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Question:

The half-life period of a first-order reaction is 15 minutes. The amount of substance left after one hour will be:

14of the original amount

18of the original amount

116of the original amount

132of the original amount

Solution:

For first order reaction, At/A0 = e^-kt
t1/2 = 0.693/k
k = 0.693/15 min = 0.046 min^-1
At/A0 = e^-0.046 x 60
At/A0 = 1/16
At = A0/16
Hence, option A is correct.