Acid Strength Trends in Oxyacids of P, S and Cl

By Aniket Bhardwaj · 28 October 2026 · Inorganic

Oxoacids of p-block elements are widely tested in CBSE Class 12, NEET, JEE Main, and CUET examinations. Questions regularly require students to rank oxyacids of phosphorus, sulfur, and chlorine by acid strength, basicity, and reducing power. Understanding the molecular principles behind these trends avoids confusion and secures high marks.

Fundamental Factors Governing Oxyacid Strength

An oxyacid contains ionisable hydrogen bonded to oxygen, written as (O)pE(OH)q, where p is the number of terminal oxo-oxygens (=O) and q is the number of hydroxyl groups (−OH). Acid strength depends on how easily the O−H bond ionises to release H+ and how stably the conjugate base accommodates negative charge.

Key factors governing oxyacid strength include:

Electronegativity of the central atom: Across a period (P < S < Cl), higher electronegativity polarises the O−H bond, assisting proton release.
Terminal oxo-oxygen atoms (p): Strongly electronegative terminal oxo-oxygens withdraw electrons inductively and delocalise negative charge in the conjugate base.
Oxidation state: For oxyacids of the same element, a higher oxidation state increases electron withdrawal, strengthening the acid.
Conjugate base resonance: Distributing negative charge across multiple equivalent oxygens produces a more stable conjugate base and a stronger acid.

Pauling Rules for Oxyacid Dissociation:

For an oxyacid written as (O)pE(OH)q:

pKa1 ≈ 8 − 5p

Where:

p = number of terminal oxo-oxygen atoms (=O) bonded to central atom E

q = number of ionisable hydroxyl groups (−OH)

pKa1 = negative logarithm of the first acid dissociation constant Ka1

Negative charge per oxygen in conjugate base = −1 / (p + 1)

Successive ionisations: pKa(n+1) ≈ pKan + 5

Chlorine Oxyacids: Trend Across Oxidation States

Chlorine forms four oxyacids: hypochlorous (HClO), chlorous (HClO2), chloric (HClO3), and perchloric acid (HClO4). As the oxidation state of chlorine rises from +1 to +7, acid strength increases sharply:

HClO < HClO2 < HClO3 < HClO4

In HClO, p = 0, leaving negative charge in ClO− localised on one oxygen. In HClO2, p = 1, sharing charge between two oxygens (−0.50 each). In HClO3, p = 2, distributing charge across three oxygens (−0.33 each). In HClO4, p = 3, distributing charge across four oxygens (−0.25 each). Extensive resonance makes perchloric acid extremely strong.

Sulfur Oxyacids: Sulfuric vs Sulfurous Acid

Sulfur forms sulfuric acid (H2SO4) and sulfurous acid (H2SO3). In H2SO4, sulfur has a +6 oxidation state with structure (O)2S(OH)2. With p = 2, it is a strong diprotic acid with pKa1 < 0, delocalising charge across three oxygens in HSO4−.

In H2SO3, sulfur has a +4 oxidation state with structure (O)S(OH)2. With p = 1, sulfurous acid is a weak acid with pKa1 ≈ 1.8. Down Group 16, H2SO4 is stronger than H2SeO4 because sulfur is more electronegative than selenium.

Phosphorus Oxyacids: Basicity vs Acid Strength

Phosphorus forms orthophosphoric acid (H3PO4), phosphorous acid (H3PO3), and hypophosphorous acid (H3PO2). Only P−OH hydrogens ionise; P−H hydrogens do not ionise because P and H have similar electronegativity.

This creates three vital exam distinctions:

Basicity: H3PO4 has three −OH groups (tribasic). H3PO3 has two −OH groups and one P−H bond (dibasic). H3PO2 has one −OH group and two P−H bonds (monobasic).
Reducing power: Reducing power depends on P−H bonds: H3PO2 (2 P−H) > H3PO3 (1 P−H) > H3PO4 (0 P−H).
Acid strength (pKa1): All three acids have one terminal oxo-oxygen: (O)P(OH)3, (O)P(H)(OH)2, and (O)P(H)2(OH). Because p = 1 for all three, their first dissociation constants are nearly equal: H3PO2 (pKa1 ≈ 1.2), H3PO3 (pKa1 ≈ 1.3), and H3PO4 (pKa1 ≈ 2.15). H3PO2 is marginally stronger because −OH groups donate electron density to phosphorus, reducing positive character compared to neutral P−H bonds.

