Four gases P, Q, R andS have almost same values of 'b' but their 'a' values (a, b are van der Waals constants) are in the order `Q lt R lt S lt P`. At a particular temperature, among the four gases the most easily liquefiable one is

Options

A. P
B. Q
C. R
D. S

Explanation

To determine which of the four gases (P, Q, R, and S) is the most easily liquefiable, we need to analyze the van der Waals constants 'a' and 'b'. The van der Waals constant 'a' represents the attractive forces between gas molecules, while 'b' represents the volume occupied by the gas molecules.

### Step-by-Step Solution:

1. **Understanding the Constants**:
- The van der Waals constant 'a' indicates the strength of intermolecular attractions. A higher 'a' value means stronger attractions between molecules, which facilitates the process of liquefaction.
- The constant 'b' is related to the volume occupied by the gas molecules and is said to be almost the same for all four gases in this case.

2. **Given Information**:
- The order of 'a' values for the gases is:
- Q < R < S < P
- This means:
- Q has the lowest attractive forces.
- P has the highest attractive forces.

3. **Determining Liquefaction**:
- Since liquefaction occurs more easily when intermolecular forces are stronger, we look for the gas with the highest 'a' value.
- Among the gases, gas P has the highest 'a' value, indicating it has the strongest intermolecular attractions.

4. **Conclusion**:
- Therefore, the gas that is most easily liquefiable among P, Q, R, and S is gas P.

### Final Answer:
The most easily liquefiable gas is **P**.