The<unk>  (abbreviated Mr) of a substance, formerly also called<unk>  weight and abbreviated as<unk> , is the<unk>  of one molecule of that substance, relative to the unified atomic<unk>  unit u (equal to 1/12 the<unk>  of one atom of carbon-12). Due to this relativity, the<unk>  of a substance is commonly referred to as the relative<unk> , and abbreviated to Mr.
Although this term appears well-defined, there are varying interpretations of this definition. It is interpreted by many, including many chemists, to be a synonym of molar<unk>  differing only in units (see average<unk>  below). This is inconsistent with a strict interpretation of the definition because it neglects that the<unk>  of a single molecule is not the same as the average of an ensemble. A mole of molecules most often contains a variety of<unk> es due to natural isotopes and the average is usually not identical to any single molecule. 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<unk>  in any bulk stoichiometric calculations.
The average<unk>  (sometimes abbreviated as average<unk>  is another variation on the use of the term<unk> . The average<unk>  is the abundance weighted mean (average) of the<unk> es in a sample. This is often closer to what is meant when<unk> " and "molar<unk> " are used synonymously and may have derived from shortening of this term. The average<unk>  and the molar<unk>  of a particular substance in a particular sample are in fact numerically identical and may be interconverted by avogadro's number. It should be noted, however, that the molar<unk>  is almost always a computed figure derived from the standard atomic weights, whereas the average<unk> , in fields that need the term, is often a measured figure specific to a sample. Therefore, they often vary since one is theoretical and the other is measured. Specific samples may vary significantly from the expected isotopic composition due to real deviations from earth average isotopic abundances.
Molecular mass
Fluency
Density
Chemical polarity
Spin quantum number
Gas constant
Mass versus weight
Saturation 