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Question

A first order reaction has a rate constant of 102sec1. How long will it take for a 20 g sample to become 5 g?
  1. 138.6

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Solution

The correct option is A 138.6
k = 102 = 0.693t12
t12= 0.693102

t12= 69.3 seconds

It takes two half lives for 20 g to reduce to 5 g,

therefore, the time required = 2×69.3
=138.6 seconds

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