For reaction, assuming large volume of water.
`H_(2)O(l)hArrH_(2)O(g) , "at temp". T K`
Choose correct options:

Options

A. On introduction of an inert gas at constant temperature pressure in the container remins same at equilibrium.
B. For this system `%` relative humidity always remains `100%` at constant temperature at equilibrium
C. If steam at temperature `2T` is passed into given system. After equilibrium is attained relative humidity changes.
D. This is a special case of equilibrium where pressure of `H_(2)O(g)` remains same always due to unique structural feature of `H_(2)O`.

Answer

To solve the problem regarding the equilibrium of the reaction \( H_2O(l) \rightleftharpoons H_2O(g) \) at temperature \( T \) K, we will analyze the provided options one by one.

### Step-by-Step Solution:

1. **Understanding the Reaction**:
The reaction involves the phase change of water from liquid to gas. At equilibrium, the rates of the forward and reverse reactions are equal.

2. **Analyzing the First Option**:
- The first option states that "on introduction of an inert gas at constant temperature, pressure in the container remains the same at equilibrium."
- When an inert gas is added to the system at constant temperature and pressure, the total pressure increases, but the partial pressure of water vapor decreases because the volume of the gas phase effectively increases.
- According to Le Chatelier's principle, the equilibrium will shift towards the side with fewer moles of gas to counteract the change, which in this case is the liquid side.
- **Conclusion**: This option is **incorrect**.

3. **Analyzing the Second Option**:
- The second option states that "for the system, percentage relative humidity always remains 100 percent."
- Relative humidity is defined as the ratio of the partial pressure of water vapor to the saturation pressure of water vapor at that temperature. At equilibrium, when the system is saturated, the relative humidity is indeed 100%.
- **Conclusion**: This option is **correct**.

4. **Analyzing the Third Option**:
- The third option mentions that "if steam at temperature 2T is passed into the given system after equilibrium is attained, relative humidity changes."
- If steam is added to the system, the partial pressure of water vapor will increase, which can lead to a change in relative humidity. However, if the system is already at equilibrium, the addition of steam does not guarantee that the relative humidity will remain constant.
- **Conclusion**: This option is **incorrect**.

5. **Analyzing the Fourth Option**:
- The fourth option states that "this is a special case of equilibrium where the pressure of \( H_2O \) gas remains the same due to unique structural features of water."
- The pressure of water vapor can change based on the conditions of the system (temperature, volume, etc.). The statement implies that the pressure remains constant regardless of changes, which is not true.
- **Conclusion**: This option is **incorrect**.

### Final Answer:
The correct option is **B**: "for the system, percentage relative humidity always remains 100 percent."