Amount of substance and molar mass

A macroscopic body contains an enormous number of particles, so their number is compared with the number of atoms in a standard sample: 12 g of carbon. One mole is the amount of substance containing (almost exactly) as many particles as there are atoms in 12 g of carbon-12. This number is the Avogadro constant N_A ≈ 6.02·10²³ mol⁻¹, the same for all substances; since 2019 the mole has been defined by fixing this constant at exactly 6.02214076·10²³. The amount of substance is denoted ν (nu) and measured in moles: ν = N/N_A, so N = νN_A. The mass of one mole is the molar mass M: M = m₀N_A (unit kg/mol); numerically, in g/mol, M equals M_r, e.g. M = 18 g/mol for water. For a substance of mass m, ν = m/M; the number of molecules per unit volume is the concentration n = N/V (unit m⁻³), and ρ = n·m₀.