Q1(f). Potassium sulfate activities

Answer in brief

Iₘ=0.0750 mol kg⁻¹; a Debye–Huckel limiting-law estimate gives γ_±≈0.526 and a_±≈0.0209.

Key points

• Molalities are m_(K⁺)=0.0500 and m_(SO₄²⁻)=0.0250 mol kg⁻¹. Thus Iₘ=1/2[0.0500(1)²+0.0250(2)²]=0.0750 mol kg⁻¹.
• Use the standard water value A≈0.509 kg^1/2 mol^-1/2 at 298 K. The limiting law gives log₁₀ γ_±=-A|z₊z₋|√(Iₘ)=-0.2788, so γ_±=0.526.
• The mean ionic molality is m_±=(m₊²m₋)^1/3=4^1/3(0.0250)=0.039685 mol kg⁻¹. With m°=1 mol kg⁻¹, a_±=γ_± m_±/m°=0.02089.
• Complete ionisation alone cannot determine activity coefficients.