The molecular mass of an element is defined as the sum of the masses of the elements present in the molecule. Molecular mass is obtained by multiplying the atomic mass of an element with the number of atoms in the molecule and then adding the masses of all the elements in the molecule.
O2 is a diatomic molecule.
The mass of an oxygen atom = 16 amu.
∴ Mass of O2 molecule = 2 × 16 = 32 amu.
Mass of one molecule of oxygen
⇒ 32/ Avogadro constant ( 6.023 × 1023)
⇒ 5.31 × 10-23g.