The Ksp of Fe(OH)3 is 4.0 × 10⁻³⁸ . What is the pH at which [Fe³⁺] = 1.0 × 10⁻¹⁰ M in a saturated solution?
For Fe(OH)3 Fe³⁺ + 3OH- , Ksp = [Fe³⁺][OH-]³ = 4.0 × 10⁻³⁸ . Given [Fe³⁺] = 1.0 × 10⁻¹⁰ , (1.0 × 10⁻¹⁰)[OH-]³ = 4.0 × 10⁻³⁸ , [OH-]³ = 4.0 × 10⁻²⁸ , [OH-] = (4.0 × 10⁻²⁸)¹/³ ≈ 1.59 × 10⁻⁹ . pOH = -log(1.59 × 10⁻⁹) ≈ 8.8 , pH = 14 - 8.8 = 5.2 .
Ref: NCERT Class 11 Chemistry > Chapter 6: Equilibrium > Topic: Buffer Solutions and Solubility Product and Common Ion Effect