Potassium manganate `(K_2MnO_4)` is formed when

Options

A. Chlorine is passed through aqueous `KMnO_4` solution
B. Magnanese dioxide is fused with potassium hydroxide in air
C. Formaldehyde reacts with potassium
D. Potassium permaganate reacts with `H_2SO_4`

Answer

To determine how potassium manganate \((K_2MnO_4)\) is formed, we will analyze the reactions provided in the options.

### Step-by-Step Solution:

1. **Analyze Option A: Chlorine in Aqueous K2MnO4**
- When chlorine gas is passed into an aqueous solution of potassium manganate, the manganese in \((K_2MnO_4)\) is in the +7 oxidation state. Chlorine, being a strong oxidizing agent, would not react with \((K_2MnO_4)\) as manganese does not want to be reduced further. Therefore, this reaction does not produce \((K_2MnO_4)\).
- **Conclusion**: Option A does not form \((K_2MnO_4)\).

2. **Analyze Option B: Manganese Dioxide with Potassium Hydroxide and Oxygen**
- The reaction can be written as:
\[ MnO_2 + 4KOH + O_2 \rightarrow 2K_2MnO_4 + 2H_2O \]
- This reaction shows that manganese dioxide reacts with potassium hydroxide and oxygen to produce potassium manganate. The balancing of the equation confirms that \((K_2MnO_4)\) is indeed formed.
- **Conclusion**: Option B forms \((K_2MnO_4)\).

3. **Analyze Option C: Formaldehyde with Potassium Permanganate in Strong Alkali**
- In the presence of a strong alkali, potassium permanganate \((KMnO_4)\) can be reduced. The manganese in \((KMnO_4)\) will change from +7 to +6 oxidation state, resulting in the formation of potassium manganate:
\[ 3CH_2O + 2KMnO_4 + 2KOH \rightarrow 2K_2MnO_4 + 3H2O + 2K2CO3 \]
- This indicates that \((K_2MnO_4)\) is formed in this reaction as well.
- **Conclusion**: Option C also forms \((K_2MnO_4)\).

4. **Analyze Option D: Potassium Permanganate with Concentrated Sulfuric Acid**
- When potassium permanganate reacts with concentrated sulfuric acid, it typically forms manganese(VII) oxide \((Mn_2O_7)\) and does not produce \((K_2MnO_4)\). Instead, the reaction leads to the formation of manganese dioxide and other products, but not \((K_2MnO_4)\).
- **Conclusion**: Option D does not form \((K_2MnO_4)\).

### Final Answer:
The correct options that lead to the formation of potassium manganate \((K_2MnO_4)\) are **Option B** and **Option C**.

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