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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 · 2016 · Mains Paper II

Q1(g). Ammonium chloride pH

Question

Calculate the pH of 0.01 M NH₄Cl at 25° C, given pKb=4.74 for aqueous ammonia and Kw=10⁻¹⁴.

Answer in brief

pH=5.63.

Key points

  • NH₄⁺ is a weak acid: NH₄⁺ + H₂O ⇌ NH₃ + H₃O⁺. Chloride does not appreciably hydrolyse.
  • pKₐ=14.00-4.74=9.26, so Kₐ=5.50×10⁻¹⁰.
  • With C=0.010 M, [H⁺]≃√(KₐC)=2.34×10⁻⁶ M and pH=-log [H⁺]=5.63. The hydrolysed fraction is only 2.34×10⁻⁴, supporting the weak-acid approximation.

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(g), 2016 Mains Paper II: solution pages 3 and 4
Solution pages 3 and 4 of 19
Read the whole 2016 solution2016 question paper

CoversAmmonium chloride pHIonic equilibrium

All parts of Question 1