The brown ring test for `NO_(3)^(-)` is due to the formation of the complex ion with formula

Options

A. `[Fe(H_(2)O)_(6)]^(2+)`
B. `[Fe(NO(CN)_(5)]^(2-)`
C. `[Fe(H_(2)O)_(5)NO]^(2+)`
D. `[Fe(H_(2)O)(NO)_(5)]^(2+)`

Answer

To solve the question regarding the brown ring test for the nitrate ion \((NO_3^−)\), we need to understand the chemical reactions involved and the formation of the complex ion responsible for the brown color.

### Step-by-Step Solution:

1. **Understanding the Brown Ring Test**:
The brown ring test is a qualitative analysis method used to detect the presence of nitrate ions in a solution. The test involves the reaction of ferrous ions \((Fe^{2+})\) with nitrate ions.

2. **Reaction of Ferrous Ion with Nitrate**:
When ferrous ions are mixed with nitrate ions, the ferrous ion is oxidized to ferric ion \((Fe^{3+})\), while the nitrate ion is reduced to nitric oxide \((NO)\):
\[ 3Fe^{2+} + 2NO_3^{-} + 4H^+ \rightarrow 3Fe^{3+} + 2NO + 2H_2O \]

3. **Formation of the Complex Ion**:
The nitric oxide produced in the reaction then interacts with the ferric ion \((Fe^{3+})\) in the presence of water. The nitric oxide coordinates with the ferric ion, replacing one of the water molecules in the coordination sphere. The complex formed is:
\[ [Fe(H_2O)_{5}(NO)]^{2+} \]
This complex is responsible for the characteristic brown color observed in the test.

4. **Conclusion**:
The brown ring observed in the test is due to the formation of the complex ion \([Fe(H_2O)_{5}(NO)]^{2+}\). Therefore, the correct answer to the question is that the brown ring test for \((NO_3^−)\) is due to the formation of this complex ion.

### Final Answer:
The complex ion responsible for the brown ring test is \([Fe(H_2O)_{5}(NO)]^{2+}\).

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