The number of molecules in 4.25 g of ammonia are:

Molecular weight of NH₃ is 17
According to the mole concept
$$\begin{gathered} 17\mathrm{gm}{\mathrm{NH}}_3\text{ has molecules }=6.02\times {10}^{23} \\ \therefore 1\mathrm{gm}{\mathrm{NH}}_3\text{ has molecules }=\frac{6.02\times {10}^{23}}{17} \\ \therefore 4.25\mathrm{gm}{\mathrm{NH}}_3\text{ has molecules } \\ =\frac{6.02\times {10}^{23}\times 4.25}{17}=1.5\times {10}^{23}\text{ molecule } \end{gathered}$$