`X_(2)S_(n)+water to X(OH)_(n) darr+ underset((Gas))(Y uarr)overset(Pb(CH_(3)COO)_(2))tounderset(("Black ppt."))(Z darr)`
Then (X) cation can not be:

Options

A. `Fe^(3+)`
B. `Al^(3+)`
C. `Cr^(3+)`
D. `Mg^(2+)`

Answer

To determine which cation (X) cannot produce hydrogen sulfide (H₂S) when reacting with water, we will analyze the reactions of various cations with water and check if H₂S is produced.

### Step-by-Step Solution:

1. **Identify the Reaction**:
The initial reaction is:
\[ X_2S_n + \text{water} \rightarrow X(OH)_n + Y \text{ (gas)} \]
Here, Y is identified as H₂S gas.

2. **Analyze Cations**:
We need to analyze different cations to see if they produce H₂S when reacting with water.

3. **Cation Mg²⁺**:
- Reaction:
\[ MgS + \text{water} \rightarrow Mg(OH)_2 + H_2S \]
- H₂S is produced. Therefore, Mg²⁺ cannot be the answer.

4. **Cation Cr³⁺**:
- Reaction:
\[ Cr_2S_3 + \text{water} \rightarrow Cr(OH)_3 + H_2S \]
- H₂S is produced. Therefore, Cr³⁺ cannot be the answer.

5. **Cation Al³⁺**:
- Reaction:
\[ Al_2S_3 + \text{water} \rightarrow Al(OH)_3 + H_2S \]
- H₂S is produced. Therefore, Al³⁺ cannot be the answer.

6. **Cation Fe³⁺**:
- Reaction:
\[ Fe_2S_3 + \text{water} \rightarrow Fe(OH)_3 + \text{(no H₂S)} \]
- H₂S is **not** produced. Therefore, Fe³⁺ is the cation that cannot produce H₂S.

7. **Conclusion**:
The cation (X) that cannot produce H₂S when reacting with water is **Fe³⁺**.

### Final Answer:
The cation (X) cannot be: **Fe³⁺**.

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