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UPSC Combined Geo-Scientist

Chemist

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

Mains Paper III covers analytical and organic chemistry.

UPSC Combined Geo-Scientist, Mains · 2019 · Mains Paper III

Q1(a). Activity-coefficient trends

Question

Correct the statements for ionic strength 0–0.1 M: activity coefficients increase with (i) ionic strength and (ii) ionic charge.

Answer in brief

In the dilute-solution trend, activity coefficients decrease as ionic strength or charge magnitude increases.

Key points

  • The Debye–Huckel limiting law gives log γᵢ=-Azᵢ²√I. At infinite dilution γᵢ→1.
  • Increasing I makes the logarithm more negative, so γᵢ falls below one.
  • At fixed I, larger |zᵢ| gives a stronger departure from ideality because the term contains zᵢ².
  • The limiting law is not quantitatively reliable throughout 0–0.1 M.

The full model answer

As written for the answer booklet: the diagram carries the working, the lines stay short. Read here only; it is not offered as a download.

Model answer to Q1(a), 2019 Mains Paper III: solution pages 1 and 2
Solution pages 1 and 2 of 19
Read the whole 2019 solution2019 question paper

CoversActivity-coefficient trendsElectrolyte solutions

All parts of Question 1