One mole = 6.022 x 10^23 entities (Avogadro's number, $N_A$). This number connects the atomic scale to the macroscopic scale. Molar mass $\frac{g}{mol}$ = mass of 1 mole = numerically equal to atomic/molecular mass in amu. Key relationships: moles = mass$\frac{g}{molar}$ mass = number of particles/$N_A$ = volume at STP/22.4 L (gases only). 1 amu = 1.66 x 10^-24 g = 1/$N_A$ grams. Mass of one molecule = molar mass/$N_A$. At STP (273.15 K, 1 atm): 1 mole of any ideal gas = 22.4 L (molar volume). At 25 C, 1 bar (new IUPAC STP): molar volume = 24.79 L. JEE typically uses 22.4 L at 0 C, 1 atm.