Which of the following combines with `Fe^(2+)` ions to form brown complex ?

Options

A. `N_(2)O`
B. `NO`
C. `N_(2)O_(3)`
D. `N_(2)O_(5)`

Answer

To solve the question "Which of the following combines with Fe^(2+) ions to form a brown complex?", we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Context**: We need to identify which of the given compounds can react with Fe^(2+) ions to form a brown complex. The options provided are:
- Option 1: N2O
- Option 2: NO
- Option 3: N2O3
- Option 4: N2O5

2. **Recall the Brown Ring Test**: The question refers to a well-known qualitative analysis test called the "Brown Ring Test". This test is used to detect the presence of nitrate ions (NO3^-) in a solution.

3. **Identify the Reactants**: In the Brown Ring Test, ferrous sulfate (FeSO4) is mixed with concentrated sulfuric acid (H2SO4) and the compound containing nitrate ions (NO3^-). The ferrous ions (Fe^(2+)) react with the nitrate ions.

4. **Determine the Reaction**: The nitrate ion reacts with Fe^(2+) ions and protons (H^+) from the sulfuric acid to produce nitric oxide (NO) and ferric ions (Fe^(3+)). The nitric oxide (NO) then forms a complex with the ferrous ions.

5. **Identify the Complex**: The resulting complex formed is [Fe(H2O)5(NO)]^(2+), which is brown in color.

6. **Analyze the Options**:
- N2O does not typically react to form a brown complex.
- NO (Nitric Oxide) is known to react with Fe^(2+) to form the brown complex.
- N2O3 and N2O5 do not form a brown complex with Fe^(2+).

7. **Conclusion**: Based on the above analysis, the correct answer is **Option 2: NO**.

### Final Answer:
The compound that combines with Fe^(2+) ions to form a brown complex is **NO**.

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