Question -
A first order reaction is half completed in 45 minutes. How long does it need for 99.9% of the reaction to be completed?
Options:
A .  20 hours
B .  10 hours
C .  7.5 hours
D .  5 hours
Answer: Option C : C Each half life would reduce the concentration to half of its original value. So, 99.9% completion takes place in 10 half-lives. Remember 210=1024 and 1/1024≈0.001 Hence time taken = 45 × 10 minutes = 450 minutes = 7.5 hours.
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