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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 II covers organic chemistry and spectroscopy.

UPSC Civil Services (Mains), Chemistry optional · 2025 · Paper II

Q5(d). Rotational spectroscopy

Question

(i) Which of the following molecules is/are active to rotational spectroscopy and why? CH₄, H₂O, NH₃, BCl₃, XeF₄ (5 marks) (ii) The spacing between lines in the microwave spectrum of CO decreases by substituting ¹²C by ¹³C. Why? (5 marks)

Answer in brief

Only H₂O and NH₃ are microwave active; line spacing 2B falls from 3.85 to 3.68 cm⁻¹ from ¹²C¹⁶O to ¹³C¹⁶O because the reduced mass rises.

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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.

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Topic notes: rotational (microwave) and Raman spectra, with every Civil Services, IFoS and Geo-Scientist PYQ →

Coversmicrowave spectroscopyrotational spectrumpermanent dipole momentisotope effectreduced massrotational constant

All parts of Question 5