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The mole is the SI unit for amount of substance. One mole = 6.022 x 10^23 entities (Avogadro's number). This bridges the atomic and macroscopic worlds. Molar mass (g/mol) = mass of one mole of a substance, numerically equal to atomic/molecular mass in amu. Core conversions: moles = mass/molar mass = particles/N_A = volume(STP)/22.4 L. Mass of one atom/molecule = molar mass/N_A. 1 amu = 1.66 x 10^-24 g. At STP (0 C, 1 atm), any ideal gas occupies 22.4 L per mole. Average atomic mass considers isotopic abundances: M_avg = sum(fraction_i x mass_i). This is why periodic table masses are not integers. For molecules: molecular mass = sum of constituent atomic masses. For ionic compounds: use formula mass (sum of atomic masses in formula unit). Vapour density = M/2 (relative to H2). These relationships form the foundation of all quantitative chemistry — every stoichiometric calculation begins with converting to moles. JEE frequently tests multi-step conversions: given grams, find molecules, or given volume of gas, find mass of another reactant.