- Calculate the number of oxygen atoms in 10 g of CaCO
_{3}.

[Atomic mass of Ca = 40 u, C = 12 u and 0=16 u]
- Calculate the mass of O
_{2} molecules which contain same number of molecules as 1.4 g of nitrogen (N_{2}).

- Molar mass of CaCO
_{3} = Atomic mass of (Ca + C + 3 x O)

= 40 + 12 + 3 x 16=100 g

∵ 100g CaCO3 contains 3 x 6.022 x 10^{23} atoms of oxygen.

- Molar mass of N
_{2} = 2 x 14 g = 28 g

Molar mass of O_{2} = 2 x 16 g = 32 g

∵ 28 g N_{2} contains 6.022 x 10^{23} molecules

∵ 1.4 g N_{2} will contain = molecules