Read P-H Bonds as the Marker of Reducing Power
The phosphorus oxoacid family (H3PO2, H3PO3, H3PO4 and their salts) is organised by two counts on each molecule — how many P-OH groups (basicity) and how many P-H bonds (reducing power) — and these two counts don't move together.
Chemistry questions first require the material, particles or reaction condition to be identified before a formula or rule is selected.
Basicity counts P-OH groups, not total hydrogens
Hypophosphorous acid (H3PO2) is monobasic despite its formula showing 3 H atoms, because only ONE is attached via O-H — the other two are direct P-H bonds, which don't ionise. Phosphorous acid (H3PO3) is dibasic (2 P-OH, 1 P-H), and orthophosphoric acid (H3PO4) is tribasic (3 P-OH, 0 P-H) — always count P-OH bonds specifically, never just the hydrogen subscript.
P-H bonds are the source of reducing power
More P-H bonds means stronger reducing character: H3PO2 (2 P-H bonds) is the strongest reducing agent among the common phosphorus oxoacids, more reducing than H3PO3 (1 P-H bond), which in turn is more reducing than H3PO4 (0 P-H bonds, non-reducing) — this maps directly onto NaH2PO2 being more reducing than Na2HPO3.
P4O10's structure explains its short and long P-O bonds
P4O10 has 4 short, terminal P=O bonds (one per phosphorus, double-bond character) and 6 longer P-O-P bridging bonds connecting the phosphorus atoms in the underlying P4 tetrahedral cage — total lone pairs plus bridging linkages in the dimer (P4O10)2-equivalent framework follow directly from this cage-plus-terminal-oxygen structure.
Tracing a reaction sequence: P4 through halides to oxoacids
P4 + limited Cl2 gives PCl3; PCl3 hydrolyses to H3PO3. P4 + excess Cl2 gives PCl5; PCl5 hydrolyses (via POCl3) ultimately to H3PO4. Keeping straight which halide (PCl3 vs PCl5) forms under which Cl2 stoichiometry, and which acid each hydrolyses to, resolves most multi-step 'identify the products' questions in this area.
OlympIQ exam lens
- ✓Basicity = number of P-OH groups specifically, not total H atoms in the formula — H3PO2 is monobasic, H3PO3 is dibasic, H3PO4 is tribasic.
- ✓More P-H bonds = stronger reducing power: H3PO2 > H3PO3 > H3PO4 (non-reducing).
- ✓P4O10 has 4 short terminal P=O bonds and 6 longer bridging P-O-P bonds within its tetrahedral cage.
- ✓P + limited Cl2 → PCl3 → (hydrolysis) → H3PO3; P + excess Cl2 → PCl5 → (hydrolysis) → H3PO4.
Don't count basicity from the hydrogen subscript in the formula — H3PO2's '3' includes two non-ionisable P-H hydrogens, so it's monobasic despite looking tribasic on paper.
Ready to try the pattern?
100% coverage across the 9 linked questions.
Practise 9 linked questions