(a) The number of atoms or molecules in 12 gm of carbon-12 ( in a mole of a substance) is called as Avogadro constant (6.022 × 10^{23} )

(b) Number of molecules(N) = Given mass X Avgadros no / Molar mass

Molar mass of 1 mole of N_{2}= 14 × 2 =28

N = 56/ 28 ×6.022 × 10^{23}

N = 12.044 × 10^{23 }

Hence there are 12.044 × 10^{23} particles present in 56 gm of N_{2}