Chemistry / Unit 10 Chem 1862 worked example, 60 to 90 seconds
Determining pH of a Salt Solution via Hydrolysis
The idea
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.
In practice
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.
Where people slip
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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