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

Options

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

Explanation

To solve the question regarding the constant 'a' in van der Waals' equation, we need to understand what 'a' represents and how it varies among different gases.

### Step-by-Step Solution:

1. **Understanding van der Waals' Equation**:
The van der Waals equation is given by:
\[ \left(P + \frac{a}{V^2}\right)(V - b) = RT \]
Here, \(P\) is the pressure, \(V\) is the volume, \(T\) is the temperature, \(R\) is the gas constant, and \(a\) and \(b\) are van der Waals constants. The constant 'a' accounts for the attractive forces between gas molecules.

**Hint**: Remember that 'a' is related to the intermolecular forces present in the gas.

2. **Identifying the Role of 'a'**:
The value of 'a' indicates the strength of the attractive forces between the molecules of the gas. A higher value of 'a' means stronger intermolecular attractions.

**Hint**: Think about how molecular structure affects intermolecular forces.

3. **Comparing Different Gases**:
- **Helium (He)**: Being a noble gas, it has negligible intermolecular forces, so 'a' is very low.
- **Hydrogen (H2)**: Hydrogen molecules have weak Van der Waals forces but no significant polarity, so 'a' is low.
- **Oxygen (O2)**: Similar to hydrogen, oxygen has weak Van der Waals forces due to its non-polar nature, resulting in a low 'a'.
- **Ammonia (NH3)**: Ammonia has significant hydrogen bonding due to the presence of nitrogen, which is highly electronegative. This results in stronger intermolecular attractions, leading to a higher value of 'a'.

**Hint**: Consider the types of bonding and polarity in each molecule.

4. **Conclusion**:
Among the gases listed, ammonia (NH3) has the strongest intermolecular forces due to hydrogen bonding, which leads to the highest value of the constant 'a'. Therefore, the answer to the question is that 'a' is maximum in ammonia.

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