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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 · 2018 · Mains Paper II

Q1(h). Very dilute hydrochloric acid

Question

At 25° C, why is the pH of 10⁻⁸ mol L⁻¹ aqueous HCl less than 7.0?

Answer in brief

Including water ionisation gives [H⁺]=1.051×10⁻⁷ M and pH =6.978.

Key points

  • HCl contributes C=10⁻⁸ M chloride. Charge balance gives h=[H⁺]=C+[OH⁻], and h[OH⁻]=Kw=10⁻¹⁴.
  • Hence h²-Ch-Kw=0, whose physical solution is h=(C+√(C²+4Kw))/2.
  • Substitution yields h=1.05125×10⁻⁷ M and pH 6.9783.

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(h), 2018 Mains Paper II: solution pages 3 and 4
Solution pages 3 and 4 of 17
Read the whole 2018 solution2018 question paper

CoversVery dilute hydrochloric acidIonic equilibrium

All parts of Question 1