`H_(2)S` would separate the following in `pH lt 7`

Options

A. `Zn^(2+),Co^(2+)`
B. `Cu^(2+),Cd^(2+)`
C. `Cu^(2+),Cr^(3+)`
D. `Cu^(2+),As^(3+)`

Answer

To solve the problem of which cations would be separated by H₂S in a pH less than 7, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Conditions**:
- We are dealing with H₂S (hydrogen sulfide) in an acidic medium (pH < 7). In this environment, H₂S acts as a reagent that can precipitate certain metal cations as sulfides.

2. **Identify Group II Cations**:
- In acidic conditions, H₂S is known to precipitate Group II cations. The common cations that fall under this category include:
- Lead (Pb²⁺)
- Copper (Cu²⁺)
- Cadmium (Cd²⁺)
- Mercury (Hg²⁺)
- Arsenic (As³⁺)

3. **Analyze the Options**:
- We need to check the provided options to see which ones contain cations that can form precipitates with H₂S in an acidic medium.
- The options provided in the question include combinations of cations.

4. **Evaluate Each Option**:
- **Option A**: (not specified, but let's assume it doesn't contain Group II cations)
- **Option B**: Copper (Cu²⁺) and Cadmium (Cd²⁺) - Both are Group II cations and will precipitate with H₂S.
- **Option C**: Copper (Cu²⁺) and Chromium (Cr³⁺) - Chromium is a Group III cation and will not precipitate in acidic medium.
- **Option D**: Copper (Cu²⁺) and Arsenic (As³⁺) - Arsenic is a Group II cation and will precipitate with H₂S.

5. **Conclude the Answer**:
- The correct options that would be separated by H₂S in a pH less than 7 are:
- **Option B**: Copper and Cadmium
- **Option D**: Copper and Arsenic

### Final Answer:
The cations that would be separated by H₂S in a pH less than 7 are:
- **Option B**: Copper and Cadmium
- **Option D**: Copper and Arsenic