Question

The numerical value of $a$, the Van der Waals constant is maximum for:
(A) $He$
(B) ${H_2}$
(C) ${O_2}$
(D) $N{H_3}$

Answer 1

Van der Waals constant $a$ is the magnitude of intermolecular attractive forces with the gas. It depends on how strongly the molecules of gas attract each other, which in turn helps in the ease liquefaction of a gas. Higher the value of $a$ , greater is the intermolecular force and easier the liquefaction.

Complete answer:
$N{H_3}$ is a polar molecule and exhibits three of the polar molecules: dipole-dipole attraction, induced attraction and London dispersion forces. Since $H$ is bonded to $N$ , it exhibits hydrogen bonding.
Due to the dipole-dipole attraction, $N{H_3}$ is easily liquefiable compared to other molecules.
Here are the values of $a$ for the given molecules in ascending order:
$He(0.0346) < {H_2}(0.2476) < {N_2}(1.370) < N{H_3}(4.225)$ .
Option (D) is correct.

Note:
Intermolecular interactions are electrostatic i.e. between delta negatively and delta positively charged atoms. In $N{H_3}$ , the interaction is between delta positive $H{\text{s}}$ and delta negative $N$ which is attractive.
The intermolecular attractive forces can vary to some extent with temperature and pressure and are always minimum for inert gases.