The compound that cannot act both as oxidising and reducing agent is

Options

A. `HNO_2`
B. `H_2O_2`
C. `H_2SO_3`
D. `H_3PO_4`

Answer

To determine which compound cannot act both as an oxidizing and reducing agent, we need to analyze the oxidation states of the elements in the given compounds.

### Step-by-Step Solution:

1. **Understanding Oxidizing and Reducing Agents**:
- An **oxidizing agent** is a substance that gains electrons in a reaction (is reduced) and thus causes another substance to be oxidized.
- A **reducing agent** is a substance that loses electrons in a reaction (is oxidized) and thus causes another substance to be reduced.

2. **Identifying Oxidation States**:
- We need to find the oxidation states of the central atom in each compound to see if it can both gain and lose electrons.

3. **Analyzing Each Compound**:
- **HNO2 (Nitrous Acid)**:
- Nitrogen in HNO2 has an oxidation state of +3. It can gain up to 4 electrons (to reach -1) or lose up to 3 electrons (to reach +6). Therefore, it can act as both an oxidizing and reducing agent.
- **H2SO3 (Sulfurous Acid)**:
- Sulfur in H2SO3 has an oxidation state of +4. It can gain up to 2 electrons (to reach +2) or lose up to 6 electrons (to reach +10). Thus, it can also act as both.
- **H3PO4 (Phosphoric Acid)**:
- Phosphorus in H3PO4 has an oxidation state of +5. It can gain up to 3 electrons (to reach +2) or lose 5 electrons (to reach +10). Hence, it can also act as both.
- **Final Compound**:
- The question does not specify the fourth compound, but based on the context, we need to find a compound that cannot act as both.

4. **Conclusion**:
- The compound that cannot act both as an oxidizing and reducing agent is typically one that is in its highest oxidation state and cannot gain any more electrons. For example, a compound like **K2Cr2O7 (Potassium Dichromate)**, where chromium is in the +6 oxidation state, can only act as an oxidizing agent and cannot be reduced further.

### Final Answer:
The compound that cannot act both as an oxidizing and reducing agent is **K2Cr2O7 (Potassium Dichromate)**.