Q.17 Diphosphonic acid (H4P2O5) has no P–P bond. This acid is
Diphosphonic acid (H₄P₂O₅) has no P–P bond. This acid is
Final Answer: (A) tetrabasic
Molecular Structure of H₄P₂O₅
(HO)₂P(O)-O-P(O)(OH)₂ → No P-P bond, P-O-P bridge
Key Features:
- Two phosphonic acid units:
-PO(OH)₂ - Each unit provides 2 ionizable H⁺
- Total basicity: 2 × 2 = 4 → tetrabasic
- Molecular formula
H₄P₂O₅confirms 4 acidic hydrogens
Step-by-Step Ionization (4 Steps)
H₄P₂O₅ ⇌ H₃P₂O₅⁻ + H⁺H₃P₂O₅⁻ ⇌ H₂P₂O₅²⁻ + H⁺H₂P₂O₅²⁻ ⇌ HP₂O₅³⁻ + H⁺HP₂O₅³⁻ ⇌ P₂O₅⁴⁻ + H⁺- Result: H₄P₂O₅ → P₂O₅⁴⁻ + 4H⁺ (tetrabasic)
Phosphonic vs Phosphoric Acids Comparison
| Acid | Formula | Structure | Basicity |
|---|---|---|---|
| Phosphoric | H₃PO₄ | (HO)₃P=O | Tribasic |
| Diphosphonic | H₄P₂O₅ | (HO)₂P(O)-O-P(O)(OH)₂ | Tetrabasic |
| Pyrophosphoric | H₄P₂O₇ | (HO)₂(O)P-O-P(O)(OH)₂ | Tetrabasic |
- Count -PO(OH)₂ groups: 2 × 2H⁺ = 4 (tetrabasic)
- No P-P bond = oxygen bridge structure
- H₄ formula directly indicates tetrabasic nature
- Eliminate B/C/D: Insufficient acidic hydrogens
