Question

Arrange the following gases in increasing order of van der Waals constant 'a'
A. Ar
B. CH4
C. H₂O
D. C6H6
Choose the correct option from the following.

• A. A, B, C and D
• B. D, C, B and A
• C. B, C, D and A
• D. C, D, B and A

Hint:

Van der Waals constant a is proportional to the strength of intermolecular forces. Hy drogen bonding and larger molecular size significantly increase the value of a.

Answer 1

The Correct Option is A

The van der Waals constant \( a \) represents the magnitude of intermolecular attractive forces. It increases with molecular size, molecular mass, and the presence of hydrogen bonding. Let us analyze the given gases:

1. Argon (Ar): Being a noble gas, Ar has the weakest intermolecular forces and the lowest value of \( a \).
2. Methane (\( \text{CH}_4 \)): \( \text{CH}_4 \) has stronger London dispersion forces than Ar due to its larger molecular size.
3. Water (\( \text{H}_2\text{O} \)): \( \text{H}_2\text{O} \) has significant hydrogen bonding, increasing its \( a \) value further.
4. Benzene (\( \text{C}_6\text{H}_6 \)): \( \text{C}_6\text{H}_6 \) is a large aromatic molecule with strong dispersion forces, giving it the highest \( a \) value among the given gases.

Thus, the increasing order of \( a \) is:

\(Ar (A) < CH_4 (B) < H_2O (C) < C_6H_6 (D) .\)