Molar mass

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Molar mass, symbol M,[1] is a physical property characteristic of a given substance (chemical element or chemical compound), namely its mass per amount of substance.[2] The base SI unit for mass is the kilogram and that for amount of substance is the mole.[1] Thus, the derived unit for molar mass is kg/mol. However, for both practical and historical reasons, molar masses are almost always quoted in grams per mole (g/mol or g mol−1), especially in chemistry.

Molar mass is closely related to the relative molar mass (M
r
) of a compound, the older term formula weight and to the standard atomic masses of its constituent elements. However, it should be distinguished from the molecular mass (also known as molecular weight), which is the mass of one molecule (of any single isotopic composition) and is not directly related to the atomic mass, the mass of one atom (of any single isotope). The dalton, symbol Da, is also sometimes used as a unit of molar mass, especially in biochemistry, with the definition 1 Da = 1 g/mol, despite the fact that it is strictly a unit of molecular mass (1 Da = 1.660 538 782(83)×10−27 kg).[3][4]

Molar masses are almost never measured directly. They may be calculated from standard atomic masses, and are often listed in chemical catalogues and on material safety data sheets (MSDS). Molar masses typically vary between:

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