Which of the following are correct statements ?
X) Vander Waal's constant a is a measure of attractive force
Y) Vander Waal's constant b is also called co-volume or excluded volume
Z) b is expressed in L `mol^(-1)`

Options

A. X,Y
B. Y,Z
C. X,Z
D. X,Y,Z

Answer

To determine which statements are correct regarding the van der Waals constants, we will analyze each statement one by one.

### Step-by-Step Solution:

1. **Understanding Van der Waals Constants:**
The van der Waals equation is given by:
\[ \left( P + \frac{a n^2}{V^2} \right)(V - nb) = nRT \]
Here, \( a \) and \( b \) are the van der Waals constants.

2. **Analyzing Statement X:**
- **Statement:** "Van der Waals constant \( a \) is a measure of attractive force."
- **Explanation:** The constant \( a \) quantifies the magnitude of attractive forces between the molecules. Therefore, this statement is **correct**.

3. **Analyzing Statement Y:**
- **Statement:** "Van der Waals constant \( b \) is also called co-volume or excluded volume."
- **Explanation:** The constant \( b \) represents the volume occupied by the molecules themselves, which is often referred to as the co-volume or excluded volume. Thus, this statement is also **correct**.

4. **Analyzing Statement Z:**
- **Statement:** "b is expressed in L mol\(^{-1}\)."
- **Explanation:** The unit of \( b \) is indeed liters per mole (L mol\(^{-1}\)), as it represents the volume per mole of the substance. Hence, this statement is **correct**.

5. **Conclusion:**
Since all three statements (X, Y, and Z) have been verified to be correct, the final answer is that all statements are correct.

### Final Answer:
All statements X, Y, and Z are correct.