Which one of the following species is stable in aqucous solution ?

Options

A. `Cr^(2+)`
B. `MnO_(4)^(2-)`
C. `MnO_(4)^(3-)`
D. `Cu^(+)`

Explanation

To determine which species is stable in aqueous solution among the given options (Cr²⁺, MnO₄²⁻, MnO₄³⁻, and Cu⁺), we can analyze the oxidation states and stability of each ion.

### Step-by-Step Solution:

1. **Identify the oxidation states of each species:**
- **Cr²⁺:** Chromium is in the +2 oxidation state.
- **MnO₄²⁻:** Manganese is in the +6 oxidation state (since 4 oxygen atoms contribute -8, and the overall charge is -2, we have +6 for manganese).
- **MnO₄³⁻:** Manganese is in the +5 oxidation state (4 oxygen atoms contribute -8, and the overall charge is -3, so +5 for manganese).
- **Cu⁺:** Copper is in the +1 oxidation state.

2. **Assess the stability of each species in aqueous solution:**
- **Cr²⁺:** This ion can be somewhat stable, but it is less stable compared to Cr³⁺, which is more common in aqueous solutions.
- **MnO₄²⁻:** Manganese in the +6 oxidation state is highly stable in strongly alkaline solutions (pH > 13). This species is known to be stable in aqueous solutions under certain conditions.
- **MnO₄³⁻:** Manganese in the +5 oxidation state is generally unstable and can easily reduce to lower oxidation states.
- **Cu⁺:** The Cu⁺ ion is less stable in aqueous solution compared to Cu²⁺ due to its tendency to oxidize to Cu²⁺.

3. **Conclusion:**
- Among the given species, **MnO₄²⁻** is the most stable in aqueous solution due to the stability of manganese in the +6 oxidation state.

### Final Answer:
**MnO₄²⁻ is stable in aqueous solution.**