At 298 K, molar conductivities at infinite dilution of NH4Cl, NaOH and NaCl are
129.8, 217.4 and 108.9 ohm-1 cm2 respectively. If molar conductivity ofcentinormal solution of NH4OH is 9.33 ohm-1 cm2, what is the degree ofdissociation if NH4OH solution?
From Kohlrausch’s law, the molar conductivity at infinite dilution of NH4OH can be calculated as:
∧∞m (NH4OH) = ∧∞m (NH4Cl) + ∧∞m (NaOH) - ∧∞m (NaCl)
= 129.8 + 217.4 – 108.9
= 238.3 ohm/cm2
From Ostwald’s dilution law, ionization constant Kb of weak base is
Kb = cx2 / (1-x) where,
x = degree of ionisation = ∧cm / ∧∞m = 9.33 / 238.3 = 0.0392
given that ∧cm (0.01N) = 9.33 S ohm/cm2 C = 0.01
Kb = (0.01) x (0.0391)2 / (1-0.0392)
= 1.6 x 10-5 Degree of ionisation