Gases tend to behave non-ideally at low temperatures and high pressures. The deviation from ideal behaviour can be explained by considering two types of corrections. They are volume correction and pressure correction.
Following represents equation of state for n moles of real gas  `[P + (n^2a)/(V^2)][V-nb] = nRT`. Select incorrect statement for a real gas

Options

A. Constant 'a' is a measure of force of attraction among gas molecules
B. a is expressed in atm `L^(2) "mol"^(-2)` b is expressed in L `"mol"^(-1)`
C. At high pressure, compression factor is `(1+ (Pb)/(RT))`
D. `(n^(2)a)/V^(2)` is also called internal volume.

Answer

To solve the question regarding the incorrect statement for a real gas based on the Van der Waals equation, we will analyze each statement step by step.

### Step-by-Step Solution:

1. **Understanding the Van der Waals Equation**:
The Van der Waals equation for n moles of a real gas is given by:
\[ \left[P + \frac{n^2a}{V^2}\right](V - nb) = nRT \]
Here, \(a\) and \(b\) are constants specific to each gas, where \(a\) accounts for the attractive forces between molecules and \(b\) accounts for the volume occupied by the gas molecules themselves.

2. **Analyzing the First Statement**:
- **Statement**: "Constant A is a measure of force of attraction among gas molecules."
- **Evaluation**: This statement is correct because the constant \(a\) in the equation represents the strength of intermolecular forces.

3. **Analyzing the Second Statement**:
- **Statement**: "A is expressed in atmosphere liter square per mole square and B is expressed in liter per mole."
- **Evaluation**: This statement is also correct. The unit of \(a\) can be derived as follows:
\[ \text{Units of } a = \text{Pressure} \times \text{Volume}^2 / \text{Moles}^2 = \text{atm} \cdot \text{L}^2/\text{mol}^2 \]
And for \(b\):
\[ \text{Units of } b = \text{Volume}/\text{Moles} = \text{L/mol} \]

4. **Analyzing the Third Statement**:
- **Statement**: "At high pressure, compression factor is \(1 + \frac{Pb}{RT}\)."
- **Evaluation**: This statement is correct. At high pressures, the term \( \frac{n^2a}{V^2} \) becomes negligible, and the equation simplifies to the ideal gas law, allowing us to express the compressibility factor \(Z\) correctly.

5. **Analyzing the Fourth Statement**:
- **Statement**: "n square A by V square is called internal volume."
- **Evaluation**: This statement is incorrect. The term \( \frac{n^2a}{V^2} \) is a pressure correction term, not an internal volume. It accounts for the attractive forces between molecules, which affect the pressure exerted by the gas.

### Conclusion:
The incorrect statement is the fourth one: "n square A by V square is called internal volume."

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