stoffmengde

The amount of substance, known in Norwegian as stoffmengde, is a base quantity in the International System of Units (SI) used to specify the number of specified elementary entities in a sample. Its symbol is n and its unit is the mole (mol). One mole contains exactly Avogadro's number of entities, N_A = 6.02214076 × 10^23, whether they are atoms, molecules, ions, or formula units. The amount of substance is distinct from mass, but it connects to mass through molar mass.

The amount of substance is related to mass by the molar mass M, defined as the mass per mole of a substance (in g/mol). For a sample with mass m, n = m / M; conversely, m = n × M. The concept enables meaningful comparisons between substances that have different masses by counting how many particles are present rather than relying solely on mass.

In chemical reactions the coefficients in balanced equations express molar ratios; these ratios allow conversions between substances using n_A × (coeff_B / coeff_A). In solution chemistry, common quantities derived from n include molarity (M) = n / V, molality (m) = n_solute / kg solvent, and mole fraction x_i = n_i / ∑ n. For gases, PV = nRT relates the amount of substance to pressure, volume and temperature, with a standard molar volume around 22.414 L/mol at 0 °C and 1 atm.

Stoffmengde provides a consistent foundation for quantitative chemistry, linking mass, particle count, reaction stoichiometry, and solution concentration. The SI definition fixes Avogadro's number, making the mole a precise amount of substance independent of the type of particle.