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UPSC Combined Geo-Scientist

Chemist

  • 3 descriptive papers
  • 200 marks each
  • 3 h per paper

Mains Paper II covers physical chemistry.

UPSC Combined Geo-Scientist, Mains · 2025 · Mains Paper II

Q5. Debye–Hückel limiting law and the mean activity coefficient

Question

The Debye–Hückel limiting equation is log₁₀ γi = −A Zi² √I. (a) Write down the significance of each symbol with unit. (b) What are the factors on which A depends? (c) Derive the equation log₁₀ γ± = −A |Z₊Z₋| √I.

Answer in brief

γi is the ion's activity coefficient, Zi its charge number, I the ionic strength and A a solvent constant (0.509 for water at 298 K) fixed by εr, T and ρ.

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Topic notes: electrolytic conductance, Kohlrausch's law, transport numbers and the Debye–Hückel limiting law, with every Civil Services, IFoS and Geo-Scientist PYQ →

CoversDebye–Hückel limiting lawmean activity coefficientionic strengthactivity coefficientelectroneutrality