Prussian blue is formed when

Options

A. ferrous sulphate reacts with `FeCl_(3)`
B. ferric sulphate reacts with `Na_(4)[Fe(CN)_(6)]`
C. ferrous ammonium sulphate reacts with `FeCl_(3)`
D. ammonium sulphate reacts with `FeCl_(3)`

Explanation

To determine how Prussian blue is formed, we need to analyze the given reactions and identify which one leads to the formation of Prussian blue, whose chemical formula is Fe₄[Fe(CN)₆]₃.

### Step-by-Step Solution:

1. **Identify the Chemical Formula of Prussian Blue**:
- Prussian blue has the formula Fe₄[Fe(CN)₆]₃. This indicates that cyanide ions (CN⁻) are involved in its formation.

2. **Examine the Given Reactions**:
- **Reaction A**: Ferrous sulfate reacts with FeCl₃.
- **Reaction B**: Ferric sulfate reacts with sodium hexacyanoferrate.
- **Reaction C**: Ferrous ammonium sulfate reacts with ferric chloride.
- **Reaction D**: Ammonium sulfate reacts with ferric chloride.

3. **Identify the Presence of Cyanide**:
- From the reactions, only **Reaction B** includes sodium hexacyanoferrate, which contains cyanide ions (CN⁻). The other reactions do not involve cyanide.

4. **Write the Reaction for Reaction B**:
- The reaction can be written as:
\[ \text{Fe}_2(SO_4)_3 + 3 \text{Na}_2[Fe(CN)_6] \rightarrow 3 \text{Na}_2SO_4 + \text{Fe}_4[Fe(CN)_6]_3 \]
- This is a double displacement reaction where the iron ions displace each other, leading to the formation of sodium sulfate and Prussian blue.

5. **Balance the Reaction**:
- To balance the reaction:
- Start with the iron atoms: 4 Fe on the product side means we need 2 Fe from Fe₂(SO₄)₃ and 3 from sodium hexacyanoferrate.
- Balance sodium and sulfate ions accordingly.
- The balanced equation would be:
\[ \text{Fe}_2(SO_4)_3 + 3 \text{Na}_2[Fe(CN)_6] \rightarrow 3 \text{Na}_2SO_4 + \text{Fe}_4[Fe(CN)_6]_3 \]

6. **Conclusion**:
- Therefore, the formation of Prussian blue occurs when **ferric sulfate reacts with sodium hexacyanoferrate**.

### Final Answer:
Prussian blue is formed when **ferric sulfate reacts with sodium hexacyanoferrate** (Option B).