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MHT CET202611 April 2026Morning ShiftChemistryChemical KineticsActual

A first order reaction complete 60 % in 20 minutes. How long will the reaction take to complete 84 %?

Options

  1. A54 min
  2. B68 min
  3. C40 min
  4. D76 min

Correct answer

C. 40 min

Step-by-step solution

For a first order reaction, the rate constant is given by k = 2.303 t ( a a-x ) For 60 % completion in 20 minutes: k = 2.303 20 ( 100 100-60 ) = 2.303 20 (2.5) For 84 % completion in time t : k = 2.303 t ( 100 100-84 ) = 2.303 t (6.25) Equating the two expressions for k : 1 20 (2.5) = 1 t (2.5^2) 1 20 (2.5) = 2 t (2.5) t = 40 min Answer: 40 min

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