Molecular mass or molecular wicht refers tae the mass o a molecule. It is calculatit as the sum o the mass o each constituent atom multiplied bi the nummer o atoms o that element in the molecular formula. The molecular mass o smaa tae medium size molecules, measured bi mass spectrometry, determines stoichiometry. For lairge molecules such as proteins, methods based on viscosity an licht-scatterin can be uised tae determine molecular mass when crystallographic data are nae available.