Acids and Bases for JEE Main 2026: Complete Guide

Acids and bases permeate every branch of chemistry, and JEE Main tests them through pH calculations, buffer problems, titration curves, and the conceptual frameworks from three theories. The numerical problems are highly predictable — once you know the formula sequence for each type, the calculation becomes routine. This guide organises everything by problem type.

Three Acid-Base Theories

Arrhenius: acids produce H⁺ in water; bases produce OH⁻. Brønsted-Lowry: acids are proton donors; bases are proton acceptors — each acid has a conjugate base (losing H⁺) and each base has a conjugate acid (gaining H⁺). Lewis: acids are electron-pair acceptors; bases are electron-pair donors — this is the broadest definition, covering coordination compounds and reactions with no proton transfer. JEE tests all three, most often asking which species is the Brønsted acid/base in a reaction or identifying a Lewis acid. See our chemical equilibrium guide for the equilibrium connection.

pH, Ka, and Kb Calculations

For a strong acid (e.g. HCl, 0.01 M): [H⁺] = 0.01 M, pH = 2. For a weak acid HA with Ka: [H⁺] = √(Ka × C) (valid when Ka << C); pH = −log[H⁺]. For a weak base BOH with Kb: [OH⁻] = √(Kb × C); pOH = −log[OH⁻]; pH = 14 − pOH. The relationship Ka × Kb = Kw = 10⁻¹⁴ at 25°C connects a weak acid and its conjugate base. If Ka = 1.8 × 10⁻⁵ for acetic acid, its conjugate base (acetate) has Kb = 10⁻¹⁴/(1.8 × 10⁻⁵) = 5.6 × 10⁻¹⁰.

Buffer Solutions

Henderson-Hasselbalch equation: pH = pKa + log([A⁻]/[HA]). A buffer resists pH change because it contains both a weak acid and its conjugate base in comparable amounts. At the half-equivalence point in a titration, [HA] = [A⁻] and pH = pKa — a key reference point. Buffer capacity is maximum at pH = pKa and degrades beyond pH = pKa ± 1. For detailed buffer treatment see our buffer solutions guide.

Hydrolysis of Salts

Salt of strong acid + strong base (NaCl): pH = 7 (neither ion hydrolyses). Salt of strong acid + weak base (NH₄Cl): cation hydrolyses, solution is acidic; pH = 7 − ½(pKb + log C). Salt of weak acid + strong base (CH₃COONa): anion hydrolyses, solution is basic; pH = 7 + ½(pKa + log C). Salt of weak acid + weak base: both ions hydrolyse; pH depends on relative Ka and Kb. Memorise these four cases and their pH expressions — JEE commonly gives a salt formula and asks for the approximate pH.

Indicators and Titration Curves

An indicator changes colour over a pH range of approximately pKa(indicator) ± 1. Phenolphthalein: colour change pH 8.2–10.0 (use for strong acid-strong base or weak acid-strong base titrations). Methyl orange: colour change pH 3.1–4.4 (use for strong acid-weak base titrations). The choice of indicator must match the equivalence-point pH of the titration. Titration curves show sharp equivalence points for strong acid-strong base (at pH 7) and gradual curves for weak acid-strong base (at pH > 7). After reviewing all acid-base types, take a free mock test on ionic equilibrium.