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The molecular mass (abbreviated m) of a substance, more commonly referred to as 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). This is distinct from the relative molecular mass of a molecule, which is the ratio of the mass of that molecule to 1/12 of the mass of carbon 12 and is a dimensionless number. Relative molecular mass is abbreviated to Mr.

Molecular mass differs from more conventional measurements by taking into account different isotopic compositions, and as a result Molecular mass is generally more accurate then molar mass, but less used.

Conversion Factor of average molecular mass to molar mass:

- molar mass = average molecular mass * ((1/6.022)*10
^{23}g/u)*(6.022*10^{23}/mol)

- or

- molar mass in g/mol= average molecular mass in u

The average atomic mass of natural hydrogen is 1.00794 u and that of natural oxygen is 15.9994 u;
therefore, the molecular mass of natural water with formula H_{2}O is (2 × 1.00794 u) + 15.9994 u = 18.01528 u.
Therefore, one mole of water has a mass of 18.01528 grams. However, the exact mass of hydrogen-1 (the most common hydrogen isotope) is 1.00783, and the exact mass of oxygen-16 (the most common oxygen isotope) is 15.9949, so the mass of the most common molecule of water is 18.01056 u. The difference of 0.00472 u or 0.03% comes from the fact that natural water contain traces of water molecules containing, oxygen-17, oxygen-18 or hydrogen-2 (Deuterium) atoms. Although this difference is trivial in bulk chemistry calculations, it can result in complete failure in situations where the behavior of individual molecules matters, such as in mass spectrometry and particle physics (where the mixture of isotopes does not act as an average).

There are also situations where the isotopic distributions are not typical such as with heavy water used in some nuclear reactors which is artificially enriched with Deuterium. In these cases the computed values of molar mass and average molecular mass, which are ultimately derived from the standard atomic weights, will not be the same as the actual molar mass or average molecular mass of the sample. In this case the mass of deuterium is 2.0136 u and the average molecular mass of this water (assuming 100% deuterium enrichment) is (2 × 2.0136 u) + 15.9994 u = 20.0266 u. This is a very large difference of ~11% error from the expected average molecular mass based on the standard atomic weights. Furthermore the most abundant molecular mass is actually slightly less than the average molecular mass since oxygen-16 is still the most common. (2 × 2.0136 u) + 15.9949 u = 20.0221 u. Although this is an extreme artificial example, natural variation in isotopic distributions do occur and are measurable. For example, the atomic weight of lithium as found by isotopic analysis of 39 lithium reagents from several manufacturers varied from 6.939 to 6.996

- Learning by Simulations Calculation of Molecular Formulas from Molecular Masses
- Molecular Mass Calculator An online molecular mass calculator
- Free online calculations for mol weight and elemental composition using ChemAxon's Marvin and Calculator Plugins - requires Java
- Molecular Mass Calculator Online Molecular Mass and Elemental Composition Calculator

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Last updated on Wednesday October 08, 2008 at 10:29:09 PDT (GMT -0700)

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This article is licensed under the GNU Free Documentation License.

Last updated on Wednesday October 08, 2008 at 10:29:09 PDT (GMT -0700)

View this article at Wikipedia.org - Edit this article at Wikipedia.org - Donate to the Wikimedia Foundation

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