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The average molecular mass is often used for larger molecules since molecules with many atoms are unlikely to be composed exclusively of the most abundant isotope of each element. The masses used to compute the monoisotopic molecular mass are found on a table of isotopic masses and are not the same as found on a typical periodic table. Note that this also differs subtly from the molecular mass in that the choice of isotopes is defined. In mass spectrometry, the molecular mass of a small molecule is usually reported as the monoisotopic mass, that is, the mass of the molecule containing only the most common isotope of each element. The molecular mass can also be measured directly using mass spectrometry. Note that any given molecule may contain any given combination of isotopes, so there may be multiple molecular masses for each chemical compound. Note that the use of average atomic masses as found on a standard periodic table will result in an average molecular mass, whereas the use of isotopic masses will result in a molecular mass consistent with the strict interpretation of the definition, i.e. This is possible because molecules are created by chemical reactions which, unlike nuclear reactions, have very small binding energiesĬompared to the rest mass of the atoms ( < 10 -9) and therefore create a negligible mass defect.
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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 measured. 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 actual difference numerically is very small and 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.
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A mole of molecules most often contains a variety of molecular masses due to natural isotopes and the average is usually not identical to any single molecule. This is inconsistent with a strict interpretation of the definition because it neglects that the mass of a single molecule is not the same as the average of an ensemble. It is interpreted by many, including many chemists, to be a synonym of molar mass differing only in units (see average molecular mass below). Due to this relativity, the molecular mass of a substance is commonly referred to as the relative molecular mass, and abbreviated to Mr.Īlthough this term appears well-defined, there are varying interpretations of this definition. The molecular mass (abbreviated Mr) of a substance, formerly also called molecular weight and abbreviated as MW, is the mass of one molecule of that substance, relative to the unified atomic mass unit u (equal to 1/12 the mass of one atom of carbon-12).