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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 · 2025 · Paper I

Q5(b). Light absorbed by a dye solution

Question

A dye solution containing 1 g per 100 cc transmitted 60% of the blue light in a cell 1 cm thick. (i) What percentage of light would be absorbed by a solution containing 2 g per 100 cc in the same cell? (ii) What should be the cell thickness so that 90% of the light is absorbed by the original solution?

Answer in brief

(i) 64 % absorbed; (ii) 4.5 cm.

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Topic notes: photochemistry (quantum yield, Jablonski diagram, H₂–halogen reactions), with every Civil Services, IFoS and Geo-Scientist PYQ →

CoversBeer–Lambert lawabsorbancetransmittancepath lengthmolar absorption coefficient

All parts of Question 5