What happens when ?
(a)To a `Zn^(2+)` ions solution faintly acidified with `2M` acetic acid, `0.1 mL` of `0.25 M CuSO_(4)` solution and `2 mL` of ammonium tetrathiocyanatomercurate `(II)` reagents is added.
(b) The above test is performed in absence of `CuSO_(4)` solution.

Answer

### Step-by-Step Solution

#### (a) Reaction of Zn²⁺ ions with CuSO₄ and ammonium tetrathiocyanatomercurate(II)

1. **Starting Solution**: We have a solution of Zn²⁺ ions that is faintly acidified with 2M acetic acid (CH₃COOH).

2. **Addition of CuSO₄**: When 0.1 mL of 0.25 M CuSO₄ solution is added to the Zn²⁺ solution, the copper ions (Cu²⁺) can interact with the zinc ions.

3. **Addition of Ammonium Tetrathiocyanatomercurate(II)**: Next, we add 2 mL of ammonium tetrathiocyanatomercurate(II) (NH₄)₂[Hg(SCN)₄].

4. **Formation of Complexes**: The reaction between Zn²⁺ ions and the ammonium salt leads to the formation of two complex compounds:
- **Zn(Hg(SCN)₄)²⁻**: This complex is formed from the interaction of Zn²⁺ with the ammonium tetrathiocyanatomercurate(II).
- **Cu(Hg(SCN)₄)²⁻**: This complex is formed from the interaction of Cu²⁺ with the ammonium tetrathiocyanatomercurate(II).

5. **Color of the Complexes**: Both of these complexes exhibit a dark violet or blackish-purple color.

#### (b) Reaction of Zn²⁺ ions with ammonium tetrathiocyanatomercurate(II) in absence of CuSO₄

1. **Starting Solution**: The same faintly acidified Zn²⁺ solution is used, but this time we do not add CuSO₄.

2. **Addition of Ammonium Tetrathiocyanatomercurate(II)**: We directly add 2 mL of ammonium tetrathiocyanatomercurate(II) to the Zn²⁺ solution.

3. **Formation of Complex**: The reaction will still occur, leading to the formation of the complex:
- **Zn(Hg(SCN)₄)²⁻**: This complex is formed from the interaction of Zn²⁺ with the ammonium salt.

4. **Color of the Complex**: In this case, the resulting complex will be white in color, indicating a different interaction without the presence of Cu²⁺ ions.

### Summary of Results
- **With CuSO₄**: Dark violet or blackish-purple complexes are formed (Zn(Hg(SCN)₄)²⁻ and Cu(Hg(SCN)₄)²⁻).
- **Without CuSO₄**: A white precipitate (Zn(Hg(SCN)₄)²⁻) is formed.