Daltons law of partial pressures cannot be applied to the following pair of gases , when they are mixed

Options

A. `H_(2)` & `O_(2)`
B. `H_(2)` & `N_(2)`
C. `CO_(2)` & `O_(2)`
D. `NH_(3)` & HCl

Answer

To determine which pair of gases Dalton's law of partial pressures cannot be applied to when mixed, we need to understand the conditions under which Dalton's law is applicable.

### Step-by-Step Solution:

1. **Understanding Dalton's Law of Partial Pressures**:
- Dalton's law states that in a mixture of non-reacting gases, the total pressure is equal to the sum of the partial pressures of each gas. Mathematically, it can be expressed as:
\[ P_{total} = P_1 + P_2 + P_3 + \ldots \]
- This law is only applicable when the gases do not react with each other.

2. **Analyzing Each Option**:
- **Option A: H2 and O2**:
- Hydrogen (H2) and oxygen (O2) can react to form water (H2O) under certain conditions, but at room temperature, they do not spontaneously react. Therefore, Dalton's law is applicable here.

- **Option B: H2 and N2**:
- Hydrogen (H2) and nitrogen (N2) do not react under normal conditions. The activation energy required for their reaction is very high, making Dalton's law applicable.

- **Option C: CO2 and O2**:
- Carbon dioxide (CO2) and oxygen (O2) do not react under normal conditions. Therefore, Dalton's law is applicable to this pair as well.

- **Option D: NH3 and HCl**:
- Ammonia (NH3) and hydrochloric acid (HCl) react to form ammonium chloride (NH4Cl). This is a spontaneous reaction that occurs readily at room temperature. Therefore, Dalton's law is not applicable to this pair.

3. **Conclusion**:
- Based on the analysis, the pair of gases for which Dalton's law of partial pressures cannot be applied is **NH3 and HCl** (Option D).

### Final Answer:
The correct answer is **D: NH3 and HCl**.