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 determine the total number of metal cations that are precipitated as metal sulfides when passing H₂S gas through a metal salt solution, we need to analyze the behavior of various metal cations in both acidic and basic media.

### Step-by-Step Solution:

1. **Understanding the Dissociation of H₂S**:
H₂S dissociates in solution to produce H⁺ and S²⁻ ions. The presence of these ions will influence whether metal cations will precipitate as sulfides.

2. **Effect of Acidic Medium**:
- In acidic conditions, the concentration of H⁺ ions increases. According to Le Chatelier's principle, this will shift the equilibrium to the left, reducing the concentration of S²⁻ ions.
- Only metal cations with a Ksp (solubility product constant) value less than 10⁻³⁰ will precipitate as sulfides in acidic medium.

3. **Identifying Cations in Acidic Medium**:
- The cations that will precipitate in acidic medium include:
- Pb²⁺ (Lead)
- Hg²⁺ (Mercury)
- Cu²⁺ (Copper)
- Ni²⁺ (Nickel)
- Cations like Sn²⁺, Cr³⁺, Mg²⁺, Ag⁺, and Al³⁺ do not precipitate as sulfides in acidic medium.

4. **Effect of Basic Medium**:
- In basic conditions, the concentration of H⁺ ions decreases, which increases the concentration of S²⁻ ions.
- Metal cations with Ksp values between 10⁻²⁰ and 10⁻³⁰ will precipitate as sulfides in basic medium.

5. **Identifying Cations in Basic Medium**:
- The cations that will precipitate in basic medium include:
- Pb²⁺ (Lead)
- Ni²⁺ (Nickel)
- Mn²⁺ (Manganese)
- Fe³⁺ (Iron)
- Zn²⁺ (Zinc)
- Co²⁺ (Cobalt)
- Cations like Sn²⁺, Cr³⁺, Mg²⁺, and Ag⁺ do not precipitate as sulfides in basic medium.

6. **Counting the Total Unique Cations**:
- From the acidic medium, we have: Pb²⁺, Hg²⁺, Cu²⁺, Ni²⁺ (4 cations).
- From the basic medium, we have: Pb²⁺, Ni²⁺, Mn²⁺, Fe³⁺, Zn²⁺, Co²⁺ (6 cations).
- However, we must ensure not to double count cations that appear in both lists. The unique cations that precipitate are:
- Pb²⁺, Hg²⁺, Cu²⁺, Ni²⁺, Mn²⁺, Fe³⁺, Zn²⁺, Co²⁺.
- Thus, the total number of unique cations is 8.

### Final Answer:
The total number of metal cations that are precipitated as metal sulfides when H₂S gas is passed through the metal salt solution is **8**.