For a real gas, the van der Waals constant $a$ represents the magnitude of intermolecular forces. Which of the following gases has the highest value of $a$?

Solution

1. The constant $a$ measures the strength of attractive forces between gas molecules.
2. Larger molecules with more electrons generally exhibit stronger London dispersion forces.
3. Comparing the molar masses and molecular sizes, ${\text{CH}}_4$ is the largest and most polarizable among the choices.
4. Therefore, ${\text{CH}}_4$ has the highest value of $a$.
5. Hence the answer is (B).