Molecular weight of ammonia $\left( NH _{3}\right)=14+3=17$
Molecular weight of nitrogen gas $\left( N _{2}\right)=14+14=28$
Smaller the molecular weight of a gas, higher the value of van der Waals' constant ' $a$ ' for it. Thus, between $NH _{3}$ and $N _{2}$, the value of van der Waals' constant ' $a$ ' is higher for $NH _{3}$.