Molecular mass

In chemistry and physics, the molecular mass (formerly molecular weight) 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, which is the case 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 IUPAC Goldbook does not contain an entry molecular mass. It contains molecular weight as a synonym of relative molecular mass, M_r, which has the same numerical value as the molecular mass defined above. Formally it is the ratio (and is accordingly dimensionless) of the mass of the molecule to the mass of one-twelfth of the mass of ¹²C.