Number of ${\mathrm{N}}_2$ molecules present in 1 litre of vessel at 273K when compressibility factor is 1.2.

Here;

$\Rightarrow \mathrm{n}=\frac{\mathrm{PV}}{\mathrm{zRT}}=\frac{101325\times 1\times 0.001}{1.2\times 8.314\times 293.15}=0.0346$

1 mole contains ${\mathrm{N}}_{\mathrm{a}}=6.023\times {10}^{23}$ molecules

0.0346 mole contains $0.0346\times 6.023\times {10}^{23}=2.2\times {10}^{22}$ molecules