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Molecular mass or molecular weight is the mass of a molecule. It is calculated as the sum of the atomic 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.

Both atomic and molecular masses are usually obtained relative to the mass of the isotope 12C (carbon 12), which by definition[1] is equal to 12. For example, the molecular weight of methane, whose molecular formula is CH4, is calculated as follows:
Atomic massTotal mass
C12.01112.011
H1.007944.03176
CH416.043
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 112 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.
In the example above the atomic weight of carbon is given as 12.011, not 12. This is because naturally occurring carbon is a mixture of the isotopes 12C, 13C and 14C which have relative atomic masses of 12, 13 and 14 respectively. Moreover, the proportion of the isotopes varies between samples, so 12.011 is an average value. By contrast, there is less variation in naturally occurring hydrogen so the average atomic weight is known more precisely. The precision of the molecular mass is determined by precision of the least precise atomic mass value, in this case that of carbon. In high-resolution mass spectrometry the isotopomers 12C1H4 and 13C1H4 are observed as distinct molecules, with molecular weights of 16 and 17, respectively. The intensity of the mass-spectrometry peaks is proportional to the isotopic abundances in the molecular species. 1221H3 can also be observed with molecular weight of 17.

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