On this page
Oxoacid questions look like memory tests: is H3PO3 dibasic or tribasic, what is the oxidation state of sulphur in H2S2O6? In fact almost all of them come down to one skill, drawing the structure from a few rules. This post gives those rules, works through the condensed acids most students skip, and includes a neutralisation calculation that shows why basicity matters.
Who needs this topic?
The JEE (Advanced) 2026 brochure names "oxoacids of phosphorus" under group 15 and "oxoacids of sulphur" under group 16. The JEE (Main) 2026 syllabus and the NEET (UG) 2026 syllabus cover groups 13 to 18 only at the level of general trends and the unique behaviour of the first element. The current NCERT Class 12 Part I contents have no p-block unit, and the CBSE 2026-27 chemistry curriculum assesses the Class 11 s- and p-block material only formatively. So this is mainly JEE Advanced preparation. Check the current year's documents. Our overview of p-block groups 15 to 17 covers the trends and the three simplest phosphorus acids; this post goes further.
What are the rules for drawing an oxoacid?
Phosphorus and sulphur are the central atoms, each roughly tetrahedral, with at least one double-bonded oxygen (P=O or S=O).
Acidic hydrogens are on oxygen (–OH). Hydrogen bonded directly to phosphorus is not acidic.
Larger acids are built from units joined by one of five links: P–O–P, P–P, S–O–S, S–S or O–O (a peroxide).
Basicity is the number of –OH groups.
PubChem's structures follow these rules. It names H3PO2 phosphinic acid, drawn as H2P(=O)OH, and H3PO3 phosphonic acid, drawn as HP(=O)(OH)2. Diphosphoric acid, H4P2O7, is two PO4 units sharing an oxygen (P–O–P), while hypophosphoric acid, H4P2O6, has a direct P–P bond.
Worked example 1: oxidation states of the condensed acids
Use H = +1 and O = −2, except peroxide oxygen, which is −1.
H4P2O7 (diphosphoric): 4 + 2x − 14 = 0, so x = +5. Basicity 4.
H4P2O6 (hypophosphoric): 4 + 2x − 12 = 0, so x = +4. The P–P bond is why phosphorus sits at +4. Basicity 4.
H2S2O7 (disulphuric, in oleum): 2 + 2x − 14 = 0, so x = +6. PubChem shows an S–O–S bridge. Basicity 2.
H2S2O6 (dithionic): 2 + 2x − 12 = 0, so x = +5. PubChem shows an S–S bond. Basicity 2.
H2SO5 (peroxomonosulphuric): it contains one O–O link, so three oxygens are −2 and two are −1: 2 + x − 6 − 2 = 0, giving x = +6. Treating every oxygen as −2 would give +8, which is impossible for sulphur.
H2S2O3 (thiosulphuric): one sulphur replaces an oxygen of sulphuric acid. The formula gives 2 + 2x − 6 = 0, so x = +2. That is an average; the two sulphur atoms are in different positions.
Worked example 2: why basicity changes the arithmetic
How much sodium hydroxide neutralises 8.20 g of H3PO3?
Molar mass of H3PO3 = 3(1.008) + 30.97 + 3(16.00) = 81.99 g/mol
Moles = 8.20 / 81.99 = 0.100 mol
H3PO3 has two –OH groups, so it is dibasic: NaOH needed = 2 × 0.100 = 0.200 mol
Mass of NaOH = 0.200 × 40.00 = 8.00 g
A student who treats H3PO3 as tribasic writes 12.0 g. The same logic makes NaH2PO2 a normal salt: phosphinic acid has only one acidic hydrogen, so once it is replaced, nothing acidic is left. PubChem describes phosphinic acid as "a strong monobasic acid and reducing agent".
Why are H3PO2 and H3PO3 reducing agents?
Their P–H bonds. PubChem calls phosphinic acid a powerful reducing agent and notes that heat decomposes it into phosphoric acid and phosphine. One balanced way to write that is 2H3PO2 → H3PO4 + PH3. Check the oxidation states: one phosphorus goes from +1 to +5 (up by 4) and the other from +1 to −3 (down by 4), so the reaction is a disproportionation. H3PO4 has no P–H bond and is not a reducing agent.
Which mistakes cost marks?
Counting every hydrogen as acidic. Draw the structure; count –OH only.
Missing the peroxide in H2SO5 and H2S2O8.
Confusing hypophosphoric (P–P) with diphosphoric (P–O–P). Both have four acidic hydrogens, but phosphorus is +4 in one and +5 in the other.
Reporting +2 as the oxidation state of each sulphur in thiosulphate without saying it is an average.
Confusing names: phosphorous acid (H3PO3) and phosphoric acid (H3PO4) differ by one letter and one oxygen.
Drawing is the method here, not a memory aid. A short review in Science by Ainsworth, Prain and Tytler (2011) argues that drawing should be recognised as a key element of science education. If a link such as the peroxide bond is unclear, the Fermi Concept Library covers every concept in chemistry with its own lesson, so you can fix the idea before returning to the acids.
A practice routine
Draw all eleven acids in this post from memory: H3PO2, H3PO3, H3PO4, H4P2O7, H4P2O6, H2SO3, H2SO4, H2S2O7, H2S2O8, H2SO5, H2S2O6.
Beside each, write the link type, the basicity and the oxidation state.
Do three neutralisation problems with H3PO2, H3PO3 and H4P2O7.
Repeat step 1 after three days and after a week, checking against PubChem's structures.
Today, draw H4P2O6 and H4P2O7 side by side and write one sentence explaining why phosphorus has different oxidation states in them.
