The constant 'a' in van der Waal's equation is maximum in

Options

A. Helium
B. Hydrogen
C. Oxygen
D. Ammonia.

Explanation

To determine which gas has the maximum value of the constant 'a' in the van der Waals equation, we need to analyze the intermolecular forces present in the given gases. The van der Waals equation is given by:

\[ P + \frac{a n^2}{V^2} = \frac{nRT}{V - nB} \]

Where:
- \( P \) = pressure
- \( n \) = number of moles
- \( V \) = volume
- \( R \) = universal gas constant
- \( T \) = temperature
- \( a \) = measure of the attractive forces between particles
- \( b \) = volume occupied by one mole of the gas particles

### Step-by-Step Solution:

1. **Understand the significance of 'a':**
The constant 'a' in the van der Waals equation represents the magnitude of the attractive forces between gas molecules. A higher value of 'a' indicates stronger intermolecular attractions.

2. **Analyze the given options:**
We have four gases to consider: Helium, Hydrogen, Oxygen, and Ammonia. We need to evaluate the intermolecular forces present in each of these gases.

3. **Intermolecular forces in each gas:**
- **Helium (He):** Being a noble gas, helium has very weak London dispersion forces and thus has a low value of 'a'.
- **Hydrogen (H2):** Hydrogen molecules exhibit weak Van der Waals forces, resulting in a low value of 'a'.
- **Oxygen (O2):** Oxygen has stronger Van der Waals forces than helium and hydrogen, but still not as strong as those in polar molecules.
- **Ammonia (NH3):** Ammonia exhibits hydrogen bonding due to the presence of a highly electronegative nitrogen atom bonded to hydrogen. This results in significantly stronger intermolecular forces.

4. **Conclusion:**
Since ammonia has the strongest intermolecular forces due to hydrogen bonding, it will have the maximum value of 'a' among the given options.

5. **Final Answer:**
The constant 'a' in van der Waals equation is maximum in **Ammonia (NH3)**.