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UPSC Civil Services (Mains)

Chemistry optional

  • 2 papers
  • 250 marks each
  • 3 h per paper
  • 5 of 8 questions, Q1 and Q5 compulsory

Paper I covers physical and inorganic chemistry.

UPSC Civil Services (Mains), Chemistry optional · 2021 · Paper I

Q7(c). Quantum yield for the photodecomposition of oxalic acid

Question

Radiation of wavelength 2500 Å was passed through a cell containing 10 ml of a solution which was 0.05 molar in oxalic acid and 0.01 molar in uranyl sulphate. After absorption of 80 joules of radiation energy, the concentration of oxalic acid was reduced to 0.04 molar. Calculate the quantum yield for the photochemical decomposition of oxalic acid at the given wavelength. (Given: N = 6.022 × 10²³ mol⁻¹, h = 6.626 × 10⁻³⁴ J s and c = 3 × 10⁸ m s⁻¹)

Answer in brief

Φ = molecules decomposed/photons absorbed = 6.022 × 10¹⁹/1.006 × 10²⁰ = 0.60.

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