In which of the following redox reaction precipitate is not formed?

Options

A. `Cr^(3+)(aq.)+Na_(2)O_(2)("Solution")to`
B. `Fe^(3+)(aq.)+(NH_(4))_(2)S to `
C. `Mn^(2+)(aq.)+H_(2)O_(2)+NH_(3)("Solution")to`
D. `Fe^(2+)(aq.)+Na_(2)O_(2)("solution")to`

Explanation

To determine in which of the given redox reactions a precipitate is not formed, we will analyze each reaction step by step.

### Step 1: Analyze the first reaction
**Reaction:** Cr³⁺ (aq) + Na₂O₂ (aq)
**Products:** CrO₄²⁻ (aq)

- **Observation:** The product CrO₄²⁻ is in aqueous solution, meaning it is soluble in water. Therefore, no precipitate is formed in this reaction.

### Step 2: Analyze the second reaction
**Reaction:** Fe³⁺ (aq) + (NH₄)₂S (aq)
**Products:** FeS (s)

- **Observation:** The product FeS is a solid and is insoluble in water, indicating that a precipitate is formed in this reaction.

### Step 3: Analyze the third reaction
**Reaction:** MnO₂ (s) + (NH₄)₂S (aq) + NH₃ (aq)
**Products:** MnS (s)

- **Observation:** The product MnS is also a solid and is insoluble in water, indicating that a precipitate is formed in this reaction as well.

### Step 4: Analyze the fourth reaction
**Reaction:** Fe²⁺ (aq) + Na₂O₂ (aq)
**Products:** Fe(OH)₂ (s)

- **Observation:** The product Fe(OH)₂ is a solid and is insoluble in water, indicating that a precipitate is formed in this reaction.

### Conclusion
After analyzing all four reactions, we find that:
- **No precipitate is formed in the first reaction (Cr³⁺ + Na₂O₂).**
- **Precipitates are formed in the second (Fe³⁺ + (NH₄)₂S), third (MnO₂ + (NH₄)₂S + NH₃), and fourth (Fe²⁺ + Na₂O₂) reactions.**

Thus, the correct answer is that a precipitate is not formed in the **first reaction**.

### Final Answer:
**The reaction in which a precipitate is not formed is the first reaction (Cr³⁺ + Na₂O₂).**

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