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Molar mass

Molar mass
Common symbols
mol
SI unit kg/mol
Other units
g/mol

In chemistry, the molar mass M is a physical property defined as the mass of a given substance (chemical element or chemical compound) divided by the amount of substance. The base SI unit for molar mass is kg/mol. However, for historical reasons, molar masses are almost always expressed in g/mol.

As an example, the molar mass of water: M(H2O) ≈ 18 g/mol.

The molar mass of atoms of an element is given by the standard relative atomic mass of the element multiplied by the molar mass constant, M
u
 = 1 × 10−3 kg/mol = 1 g/mol:

Multiplying by the molar mass constant ensures that the calculation is dimensionally correct: standard relative atomic masses are dimensionless quantities (i.e., pure numbers) whereas molar masses have units (in this case, grams/mole).

Some elements are usually encountered as molecules, e.g. hydrogen (H
2
), sulfur (S
8
), chlorine (Cl
2
). The molar mass of molecules of these elements is the molar mass of the atoms multiplied by the number of atoms in each molecule:

The molar mass of a compound is given by the sum of the standard atomic weight (namely, the standard relative atomic mass) of the atoms which form the compound multiplied by the molar mass constant, M
u
:


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