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The molecular mass can be calculated as the sum of the individual isotopic masses (as found in a table of isotopes) of all the atoms in any one molecule. Specific samples may vary significantly from the expected isotopic composition due to real deviations from earth average isotopic abundances. Therefore, they often vary since one is theoretical and the other is experimental. It should be noted, however, that the molar mass is almost always a computed figure derived from the standard atomic weights, whereas the average molecular mass, in fields that need the term, is often a measured figure specific to a sample. The average molecular mass and the molar mass of a particular substance in a particular sample are in fact numerically identical and may be interconverted by avogadro's number. This is often closer to what is meant when "molecular mass" and "molar mass" are used synonymously and may have derived from shortening of this term. The average molecular mass is the abundance weighted mean (average) of the molecular masses in a sample. The average molecular mass (sometimes abbreviated as average mass) is another variation on the use of the term molecular mass. The molecular mass of a molecule which happens to contain heavier isotopes than the average molecule in the sample can differ from the molar mass by several mass units.
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The size of this error becomes much larger when considering larger molecules or less abundant isotopomers. The actual numerical difference can be very small when considering small molecules and the molecular mass of the most common isotopomer in which case the error only matters to physicists and a small subset of highly specialized chemists however it is always more correct, accurate and consistent to use molar mass in any bulk stoichiometric calculations. A mole of molecules may contain a variety of molecular masses due to natural isotopes, so the average mass is usually not identical to the mass of any single molecule. A stricter interpretation does not equate the two, as the mass of a single molecule is not the same as the average of an ensemble. Many chemists use molecular mass as a synonym of molar mass, differing only in units (see average molecular mass below). There are varying interpretations of this definition. 5 Example: Average Molecular Mass versus Molecular Mass versus Molar Mass.