Molar mass

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 its 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) ≈ 6998180000000000000♠18 g/mol

Molar masses of elements

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: