In fifth group, `(NH_(4))_(2)CO_(3)` is added to precipitate out the carbonates. We do not add `Na_(2)CO_(3)` because:

Options

A. `CaCO_(3)` is soluble in `Na_(2)CO_(3)`
B. `Na_(2)CO_(3)` increases the solubility of fifth group carbonates
C. `MgCO_(3)` will be precipitated out in fifth group
D. none

Answer

To answer the question regarding why `(NH₄)₂CO₃` is added to precipitate out carbonates instead of `Na₂CO₃`, we can break down the reasoning step by step:

### Step-by-Step Solution:

1. **Understanding the Role of Carbonates**:
- In qualitative analysis, carbonates are often precipitated to separate certain ions from a solution. This is particularly important in the fifth group of cations.

2. **Identifying the Compounds**:
- We are comparing two compounds: `(NH₄)₂CO₃` (ammonium carbonate) and `Na₂CO₃` (sodium carbonate).

3. **Electrolytic Properties**:
- `Na₂CO₃` is a strong electrolyte, meaning it dissociates completely in solution. This results in a higher concentration of carbonate ions (CO₃²⁻) in the solution.
- In contrast, `(NH₄)₂CO₃` is a weak electrolyte, which means it does not dissociate completely, leading to a lower concentration of carbonate ions.

4. **Solubility Product (Ksp)**:
- The solubility product (Ksp) of a compound indicates how soluble it is in water. `Na₂CO₃` has a high Ksp, which means it can lead to the precipitation of various carbonates, including magnesium carbonate (MgCO₃).
- `(NH₄)₂CO₃`, having a low Ksp, will not cause the precipitation of certain carbonates that might interfere with the analysis.

5. **Precipitation of Carbonates**:
- If `Na₂CO₃` were added, it would cause the precipitation of MgCO₃, which is undesirable in this analysis as it would complicate the separation of ions in the fifth group.
- Therefore, `(NH₄)₂CO₃` is preferred because it selectively precipitates the desired carbonates without causing the precipitation of unwanted ones like MgCO₃.

6. **Conclusion**:
- We do not add `Na₂CO₃` because it is a strong electrolyte with a high Ksp, which would lead to the unwanted precipitation of MgCO₃, while `(NH₄)₂CO₃` is a weak electrolyte with a low Ksp, allowing for selective precipitation.