Question

In which among the following compounds, oxidation number of phosphorus is +3?

• A. \( \text{H}_4\text{P}_2\text{O}_7 \)
• B. \( \text{H}_3\text{PO}_3 \)
• C. \( \text{H}_3\text{PO}_2 \)
• D. \( \text{H}_3\text{PO}_4 \)

Hint:

Phosphorus in phosphoric acid (\( \text{H}_3\text{PO}_4 \)) has an oxidation state of +5, while in phosphorous acid (\( \text{H}_3\text{PO}_3 \)) it has an oxidation state of +3.

Answer 1

The Correct Option is B

Step 1: Understanding oxidation number of phosphorus.
The oxidation number of phosphorus can be determined by the structure of the compound. In \( \text{H}_3\text{PO}_3 \) (phosphorous acid), phosphorus has an oxidation state of +3. In other compounds like \( \text{H}_4\text{P}_2\text{O}_7 \), \( \text{H}_3\text{PO}_2 \), and \( \text{H}_3\text{PO}_4 \), the oxidation number of phosphorus differs.
Step 2: Analyzing the options.
(A) \( \text{H}_4\text{P}_2\text{O}_7 \): Incorrect. In this compound, the oxidation number of phosphorus is +5.
(B) \( \text{H}_3\text{PO}_3 \): Correct — In phosphorous acid, the oxidation number of phosphorus is +3.
(C) \( \text{H}_3\text{PO}_2 \): Incorrect. In this compound, phosphorus has an oxidation number of +3, but the oxidation state of phosphorus in other compounds is higher.
(D) \( \text{H}_3\text{PO}_4 \): Incorrect. In phosphoric acid, the oxidation number of phosphorus is +5.
Step 3: Conclusion.
The correct answer is (B) \( \text{H}_3\text{PO}_3 \), where the oxidation number of phosphorus is +3.