Amount of substance. The mole and molar mass

Amount of substance (n) expresses the number of particles (atoms, molecules, ions) in a substance, and its unit is the mole. By the SI definition in force since 2019, one mole is the amount that contains exactly 6.02214076·10²³ particles (earlier it was defined through the number of atoms in 12 g of carbon-12, which gives almost the same number); this number is the Avogadro constant, and in calculations Nₐ ≈ 6.02·10²³ mol⁻¹ is used. The mass of a substance (grams) and its amount (moles) are different quantities: the mass of one mole in grams is the molar mass M (g/mol), numerically equal to the relative atomic or molecular mass (M(H₂O) = 18 g/mol). From this follow n = m/M, m = n·M and the number of particles N = n·Nₐ; 1 kmol = 1000 mol and 1 mmol = 0.001 mol. In a chemical reaction atoms are neither lost nor created but only regrouped, so the mass of the reacting substances equals the mass of the products (law of conservation of mass). Any pure compound has a constant composition however it is obtained (law of constant composition); for example the mass fraction of carbon in CO₂ is 12/44 ≈ 27.3 %.