Question
The time required for 90\% completion of a certain first order reaction is 1 hour. Calculate the time required for 99.9\% completion of the same reaction.
MHT CET
Chemistry
Chemical Kinetics
2026
MHT CET 2026 (15 April Shift 1)
MHT CET Chemistry Question (2026) — Solution
Options
- A. 2 hours.
- B. 1 hour.
- C. 3 hours.
- D. 0.5 hour.
Answer
C. 3 hours.
Step-by-step solution
For a first order reaction, the rate constant k is given by k = 2.303 t ( a a-x ) For 90\% completion, t_ 90\% = 1 hour. k = 2.303 1 ( 100 100-90 ) = 2.303 10 = 2.303 hour ^ -1 For 99.9\% completion, the time required is: t_ 99.9\% = 2.303 k ( 100 100-99.9 ) t_ 99.9\% = 2.303 2.303 ( 100 0.1 ) t_ 99.9\% = 1000 = 3 hours. Answer: 3 hours.
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