To which of the following mixture Dalton's law of partial pressure is not applicable ?

Options

A. `H_(2)` and He
B. `NH_(3)` and HCI
C. `N_(2)` and `H_(2)`
D. `N_(2)` and `O_(2)`

Explanation

To determine which mixture Dalton's law of partial pressure is not applicable to, we need to understand the conditions under which this law holds true.

### Step-by-Step Solution:

1. **Understand Dalton's Law of Partial Pressure**:
- Dalton's law states that in a mixture of non-reacting gases, the total pressure exerted by the mixture is equal to the sum of the partial pressures of each individual gas. Mathematically, it can be expressed as:
\[ P_{total} = P_A + P_B + P_C + \ldots \]
- The partial pressure of each gas is directly proportional to its mole fraction in the mixture.

2. **Identify the Limitation of Dalton's Law**:
- The key limitation of Dalton's law is that it is only applicable to mixtures of gases that do not react with each other. If the gases react, they cannot be treated as independent contributors to the total pressure.

3. **Analyze Each Option**:
- **Option A: Hydrogen and Helium**:
- These gases do not react with each other. Therefore, Dalton's law is applicable.
- **Option B: Ammonia (NH₃) and Hydrogen Chloride (HCl)**:
- These gases react with each other to form ammonium chloride (NH₄Cl). Therefore, Dalton's law is **not applicable** here.
- **Option C: Nitrogen (N₂) and Hydrogen (H₂)**:
- These gases do not react under normal conditions. Therefore, Dalton's law is applicable.
- **Option D: Nitrogen (N₂) and Oxygen (O₂)**:
- These gases do not react with each other under normal conditions. Therefore, Dalton's law is applicable.

4. **Conclusion**:
- The only mixture where Dalton's law of partial pressure is not applicable is **Option B: Ammonia and HCl**.

### Final Answer:
The correct option is **B: Ammonia and HCl**.