Q1(e). Activity coefficient of potassium ferricyanide

Answer in brief

The Debye–Hückel limiting-law estimate is γ± ≈ 0.680.

Key points

• Complete dissociation gives K₃[Fe(CN)₆] → 3K⁺ + [Fe(CN)₆]³⁻; thus c₊ = 0.006 M and c₋ = 0.002 M.
• The ionic strength is I = ½Σcᵢzᵢ² = ½[0.006(1)² + 0.002(−3)²] = 0.012 mol L⁻¹.
• Taking the stoichiometric mean of the individual limiting laws gives log₁₀γ± = −3A√I.
• Therefore log₁₀γ± = −3(0.509)√0.012 = −0.16727, so γ± = 0.6803.