The number of molecules in 4.25 g of ammonia is

Molecular weight of NH₃ is 17
According to the mole concept
17 g NH₃ has molecules = 6.02 ×10²³

$\begin{array}{l}\therefore 1\mathrm{g}{\mathrm{NH}}_3\text{ has molecules }=\frac{6.02\times {10}^{23}}{17}\\ \therefore 4.25\mathrm{g}{\mathrm{NH}}_3\text{ has molecules }\\ =\frac{6.02\times {10}^{23}\times 4.25}{17}=1.5\times {10}^{23}\text{ molecule }\end{array}$