Question

The number of ionisable hydrogens present in the product obtained from a reaction of phosphorus trichloride and phosphonic acid is :

• A. 1
• B. 2
• C. 0
• D. 3

Hint:

In phosphorus acids, $P-H$ bonds are non-ionisable and contribute to reducing properties. Only $P-OH$ bonds are acidic.

Answer 1

The Correct Option is B

Step 1: Reaction: $PCl_3 + 3H_3PO_3 \rightarrow$ The product is Pyrophosphorous acid ($H_4P_2O_5$).
Step 2: The structure of $H_4P_2O_5$ contains two P atoms connected by an O bridge (P-O-P). Each P is bonded to one H and one OH group.
Step 3: Only Hydrogens attached to Oxygen ($P-OH$) are ionisable.
Step 4: There are 2 such $OH$ groups, so 2 ionisable hydrogens.