Salt Hydrolysis and Predicting Solution pH

NaCl, NH₄Cl and NaCH₃COO are all salts, yet their solutions have pH 7.0, about 5.1 and about 8.9 at 0.10 M. The difference lies entirely in which parent acid and base each ion came from.

First dissociate the soluble salt with correct multiplicities. A 0.100 M (NH₄)₂SO₄ solution supplies 0.200 M NH₄⁺, whereas 0.100 M NaCH₃COO supplies 0.100 M acetate. Classify each ion before choosing a pH calculation. The metal's name or the absence of H in the salt formula is not enough: hydrated Al³⁺ can acidify water, and acetate can make hydroxide despite containing no OH group.

Cl⁻, Br⁻, I⁻, NO₃⁻ and ClO₄⁻ come from strong acids and are neutral. F⁻, CH₃COO⁻, CN⁻, NO₂⁻ and CO₃²⁻ come from weak acids and are basic. HSO₄⁻ is the odd one out: it is itself a weak acid.

Write the ion's proton-transfer equation. NH₄⁺ + H₂O ⇌ NH₃ + H₃O⁺ requires K_a(NH₄⁺) = K_w/K_b(NH₃). F⁻ + H₂O ⇌ HF + OH⁻ requires K_b(F⁻) = K_w/K_a(HF). For amphiprotic HCO₃⁻, compare its acid dissociation to CO₃²⁻ with its base reaction to H₂CO₃. K_b(HCO₃⁻) comes from K_a₁ of the conjugate acid, whereas its own acid step uses K_a₂. An intermediate protonation state cannot simply be placed in the “all anions are basic” category.

For NH₄CN, compare K_a(NH₄⁺) = 5.6 × 10⁻¹⁰ with K_b(CN⁻) = 2.0 × 10⁻⁵. The base is far stronger, so the solution is basic. For ammonium acetate the two are nearly equal, so the solution is close to neutral.

When both ions react, compare K_a of the actual cation with K_b of the actual anion. For NH₄F at 25 °C, K_a(NH₄⁺) = 5.56 × 10⁻¹⁰ and K_b(F⁻) = 1.47 × 10⁻¹¹, so acidity dominates. Comparing K_a(HF) directly with K_b(NH₃) would compare the parent molecules instead of the dissolved ions. For a simple 1:1 weak-acid/weak-base salt under usual approximations, pH ≈ 1/2[pK_w + pK_a(parent acid) − pK_b(parent base)]. This estimate is not universal for polyprotic, multivalent or highly dilute systems; use full equilibria when those assumptions fail.

Once you know which ion reacts, the problem is an ordinary weak acid or weak base calculation. The salt concentration is the initial concentration of the reacting ion, and the needed constant comes from K_w divided by the partner constant.

For 0.200 M NH₄Cl, use C(NH₄⁺) = 0.200 M and K_a ≈ 5.56 × 10⁻¹⁰. The small-x estimate gives [H₃O⁺] ≈ √(KaC) = 1.05 × 10⁻⁵ M, with x/C much less than 5%. Thus pH ≈ 4.98. Chloride does not need a second hydrolysis calculation in this dilute model. The result is mildly acidic, which is consistent with a small but nonzero K_a; “weak” does not mean neutral.