Which of the following salt will not give positive brown ring test?

Options

A. `Cu(NO_(3))_(2)`
B. `Pb(NO_(3))_(2)`
C. `Zn(NO_(3))_(2)`
D. `Mg(NO_(3))_(2)`

Explanation

To determine which of the following salts will not give a positive brown ring test, we need to understand the principles behind the test and the behavior of the salts in question. The brown ring test is primarily used to detect the presence of nitrate ions (NO3-) in a solution.

### Step-by-Step Solution:

1. **Understanding the Brown Ring Test**:
- The brown ring test involves the reaction of nitrate ions with freshly prepared iron(II) sulfate (FeSO4) and concentrated sulfuric acid (H2SO4). When nitrate ions are present, a brown ring forms at the interface of the two liquids due to the formation of iron(II) nitrate complex.

2. **Identifying the Salts**:
- The salts in question are Copper Nitrate (Cu(NO3)2), Zinc Nitrate (Zn(NO3)2), Magnesium Nitrate (Mg(NO3)2), and Lead Nitrate (Pb(NO3)2).

3. **Analyzing Each Salt**:
- **Copper Nitrate**: When dissolved, it dissociates into Cu²⁺ and NO3⁻ ions. The presence of NO3⁻ allows for a positive brown ring test.
- **Zinc Nitrate**: Similarly, it dissociates into Zn²⁺ and NO3⁻ ions, also allowing for a positive brown ring test.
- **Magnesium Nitrate**: This salt dissociates into Mg²⁺ and NO3⁻ ions, leading to a positive brown ring test as well.
- **Lead Nitrate**: When dissolved, it dissociates into Pb²⁺ and NO3⁻ ions. However, when concentrated H2SO4 is added, lead sulfate (PbSO4) is formed, which is insoluble in water.

4. **Impact of Lead Sulfate Formation**:
- The formation of insoluble lead sulfate (PbSO4) precipitate interferes with the brown ring test. Since the precipitate does not dissolve, it prevents the proper interaction of iron(II) ions with the nitrate ions, leading to a failure in producing the characteristic brown ring.

5. **Conclusion**:
- Therefore, the salt that will not give a positive brown ring test is **Lead Nitrate (Pb(NO3)2)**, as the formation of insoluble lead sulfate interferes with the test.

### Final Answer:
The salt that will not give a positive brown ring test is **Lead Nitrate (Pb(NO3)2)**.

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