Find total number of metal cations which are ppted as metal sulphide on passing `H_(2)S` gas through metal salt solution.
`Pb^(2+)(aq.)," "Mn^(2+)(aq.)," "Sn^(2+)(aq.)," "Cr^(3+)(aq.)," "Mg^(2+)(aq.)," "Hg^(2+)(aq.)," "Cu^(2+)(aq.)," "Ag^(+)(aq.)," "Al^(3+)(aq.)," "Ni^(2+)(aq.)`

Answer

To find the total number of metal cations that are precipitated as metal sulfides when passing H₂S gas through metal salt solutions, we will analyze each metal cation provided in the question based on their solubility in relation to hydrogen sulfide.

### Step-by-Step Solution:

1. **Identify the Metal Cations**: The metal cations given are:
- Pb²⁺
- Mn²⁺
- Sn²⁺
- Cr³⁺
- Mg²⁺
- Hg²⁺
- Cu²⁺
- Ag⁺
- Al³⁺
- Ni²⁺

2. **Understand the Precipitation of Metal Sulfides**:
- When H₂S is passed through a solution of metal salts, metal cations that form insoluble sulfides will precipitate out of the solution.
- The solubility product (Ksp) of the metal sulfides plays a crucial role in determining whether a precipitate will form. Generally, metal sulfides with a low Ksp (less than 10⁻³⁰) will precipitate.

3. **Check Each Cation Against Known Ksp Values**:
- **Pb²⁺**: Forms PbS (Ksp < 10⁻³⁰) - Precipitates.
- **Mn²⁺**: Forms MnS (Ksp < 10⁻³⁰) - Precipitates.
- **Sn²⁺**: Forms SnS (Ksp < 10⁻³⁰) - Precipitates.
- **Cr³⁺**: Forms Cr₂S₃ (Ksp > 10⁻³⁰) - Does not precipitate.
- **Mg²⁺**: Forms MgS (Ksp > 10⁻³⁰) - Does not precipitate.
- **Hg²⁺**: Forms HgS (Ksp < 10⁻³⁰) - Precipitates.
- **Cu²⁺**: Forms CuS (Ksp < 10⁻³⁰) - Precipitates.
- **Ag⁺**: Forms Ag₂S (Ksp < 10⁻³⁰) - Precipitates.
- **Al³⁺**: Forms Al₂S₃ (Ksp > 10⁻³⁰) - Does not precipitate.
- **Ni²⁺**: Forms NiS (Ksp < 10⁻³⁰) - Precipitates.

4. **Count the Precipitating Cations**:
- The cations that precipitate as metal sulfides are:
- Pb²⁺
- Mn²⁺
- Sn²⁺
- Hg²⁺
- Cu²⁺
- Ag⁺
- Ni²⁺
- Total = 7 cations.

### Final Answer:
The total number of metal cations that are precipitated as metal sulfides on passing H₂S gas through metal salt solutions is **7**.