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Ionic Equilibrium – Complete Formula & Concept Sheet

 

Ionic Equilibrium – Complete Formula & Concept Sheet (JEE Main + Advanced)

Ionic equilibrium deals with reversible ionisation in aqueous solutions. This chapter merges equilibrium, logarithms, and electrostatics of ions.

1. Electrolytes

  • Strong electrolytes: Complete ionisation (HCl, NaOH)
  • Weak electrolytes: Partial ionisation (CH₃COOH, NH₄OH)

Trap: Strong electrolyte ≠ concentrated, weak electrolyte ≠ dilute

2. Ionisation of Water

H₂O ⇌ H⁺ + OH⁻

Kw = [H⁺][OH⁻] = 1.0 × 10−14 (at 25°C)

  • Pure water: [H⁺] = [OH⁻] = 10−7 M
  • Neutral ≠ pH 7 at all temperatures

3. pH, pOH and Scale

pH = −log[H⁺] pOH = −log[OH⁻]

pH + pOH = 14 (at 25°C)

Classic Trap: Log values must be approximated correctly (10⁻⁵.⁶ ≠ 4×10⁻⁶)

4. Weak Acids (Ka)

For a weak acid HA:

HA ⇌ H⁺ + A⁻

Ka = [H⁺][A⁻] / [HA]

If initial concentration = C and degree of ionisation = α:

Ka = Cα² / (1 − α) ≈ Cα² (if α ≪ 1)

[H⁺] = √(Ka · C)

5. Weak Bases (Kb)

BOH ⇌ B⁺ + OH⁻

Kb = [B⁺][OH⁻] / [BOH]

[OH⁻] = √(Kb · C)

6. Relation Between Ka and Kb

Ka × Kb = Kw

Trap: This relation is only for conjugate acid–base pairs

7. Salts and Hydrolysis

Salt Type Nature
Strong acid + Strong base Neutral
Strong acid + Weak base Acidic
Weak acid + Strong base Basic

8. pH of Salt Solutions (Must-Remember)

Weak acid + Strong base:

pH = 7 + ½ (pKa + log C)

Weak base + Strong acid:

pH = 7 − ½ (pKb + log C)

Trap: Students forget the ½ factor or misuse concentration

9. Buffer Solutions

A buffer resists change in pH on addition of small acid or base.

Acidic Buffer: pH = pKa + log (Salt / Acid)

Basic Buffer: pOH = pKb + log (Salt / Base)

Hidden Trap: Use mole ratio, not molarity, if volumes differ

10. Solubility Product (Ksp)

AgCl ⇌ Ag⁺ + Cl⁻

Ksp = [Ag⁺][Cl⁻]

For MX type salt: Solubility S → Ksp = S²

For MX₂ type salt: Ksp = 4S³

11. Common Ion Effect

Addition of a common ion decreases solubility or ionisation.

JEE Trap: Ksp does not change, solubility changes

12. Ultimate JEE Traps

  • Wrong log approximations
  • Using Ka instead of pKa
  • Ignoring dilution in buffers
  • Confusing ionic equilibrium with chemical equilibrium
  • Forgetting temperature dependence of Kw
StudyBeacon Insight:
Ionic equilibrium questions are never about memory. They are about conditions, approximations, and judgement.
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