Simulation · Chemistry · Class 11

pH of a strong acid and a weak acid

From the lesson Ionisation of water, pH, weak acids and weak bases in Equilibrium. Change the values and watch what happens.

The idea behind it

NCERT § "Ionization of Acids and Bases"

Water self-ionises: its ionic product Kw = [H₃O⁺][OH⁻] = 1.0 × 10⁻¹⁴ at 298 K, so pure water there has [H₃O⁺] = 1.0 × 10⁻⁷ M and pH 7.
Kw increases with temperature, so neutral water has pH below 7 when hot; neutrality means [H₃O⁺] = [OH⁻], not pH = 7.
pH = −log[H₃O⁺] and pH + pOH = pKw = 14 at 298 K.
For strong acids and bases, take complete dissociation and count all H⁺ or OH⁻ released: 0.05 M Ba(OH)₂ gives [OH⁻] = 0.10 M, pOH 1 and pH 13.
For a weak acid HA of concentration c and degree of ionisation α, Ka = cα²/(1 − α); when α is small, α ≈ √(Ka/c) and [H₃O⁺] ≈ √(Ka c).
pKa = −log Ka; a larger Ka (smaller pKa) means a stronger acid. Weak bases are treated the same way with Kb.
For a conjugate pair, Ka × Kb = Kw, so pKa + pKb = 14 at 298 K.
Polyprotic acids ionise in steps with Ka₁ > Ka₂ > Ka₃, because removing a proton from a negative ion is harder.
Acid strength of H–A: down a group, bond strength controls it (HF < HCl < HBr < HI); along a period, electronegativity of A controls it (CH₄ < NH₃ < H₂O < HF).

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