This concept can be extended to calculate molecular masses. The molecular mass of a substance is the sum of the atomic masses of all the atoms in a molecule of the substance. It is therefore the relative mass of a molecule expressed in atomic mass units (u)
(a) Calculate the relative molecular mass of water (H2O).
(b) Calculate the molecular mass of HNO3.
(a) Atomic mass of hydrogen = 1u, oxygen = 16 u
So the molecular mass of water, which
contains two atoms of hydrogen and one
atom of oxygen is = 2 × 1+ 1×16
= 18 u
(b) The molecular mass of HNO3 = the
atomic mass of H + the atomic mass of
N+ 3 × the atomic mass of O
= 1 + 14 + 48 = 63 u