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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 · 2026 · Mains Paper III

Q1(c). Percentage of amine from the absorbance of its picrate

  • 5 marks
  • Topic: Molecular spectroscopy
  • Answered
Question

An amine sample is analyzed as the picrate derivative. A 0.05 g sample is reacted with picric acid and diluted to 1 L deionized water. The solution exhibits an absorbance of 0.40 in a 1 cm cell. What is the percentage of amine in the sample? [λ_(max)=359 nm; ε=1.25×10⁴ cm⁻¹ mol⁻¹ L; formula weight of the amine = 310]

Answer in brief

Amine =19.8 % (9.92 mg in 50 mg).

Key points

  • Amount in 1 L: 3.2×10⁻⁵ mol (one amine gives one picrate), so mass =3.2×10⁻⁵×310=9.92×10⁻³ g.

The full model answer

Worked out step by step, as it should go in the answer booklet. The solution sheet follows it.

A=ε bc ⇒ c=(A)/(ε b)=0.40(1.25×10⁴)(1)=3.2×10⁻⁵ mol L⁻¹

Amount in 1 L: 3.2×10⁻⁵ mol (one amine gives one picrate), so mass =3.2×10⁻⁵×310=9.92×10⁻³ g.

% amine=9.92×10⁻³0.05×100=19.8 %

Model answer to Q1(c), 2026 Mains Paper III: solution page 4
Solution page 4 of 30
Read the whole 2026 solution2026 question paper

CoversBeer–Lambert lawPercentage of amine from the absorbance of its picrate

All parts of Question 1