The number of atoms in 4.25 g of NH₃ is approximately

$\therefore 17\mathrm{gm} {\mathrm{NH}}_3 \mathrm{contains} 6\mathrm{x}{10}^{23} \mathrm{molecules} \mathrm{of} {\mathrm{NH}}_3$
$\therefore 4.25\mathrm{gm} {\mathrm{NH}}_3 \mathrm{contains} = \frac{6\mathrm{x}{10}^{23}}{17}\mathrm{x}4.25$
$\therefore \text{ No. of atoms }=\frac{6\times {10}^{23}\times 4.25}{17}\times 4=6\times {10}^{23}\text{. }$