`Mg` is not precipitated in `V` group because:

Options

A. `MgCO_(3)` is soluble in water.
B. `K_(sp)` of `MgCO_(3)` is high.
C. `MgCO_(3)` is soluble in `NH_(4)OH`
D. None

Answer

To understand why magnesium (Mg) is not precipitated in the fifth group of qualitative analysis, we can break down the reasoning step by step.

### Step-by-Step Solution:

1. **Understanding Group V Precipitation**:
- In qualitative analysis, Group V typically includes cations that are precipitated by the addition of ammonium carbonate (NH4)2CO3 in the presence of ammonia (NH4OH).

2. **Role of Magnesium Carbonate (MgCO3)**:
- Magnesium can form magnesium carbonate (MgCO3) when reacting with carbonate ions. However, the solubility of MgCO3 is a key factor in this analysis.

3. **Solubility of MgCO3**:
- The solubility product constant (Ksp) of MgCO3 is relatively high. This means that at equilibrium, a significant amount of MgCO3 remains dissolved in solution rather than precipitating out.

4. **Decomposition of MgCO3**:
- When MgCO3 is formed, it can decompose into magnesium ions (Mg²⁺) and carbonate ions (CO3²⁻). This decomposition further contributes to the presence of Mg²⁺ ions in solution.

5. **Behavior of Alkaline Earth Metals**:
- Magnesium belongs to the alkaline earth metals, which generally have a tendency to lose electrons and form cations (Mg²⁺ in this case). This tendency means that magnesium will remain in solution rather than precipitating out as a solid.

6. **Conclusion**:
- Due to the high solubility of MgCO3 and the behavior of magnesium ions in solution, magnesium does not precipitate in Group V.

### Final Answer:
Magnesium (Mg) is not precipitated in Group V because MgCO3 is soluble in water, and its solubility product constant (Ksp) is high, preventing the formation of a precipitate.

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