Question

For which of the following oxyacids all hydrogen atoms are not replaceable in nature-

• A. H₂CO₃
• B. H₃PO₃
• C. H₄P₂O₇

Answer 1

Answer: (B)

C

Answer 2

The replaceability of hydrogen atoms in oxyacids depends on their bonding. Only hydrogen atoms bonded to oxygen (O-H bonds) are acidic and replaceable. Hydrogen atoms bonded directly to the central atom (e.g., P-H, C-H) are generally not acidic and therefore not replaceable.

Let's analyze the structure of each given oxyacid:

1. H₂CO₃ (Carbonic acid):

   • Structure: O=C(OH)₂
   • All two hydrogen atoms are bonded to oxygen atoms.
   • Therefore, all hydrogen atoms are replaceable.

2. H₃PO₃ (Phosphorous acid):

   • Structure: HPO(OH)₂ (or O=P(H)(OH)₂)
   • This acid has one hydrogen atom directly bonded to the phosphorus atom (P-H bond) and two hydrogen atoms bonded to oxygen atoms (P-O-H bonds).
   • Only the two hydrogen atoms bonded to oxygen are acidic and replaceable. The hydrogen atom bonded directly to phosphorus is not acidic.
   • Therefore, not all hydrogen atoms are replaceable.

3. H₄P₂O₇ (Pyrophosphoric acid):

   • Structure: (HO)₂P(O)-O-P(O)(OH)₂
   • All four hydrogen atoms are bonded to oxygen atoms.
   • Therefore, all hydrogen atoms are replaceable.

Based on the analysis, H₃PO₃ is the oxyacid where not all hydrogen atoms are replaceable.