Chemistry / Chem 1862 · Procedure · 60–90 seconds
Determining pH of a Salt Solution via Hydrolysis
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Determining a salt solution's pH means identifying which ion actually hydrolyzes, finding that ion's own Ka or Kb from its conjugate's already-known value via the relationship Ka·Kb = Kw, then solving that ionization with an ICE table exactly as for any ordinary weak acid or base.
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For 0.15 M NH4Cl, ammonium is the conjugate acid of NH3, Kb = 1.8 × 10⁻⁵, so Ka(NH4+) = Kw/Kb ≈ 5.6 × 10⁻¹⁰. Solving x²/0.15 = 5.6 × 10⁻¹⁰ gives x ≈ 9.2 × 10⁻⁶ M, so pH ≈ 5.04, acidic as expected for this particular salt.
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Using the conjugate acid's own Ka value directly as if it belonged to ammonia, rather than deriving ammonium's Ka through Kw, gives a badly wrong pH.
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