Molecular mass

Molecular mass or molecular weight is the mass of a molecule. It is calculated as the sum of the mass of each constituent atom multiplied by the number of atoms of that element in the molecular formula. The molecular mass of small to medium size molecules, measured by mass spectrometry, determines stoichiometry. For large molecules such as proteins, methods based on viscosity and light-scattering can be used to determine molecular mass when crystallographic data are not available.

Definitions

Both atomic and molecular masses are usually obtained relative to the mass of the isotope ¹²C (carbon 12), which by definition is equal to 12. For example, the molecular weight of methane, whose molecular formula is CH₄, is calculated as follows.

A more proper term would be "relative molecular mass". However the adjective 'relative' is omitted as it is universally assumed that atomic and molecular masses are relative to the mass of ¹²C. Relative atomic and molecular mass values are dimensionless but are given the "unit" Dalton (formerly atomic mass unit) to indicate that the number is equal to the mass of one molecule divided by ¹⁄₁₂ of the mass of one atom of ¹²C. The mass of 1 mol of substance is designated as molar mass. By definition, it has the unit gram.