In chemistry and physics, the molecular mass (formerly molecular weight and officially relative molecular mass) is the mass of a molecule expressed in unified atomic mass units. The molecular mass is equal to the sum of the atomic masses of the atoms constituting the molecule. The molecular mass may be of an isotopically pure molecule, or of a molecule consisting of isotopes in their natural abundance. The latter case is most commonly encountered in chemistry.
Take water (H2O) as an example. The two naturally occurring isotopes of the hydrogen atom are:
- H: m = 1.0078250321 u, abundance = 99.9885 %
- D: m = 2.0141017780 u, abundance = 0.0115 %
The standard atomic weight (isotopically averaged mass) of hydrogen is 1.00794 u.
The three naturally occurring isotopes of the oxygen atom are:
- 16O: m = 15.9949146221 u, abundance = 99.757%
- 17O: m = 16.99913150 u, abundance = 0.038 %
- 18O: m = 17.9991604 u, abundance = 0.205 %
The standard atomic weight (isotopically averaged mass) of oxygen is 15.9994 u.
Natural water has the molecular mass 2×1.00794 + 15.9994 = 18.01528 u. The following isotopically pure form of water (which would be very difficult to prepare, but may serve as an example): 1H–18O–D has molecular mass 1.0078250321 + 17.9991604 + 2.0141017780 = 21.0210872101 u.
The official IUPAC publication, Goldbook, does not contain an entry molecular mass. It contains molecular weight as a synonym of relative molecular mass, both having the same numerical value as the molecular mass defined above. Formally, relative molecular mass, denoted by Mr, is the (dimensionless) ratio of the mass of a molecule to the mass of one-twelfth of the mass of 12C. Note that the adjective "relative" in this definition refers very specifically to (a twelfth of) the mass of 12C.
- This usage of "relative" is not very fortunate, because the magnitude of mass of any object can be said to be relative and dimensionless. In fact, if we say a mass is w kilogram, w is by definition the ratio of the object's mass to the mass of the standard kilogram stored in Sèvres, France.