Salt HydrolysisJEE Advanced

Concept

Salts of weak parents fight back with water. The conjugate base of a weak acid (CH₃COO⁻) steals protons from water → basic solution; the conjugate acid of a weak base (NH₄⁺) donates them → acidic. Strong-strong salts stay neutral; weak-weak salts settle wherever Ka vs Kb wins.

Key formula

Derivation

Anion hydrolysis: A⁻ + H₂O ⇌ HA + OH⁻. Multiply and divide by [H⁺]: $K_h = \frac{[HA][OH^-]}{[A^-]} = \frac{K_w}{K_a}$.

ICE with h: $K_h = Ch^2$ (small h) → $h = \sqrt{K_h/C}$, $[OH^-] = Ch$, and the pH formula follows. Weak–weak: $K_h = K_w/(K_aK_b)$, pH independent of concentration!

Scenarios to explore

• Salt Hydrolysis — Why CH₃COONa is basic and NH₄Cl acidic — with pH formulas.

Real-world applications

• Why household soap solutions (salts of fatty acids) are basic.
• NH₄Cl fertiliser acidifies soil.
• Indicator choice in titrations depends on the salt formed at equivalence.

JEE exam tips

• Memory hook: the WEAK parent's conjugate does the hydrolysing, solution goes to the STRONG parent's side.
• WW salt: pH = 7 + ½(pKa − pKb) — concentration-free, a favourite trick.
• CH₃COONH₄ with pKa = pKb = 4.74 sits at exactly pH 7 — but it's NOT unhydrolysed.

Common mistakes

• Declaring every salt neutral.
• Wrong pair: WS salt is BASIC (weak-acid parent loses).
• Forgetting the ½log C term (except in WW where C cancels).

Exam traps to avoid

• Equivalence point of a weak-acid/strong-base titration is >7 — because of exactly this hydrolysis.
• Dilution pushes hydrolysis degree UP (like Ostwald) but pH TOWARD 7.