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).
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:
- 1–238 g/mol for atoms of naturally-occurring elements;
- 10–1000 g/mol for simple chemical compounds;
- 1000–5,000,000 g/mol for polymers, proteins, DNA fragments, etc.
