The mole and molar mass

The mole is the SI unit for amount of substance; one mole of anything contains 6.022 × 10²³ elementary entities (atoms, molecules, ions or formula units).

This number is the Avogadro constant, Nᴀ = 6.022 × 10²³ mol⁻¹.

Molar mass is how many grams one mole of a substance weighs; numerically it equals the atomic, molecular or formula mass in u (water: 18.02 u per molecule, 18.02 g mol⁻¹).

Number of moles n = given mass / molar mass; number of particles = n × Nᴀ.

Always check which particle is asked for: one mole of NH₃ has Nᴀ molecules but 3Nᴀ hydrogen atoms and 4Nᴀ atoms in total.

For gases, NCERT's stoichiometry example takes one mole as 22.7 L, the molar volume of an ideal gas at 273.15 K and 1 bar; the older figure of 22.4 L belongs to 273.15 K and 1 atm.