: 200
Mole concept contributes ~5% of JEE Main marks — one of the highest-weighted physical chemistry topics. Priority areas: (1) Multi-step stoichiometry — given mass of reactant, find mass/volume of product through balanced equations. (2) Limiting reagent — always check when two reactant quantities are given. (3) Empirical/molecular formula from combustion data or mass% composition. (4) Volumetric analysis — N1V1 = N2V2, back titration, and double titration problems. (5) Gas-phase calculations — combining volumes, STP conversions. Time management: most problems require 3-4 calculation steps. Practise converting between mass, moles, particles, and volume fluently. Common traps: (a) H2SO4 is diprotic (n=2) — compare equivalents, not moles. (b) STP is 0 C, not 25 C. (c) Molecular formula needs molar mass — empirical formula alone is insufficient. (d) Back titration: subtract unreacted from added, not from analyte. (e) In combustion analysis, oxygen in the compound is often overlooked. Mental math: know key molar masses by heart (H2O=18, CO2=44, NaOH=40, H2SO4=98, CaCO3=100, KMnO4=158).