Critical constants are related with van der waals' constant as follows:
`V_( c) = 3b, P_( c) =a/(27b^(2)), T_( c) =(8a)/(27 Rb)`
The pressure required to liquefy a gas at the critical temperature is called :

Options

A. reduced pressure
B. critical pressure
C. vapour pressure
D. atmospheric pressure

Answer

To solve the question regarding the pressure required to liquefy a gas at the critical temperature, we can follow these steps:

### Step-by-Step Solution:

1. **Understand Critical Constants**:
- The critical constants are defined as:
- \( V_c = 3b \) (Critical Volume)
- \( P_c = \frac{a}{27b^2} \) (Critical Pressure)
- \( T_c = \frac{8a}{27Rb} \) (Critical Temperature)

2. **Define Critical Temperature**:
- The critical temperature (\( T_c \)) is the temperature above which a gas cannot be liquefied, regardless of the pressure applied.

3. **Identify the Pressure at Critical Temperature**:
- At the critical temperature, the pressure required to liquefy a gas is known as the critical pressure (\( P_c \)). This is the maximum pressure that can be applied to a gas at its critical temperature to achieve liquefaction.

4. **Conclusion**:
- Therefore, the pressure required to liquefy a gas at the critical temperature is referred to as the **critical pressure**.

### Final Answer:
The pressure required to liquefy a gas at the critical temperature is called **critical pressure**.

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