Which of the following statements regarding van der Waals' constants a and b is not correct?

A. The constant a is a measure of van der Waals' forces.
B. A gas with a lower value of a possesses a greater tendency to get liquefied.
C. b is a measure of effective size of molecules.
D .b is the excluded volume per mole.

Options

A. The constant a is a measure of van der Waals' forces.
B. A gas with a lower value of a possesses a greater tendency to get liquefied.
C. b is a measure of effective size of molecules.
D. b is the excluded volume per mole.

Explanation

To determine which statement regarding van der Waals' constants a and b is not correct, we will analyze each statement one by one based on the principles of van der Waals' equation.

### Step-by-Step Solution:

1. **Understanding van der Waals' Equation**:
The van der Waals equation for real gases is given by:
\[ \left( P + \frac{a n^2}{V^2} \right)(V - nb) = nRT \]
Here, \( P \) is pressure, \( V \) is volume, \( n \) is the number of moles, \( R \) is the gas constant, \( T \) is temperature, \( a \) is a constant that accounts for the attractive forces between molecules, and \( b \) is a constant that accounts for the volume occupied by the gas molecules.

2. **Analyzing Statement A**:
- **Statement A**: "The constant a is a measure of van der Waals' forces."
- **Analysis**: This statement is correct. The constant \( a \) represents the strength of the attractive forces between gas molecules.

3. **Analyzing Statement B**:
- **Statement B**: "A gas with a lower value of a possesses a greater tendency to get liquefied."
- **Analysis**: This statement is incorrect. A lower value of \( a \) indicates weaker intermolecular forces, which means that the gas is less likely to be liquefied. Therefore, this statement is not true.

4. **Analyzing Statement C**:
- **Statement C**: "b is a measure of effective size of molecules."
- **Analysis**: This statement is correct. The constant \( b \) represents the volume occupied by the gas molecules, which is related to their effective size.

5. **Analyzing Statement D**:
- **Statement D**: "b is the excluded volume per mole."
- **Analysis**: This statement is also correct. The constant \( b \) accounts for the volume that is excluded due to the finite size of the gas molecules.

6. **Conclusion**:
- The only incorrect statement among the options provided is **Statement B**. Therefore, the answer to the question is **B**.

### Final Answer:
**B. A gas with a lower value of a possesses a greater tendency to get liquefied.** (This statement is not correct.)