Molekulmass

Molekulmass, or molecular mass, is the mass of a molecule. It is typically expressed in unified atomic mass units (u), where 1 u is defined as one twelfth of the mass of a carbon-12 atom. The molecular mass is obtained by summing the atomic masses of all atoms in the molecule, using the element’s isotopic composition.

To calculate the molecular mass, add the masses of each constituent atom according to the molecule’s formula. For example, water (H2O) has two hydrogen atoms and one oxygen atom. Using standard atomic masses (approximately H ≈ 1.008 u, O ≈ 15.999 u), the molecular mass of H2O is about 2 × 1.008 + 15.999 ≈ 18.015 u. More complex molecules require summing all atomic contributions in their formula.

The molar mass is the same numerical value as the molecular mass, but expressed in g/mol. Therefore, a molecular mass of 18.015 u corresponds to a molar mass of 18.015 g/mol, which is the mass of one mole of water molecules. The molecular mass can vary slightly depending on the isotopic composition of the elements present; natural samples have average masses that reflect isotopic abundances, while monoisotopic masses use the most common isotopes.

Applications and measurement: molecular mass is fundamental in stoichiometry and chemical calculations. It can be determined from standard atomic weights or measured directly by mass spectrometry. In biomolecules and polymers, approximate or average molar masses are often used due to large sizes and varied isotopic composition.