H₃PO₃ (phosphorous acid) is not tribasic because only two of its three hydrogen atoms are ionizable; one hydrogen is directly bonded to phosphorus (P-H bond) and cannot be released as H⁺.
Structure of H₃PO₃:
Actual Structure:
- Two -OH groups (hydroxyl)
- One P-H bond (hydrogen directly on phosphorus)
- Structure: (HO)₂P(=O)H or H-P(=O)(OH)₂
Ionizable vs. Non-Ionizable Hydrogens:
Ionizable (Acidic):
- Two H atoms in -OH groups
- These can dissociate as H⁺
- Results in dibasic acid (releases 2 H⁺)
Non-Ionizable:
- One H directly bonded to P (P-H bond)
- This hydrogen is covalently bonded
- Cannot dissociate as H⁺ in aqueous solution
- P-H bonds are not acidic
Basicity Determination:
- Basicity = number of ionizable H⁺ ions
- H₃PO₃: 2 ionizable H → dibasic acid
- Not 3, despite having 3 total H atoms
Comparison:
- H₃PO₄ (phosphoric acid): All 3 H in -OH groups → tribasic
- H₃PO₃ (phosphorous acid): Only 2 H in -OH groups → dibasic
- H₃PO₂ (hypophosphorous acid): Only 1 H in -OH group → monobasic
Key Takeaways:
- H₃PO₃ is dibasic, not tribasic
- One P-H bond contains non-acidic hydrogen
- Only -OH hydrogens are ionizable
- Basicity ≠ total hydrogen count