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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