Which of the following gas will have highest value of van der Waal's constant 'a' ?

Options

A. `C Cl_(4)` (g)
B. `NH_(3)` (g)
C. `CO_(2)` (g)
D. `H_(2)O` (g)

Explanation

To determine which gas has the highest value of van der Waals constant 'a', we need to analyze the intermolecular forces present in each of the given gases. The van der Waals constant 'a' is a measure of the attractive forces between molecules; a higher value indicates stronger intermolecular attractions.

### Step-by-Step Solution:

1. **Identify the Gases**: The gases given in the question are CCl4 (carbon tetrachloride), NH3 (ammonia), CO2 (carbon dioxide), and H2O (water).

2. **Analyze CCl4**:
- CCl4 is a tetrahedral molecule with polar C-Cl bonds. However, due to its symmetrical shape, the dipoles cancel out, making CCl4 a non-polar molecule.
- The only intermolecular forces present are London dispersion forces, which are the weakest type of intermolecular forces.
- **Conclusion**: CCl4 has a low value of 'a'.

3. **Analyze NH3**:
- NH3 is a polar molecule due to the electronegativity difference between nitrogen and hydrogen, resulting in a dipole moment.
- NH3 exhibits hydrogen bonding, which is a strong intermolecular force.
- **Conclusion**: NH3 has a higher value of 'a' compared to CCl4 due to hydrogen bonding.

4. **Analyze CO2**:
- CO2 is a linear molecule with polar bonds, but the symmetry causes the dipoles to cancel out, making it non-polar.
- Similar to CCl4, the only intermolecular forces present are London dispersion forces.
- **Conclusion**: CO2 also has a low value of 'a'.

5. **Analyze H2O**:
- H2O is a polar molecule with a significant dipole moment due to the high electronegativity of oxygen.
- Water exhibits strong hydrogen bonding, which is one of the strongest types of intermolecular forces.
- **Conclusion**: H2O has the highest value of 'a' among the gases considered due to its strong hydrogen bonding.

6. **Final Comparison**:
- Comparing the values of 'a':
- CCl4: low 'a' (London dispersion forces)
- NH3: moderate 'a' (hydrogen bonding)
- CO2: low 'a' (London dispersion forces)
- H2O: high 'a' (strong hydrogen bonding)
- **Final Answer**: H2O has the highest value of van der Waals constant 'a'.