Molmassen

Molmassen, in German often referred to as Molmasse, is a chemical property that describes the mass per amount of substance. It is defined as M = m/n, where m is the mass and n is the amount of substance in moles. The typical unit is grams per mole (g/mol), though the SI base unit can also be kilogram per mole (kg/mol).

For a pure substance, the molar mass is obtained by summing the atomic masses of all atoms in its formula, taking into account isotopic abundances for elements. In practice this means using standard atomic weights for elements and adding them according to the molecule or formula unit. For example, the molar mass of water (H2O) is about 18.015 g/mol, and a glucose molecule (C6H12O6) is about 180.156 g/mol. Hydrates and salts have molar masses that include all constituent atoms in their formula.

Molar mass is distinct from molecular mass. Molecular mass (often denoted Mr) is a dimensionless quantity—the mass of a molecule in atomic mass units (u). The molar mass is numerically equal to the molecular mass when expressed in g/mol, since 1 u corresponds to 1 g/mol in convention, but the two concepts apply in different units.

Applications of molmassen include stoichiometry, where masses are converted to amounts of substance to balance reactions, and vice versa. It is also used in polymer science to discuss average molar masses and distributions, and in thermodynamics and gas-phase calculations (for example via the ideal gas law, where M relates density, temperature, and pressure).

In mixtures, the molar mass depends on composition and can be calculated from the molar masses of components and their mole fractions. Hydration, salts, and mixtures with varying isotopic composition can alter the overall molar mass accordingly.