Which of the following pairs does not exhibit colour in solution ?

Options

A. `ZnCl_2, HgCl_2`
B. `CuCl_2, VOCl_2`
C. `VOCl_2, FeCl_2`
D. `MnCl_2, FeCl_3`

Answer

To determine which of the following pairs does not exhibit color in solution, we need to analyze the oxidation states and electronic configurations of the given metal ions. The presence of unpaired electrons in the d-orbitals is key to determining whether a solution will exhibit color.

### Step-by-Step Solution:

1. **Identify the Oxidation States**:
- For each metal in the pairs, identify the oxidation state.
- For example, if we have ZnCl2, Zn is in the +2 oxidation state.

2. **Write the Electronic Configurations**:
- Write the electronic configurations for the metal ions in their respective oxidation states:
- Zn²⁺: [Ar] 3d¹⁰
- Hg²⁺: [Xe] 4f¹⁴ 5d¹⁰
- Cu²⁺: [Ar] 3d⁹
- V⁴⁺: [Ar] 3d¹
- Fe³⁺: [Ar] 3d⁵
- Mn²⁺: [Ar] 3d⁵

3. **Count the Unpaired Electrons**:
- Determine the number of unpaired electrons in each case:
- Zn²⁺: 0 unpaired electrons (3d¹⁰)
- Hg²⁺: 0 unpaired electrons (5d¹⁰)
- Cu²⁺: 1 unpaired electron (3d⁹)
- V⁴⁺: 1 unpaired electron (3d¹)
- Fe²⁺: 4 unpaired electrons (3d⁶)
- Mn²⁺: 5 unpaired electrons (3d⁵)
- Fe³⁺: 5 unpaired electrons (3d⁵)

4. **Determine Color Exhibiting Behavior**:
- A solution will exhibit color if there are unpaired electrons available for d-d transitions.
- Pairs with the same number of unpaired electrons will show the same color.

5. **Identify the Pair That Does Not Exhibit Color**:
- From the analysis, Zn²⁺ and Hg²⁺ both have 0 unpaired electrons, meaning they will not exhibit color in solution.
- Other pairs like Cu²⁺ and V⁴⁺ have 1 unpaired electron, while Fe²⁺ and Fe³⁺ have multiple unpaired electrons.

### Conclusion:
The pair that does not exhibit color in solution is **Zn²⁺ and Hg²⁺**.