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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 · 2023 · Paper I

Q5(a). First-order decomposition of AB₂

Question

The decomposition of AB₂ to AB and B is a first-order reaction with k = 2.8 × 10⁻⁷ s⁻¹ at T = 1000 K. The atomic weights of A and B are 12 and 32, respectively. (i) Find the half-life of this reaction at 1000 °C. (ii) In how many days will 1 g of AB₂ decompose to the extent that 0.6 g of AB₂ remains? (iii) How much of 1 g of AB₂ would remain after 35 days?

Answer in brief

(i) t½ = ln 2/k = 2.48 × 10⁶ s = 28.7 days; (ii) 21.1 days; (iii) 0.43 g.

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Topic notes: rate laws, integrated rate equations and half-lives, with every Civil Services, IFoS and Geo-Scientist PYQ →

Coversfirst-order kineticshalf-liferate constantintegrated rate lawexponential decay

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