Solution:
(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
