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The molar mass (symbol M, SI unit kg·mol−1) is defined as the mass per unit amount of substance of a given chemical entity. In keeping with the definition of the mole, the chemical entity in question should always be specified. Usually this will be a recognized atom, molecule or ion, but any collection, or indeed fragment, of such species may be specified. The molar mass of an element E may be obtained from tables such as those in this Encyclopaedia; usually these give the dimensionless 'Atomic Weight' Ar = M(E)/(10−3 kg·mol−1). For a molecule, the molar mass is obtained by summing the molar mass of the constituent atoms. In the case of atomic or molecular ions, allowance should strictly be made for the surplus or deficit of electrons.

The term 'molecular weight' is commonly attached to the quantity M even though M is a mass (not a weight) of 1 mol (not one molecule) of a particular substance. Often the same term is incorrectly used for the dimensionless relative molar mass. In view of this confused situation, use of the term molecular weight should be abandoned in favor of the molar mass or the Relative Molar Mass, whichever is meant. Similarly, the term 'molecular mass' properly means the mass of one molecule and is therefore not a synonym for molar mass.

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