Atomic mass of calcium = 40.078 u, atomic mass of Nitrogen = 14.0067 u and that of oxygen = 15.999 u.
Molar mass of Ca(NO3)2 = 40.078+2×(14.0067+3×15.999) = 164.0854 g/mol.
Percent composition of oxygen = {(number of atoms of oxygen × atomic mass of oxygen)/molar mass of Ca(NO3)2}×100% = {(6×15.999)/164.0854} × 100% = 58.502%
