Question

An oxoacid of phosphorus 'X' reduces silver nitrate solution to metallic silver and gets oxidised to another compound Y. X and Y respectively are

• A. \(\text{HPO}_3, \text{H}_3\text{PO}_4\)
• B. \(\text{H}_3\text{PO}_2, \text{H}_3\text{PO}_4\)
• C. \(\text{H}_3\text{PO}_3, \text{H}_3\text{PO}_2\)
• D. \(\text{H}_3\text{PO}_2, \text{HNO}_3\)

Hint:

Oxoacids of Phosphorus with +1 (\(\text{H}_3\text{PO}_2\)) and +3 (\(\text{H}_3\text{PO}_3\)) oxidation states tend to disproportionate or oxidize to +5 state (\(\text{H}_3\text{PO}_4\)).

Answer 1

The Correct Option is B

Step 1: Reducing Property of P-Oxoacids:
Reducing character corresponds to the presence of P-H bonds. \(\text{H}_3\text{PO}_2\) (Hypophosphorous acid) has two P-H bonds. It is a very strong reducing agent. \(\text{H}_3\text{PO}_3\) (Orthophosphorous acid) has one P-H bond. \(\text{H}_3\text{PO}_4\) has zero P-H bonds (No reducing nature).
Step 2: Reaction with \(\text{AgNO}_3\):
\(\text{H}_3\text{PO}_2\) reduces \(\text{Ag}^+\) to metallic Silver (\(\text{Ag}\)) and itself gets oxidised to the highest oxidation state acid, Orthophosphoric acid (\(\text{H}_3\text{PO}_4\)). Reaction: \[ \text{H}_3\text{PO}_2 + 4\text{AgNO}_3 + 2\text{H}_2\text{O} \rightarrow 4\text{Ag} \downarrow + \text{H}_3\text{PO}_4 + 4\text{HNO}_3 \]
Step 3: Identification:
X = \(\text{H}_3\text{PO}_2\) Y = \(\text{H}_3\text{PO}_4\) Final Answer:
Option (B).