Question

Explain the differences between the average, nominal, and the exact molecular mass of
a molecule.

Answer 1

The average mass of a molecule is obtained by summing the average atomic masses of the constituent elements. For example, the average mass of natural water with formula H₂O is 1.00794 + 1.00794 + 15.9994 = 18.01528.

The nominal mass for an element is the mass number of its most abundant naturally occurring stable isotope, and for an ion or molecule, the nominal mass is the sum of the nominal masses of the constituent atoms. Isotope abundances are tabulated by IUPAC: for example carbon has two stable isotopes ¹²C at 98.9% natural abundance and ¹³C at 1.1% natural abundance, thus the nominal mass of carbon is 12. The nominal mass is not always the lowest mass number, for example iron has isotopes ⁵⁴Fe, ⁵⁶Fe, ⁵⁷Fe, and ⁵⁸Fe with abundances 6%, 92%, 10%, and 2%, respectively, and a nominal mass of 56. For a molecule, the nominal mass is obtained by summing the nominal masses of the constituent elements, for example water has two hydrogen atoms with nominal mass 1 and one oxygen atom with nominal mass 16, therefore the nominal mass of H₂O is 18.

The exact mass of an isotopic species (more appropriately, the calculated exact mass) is obtained by summing the masses of the individual isotopes of the molecule. For example, the exact mass of water containing two hydrogen-1 (¹H) and one oxygen-16 (¹⁶O) is 1.0078 + 1.0078 + 15.9949 = 18.0105. The exact mass of heavy water, containing two hydrogen-2 (deuterium or ²H) and one oxygen-16 (¹⁶O) is 2.0141 + 2.0141 + 15.9949 = 20.0229.