NH₃ gas is liquified more easily than N₂. Hence

$$\begin{gathered} \text{'}a\text{'} = correction for the intermolecular forces. \\ \text{'}b\text{'}= correction for finite molecular size \\ For a gas which is easily liquifiable value of \text{'}a\text{'} will be more \\ and value of \text{'}b\text{'} will be less. \\ hence a\left(N{H}_3\right)>a(N_2 ) but b(N{H}_3 )<b\left(N_2\right). \end{gathered}$$