Worked Examples with Step-by-Step Analysis

Example 1: Predicting pKa and Ranking Chlorine Oxoacids

Problem: Using Pauling rules, calculate predicted pKa1 values for HClO, HClO2, HClO3, and HClO4, and verify the order using conjugate base charge dispersion.

Step 1: Write each acid in form (O)pCl(OH)q and find p:

HClO: Cl(OH), p = 0; HClO2: (O)Cl(OH), p = 1; HClO3: (O)2Cl(OH), p = 2; HClO4: (O)3Cl(OH), p = 3.

Step 2: Calculate predicted pKa1 using pKa1 = 8 − 5p:

HClO: pKa1 = 8 − (5 × 0) = 8.0 (experimental ≈ 7.53).

HClO2: pKa1 = 8 − (5 × 1) = 3.0 (experimental ≈ 1.96).

HClO3: pKa1 = 8 − (5 × 2) = −2.0 (experimental ≈ −1.0).

HClO4: pKa1 = 8 − (5 × 3) = −7.0 (experimental ≈ −8.0).

Step 3: Calculate negative charge per oxygen in each conjugate base:

ClO−: 1 charge on 1 oxygen = −1.00; ClO2−: 1 charge on 2 oxygens = −0.50; ClO3−: 1 charge on 3 oxygens = −0.33; ClO4−: 1 charge on 4 oxygens = −0.25.

Verification by second route: Lower charge density per oxygen increases conjugate base stability, confirming the order: HClO < HClO2 < HClO3 < HClO4.

Example 2: Determining Basicity and pKa in Phosphorus Oxoacids

Problem: For orthophosphoric acid (H3PO4) and phosphorous acid (H3PO3), determine phosphorus oxidation state, basicity, and predicted pKa1 values.

Step 1: Calculate formal oxidation states of phosphorus:

H3PO4: 3(+1) + x + 4(−2) = 0 ⇒ x = +5.

H3PO3: 3(+1) + x + 3(−2) = 0 ⇒ x = +3.

Step 2: Determine ionisable protons from molecular structure:

H3PO4 structure is O=P(OH)3 with three P−OH bonds and zero P−H bonds. Basicity = 3 (tribasic).

H3PO3 structure is O=P(H)(OH)2 with two P−OH bonds and one P−H bond. Basicity = 2 (dibasic).

Step 3: Determine p value and estimate pKa1 using Pauling rule:

Both acids have one terminal oxo-oxygen (=O), so p = 1 for both.

Predicted pKa1 = 8 − (5 × 1) = 3.0 for both acids.

Verification by second route: Experimental pKa1 values are 2.15 for H3PO4 and 1.30 for H3PO3. Both lie in the weak acid regime predicted by p = 1, confirming that acid strength does not track formal oxidation state (+5 vs +3) because both have p = 1.

Common Mistakes Students Make

Assuming basicity equals total hydrogen count: Because P−H hydrogen is non-ionisable, H3PO3 is dibasic and H3PO2 is monobasic.
Confusing reducing ability with acid strength: Reducing power depends on P−H bonds (H3PO2 > H3PO3 > H3PO4), not acidity.
Assuming H3PO4 is stronger than H3PO3: Believing higher oxidation state (+5 vs +3) automatically implies higher acidity is incorrect because pKa1 values are nearly equal.
Ignoring conjugate base resonance: Assuming HClO is strongest because it has fewer oxygens is incorrect; more terminal oxygens delocalise charge, making HClO4 strongest.
Mixing oxoacid trends with binary hydrides: In binary hydrides (HF < HCl < HBr < HI), bond enthalpy dominates. In oxoacids, electronegativity and oxo-oxygen count dominate.

Exam Relevance and Summary

Oxyacid trends are evaluated across multiple competitive examinations. The table below outlines how these concepts are tested.

Examination	Question Focus	Key Skills Required
CBSE Class 12 Boards	Structure drawing, basicity, and reducing behavior	Drawing P−H and P−OH bonds and stating basicity.
JEE Main	Comparative acid strength orders and oxidation states	Applying electronegativity and terminal oxo rules.
NEET	Direct trend recall, basicity, and reducing power	Identifying monobasic, dibasic, and tribasic acids.
CUET	Conceptual matching and pKa rankings	Connecting molecular structures to conjugate base stability.

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