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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

Q3(a). Collisions of O₂ with the wall

Question

Calculate the number of collisions that oxygen makes per second on 1.00 cm² of the surface of the vessel containing them if the pressure is 1.00 × 10⁻⁶ atm and the temperature is 25 °C.

Answer in brief

2.73 × 10¹⁷ collisions per second on 1.00 cm².

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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.

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Topic notes: kinetic theory of gases, Maxwell distribution and effusion, with every Civil Services, IFoS and Geo-Scientist PYQ →

Coverscollision fluxwall collisionskinetic theory of gasesMaxwell distributionmean speed

All parts of Question 3