Which of the following pairs of sulphides are insoluble in dilute HCl?

Options

A. CoS and NiS
B. CoS and MnS
C. NiS and MnS
D. NiS and ZnS

Answer

To determine which pairs of sulfides are insoluble in dilute HCl, we will analyze the solubility of the sulfides formed from the given cations. Here’s a step-by-step solution:

### Step 1: Identify the Cations
The pairs of sulfides provided in the question contain the following cations:
- Cu²⁺ (Copper)
- Ni²⁺ (Nickel)
- Mn²⁺ (Manganese)
- Zn²⁺ (Zinc)

### Step 2: Determine the Group of Cations
All the cations mentioned belong to Group 4 of qualitative analysis, which means they can form sulfides when treated with H2S in the presence of NH4Cl and NH4OH.

### Step 3: Write the Reactions
When these cations react with H2S, they form the following sulfides:
- Cu²⁺ + H2S → CuS (Cobalt Sulfide) - Black precipitate
- Ni²⁺ + H2S → NiS (Nickel Sulfide) - Black precipitate
- Zn²⁺ + H2S → ZnS (Zinc Sulfide) - White precipitate
- Mn²⁺ + H2S → MnS (Manganese Sulfide) - Buff pink precipitate

### Step 4: Analyze Solubility in Dilute HCl
Next, we need to determine the solubility of these sulfides in dilute HCl:
- CuS (Cobalt Sulfide) - Insoluble in dilute HCl
- NiS (Nickel Sulfide) - Insoluble in dilute HCl
- ZnS (Zinc Sulfide) - Soluble in dilute HCl
- MnS (Manganese Sulfide) - Soluble in dilute HCl

### Step 5: Identify the Insoluble Pairs
From the analysis, we find that:
- CuS and NiS are insoluble in dilute HCl.
- ZnS and MnS are soluble in dilute HCl.

Thus, the pair of sulfides that are insoluble in dilute HCl is:
- **CuS and NiS**

### Conclusion
The correct answer to the question is that the pair of sulfides that are insoluble in dilute HCl is **Copper Sulfide (CuS) and Nickel Sulfide (NiS)**.

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