for the given gas before substituting.
Using Graham's law for arbitrary convective flow.
Decision / selection errorWhy it happens
Graham's relation applies to diffusion or effusion under matched conditions, not general bulk gas flow.
How it is corrected
Confirm the situation is diffusion or effusion under comparable conditions before applying the relation.
FAQ
Straight answers about how Rank Sarthi fits into serious exam preparation.
No. It is not listed as a current Chemistry unit.
Its gas laws, kinetic theory, partial pressure, diffusion, and real-gas topics appear within States of Matter: Gases and Liquids.
When an ideal-gas model is stated or is an acceptable approximation at the specified conditions, with compatible units and absolute temperature.
Gaseous State is not a named JEE Main 2026 Chemistry unit; it is covered as a merged focus within JEE Advanced 2026 States of Matter: Gases and Liquids. NCERT resources support gas laws, kinetic theory, mixtures, diffusion, and real-gas models. NIST CODATA supports the molar gas constant value. Official Advanced paper archives are cited for provenance only; no frequency, marks, or trend claims are made from them.
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