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.