Cold ferrous sulphate solution on absorption of NO develops brown colour due to the formation of

Options

A. paramagnetic `[Fe(H_(2)O)_(5)(NO)]SO_(4)`
B. diamagnetic `[Fe(H_(2)O)_(5)(N_(3))]SO_(4)`
C. paramagnetic `[Fe(H_(2)O)_(5)(NO_(3))](SO_(4))_(2)`
D. diamagnetic `[Fe(H_(2)O)_(4)(SO_(4))]NO_(3)`

Explanation

To solve the question regarding the formation of a brown color in a cold ferrous sulfate solution upon absorption of NO, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Reactants**:
- The primary reactant is ferrous sulfate, which has the formula \( \text{FeSO}_4 \).
- The gas involved in the reaction is nitric oxide (NO).

2. **Understand the Reaction Conditions**:
- The reaction occurs in the presence of a dilute acid, typically sulfuric acid (\( \text{H}_2\text{SO}_4 \)).
- This acid provides protons (\( \text{H}^+ \)) that are necessary for the reaction.

3. **Reaction of NO with Ferrous Ion**:
- When NO is absorbed in the ferrous sulfate solution, it reacts with the ferrous ions (\( \text{Fe}^{2+} \)).
- The ferrous ion can form a complex with NO, leading to the formation of a brown ring complex.

4. **Formation of Nitrogen Dioxide**:
- The reaction of NO with oxygen in the presence of ferrous ions leads to the formation of nitrogen dioxide (\( \text{NO}_2 \)), which is responsible for the brown color.
- The overall reaction can be summarized as follows:
\[ \text{Fe}^{2+} + 2 \text{NO} + 2 \text{H}_2\text{O} \rightarrow \text{[Fe(NO)]}^{2+} + 2 \text{H}^+ + \text{O}_2 \]
- The complex formed is often represented as \( \text{[Fe(NO)]}^{2+} \).

5. **Determine the Oxidation State**:
- In the complex \( \text{[Fe(NO)]}^{2+} \), the iron is in the +1 oxidation state.
- The presence of unpaired electrons in the d-orbitals of iron indicates that the complex is paramagnetic.

6. **Conclusion**:
- The brown color observed in the solution is due to the formation of the brown ring complex \( \text{[Fe(NO)]}^{2+} \), which is paramagnetic.

### Final Answer:
The brown color in the cold ferrous sulfate solution upon absorption of NO is due to the formation of the brown ring complex \( \text{[Fe(NO)]}^{2+} \).

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