Given:
Half life = 20 hrs
use relation between rate constant and half life of 2nd order reaction
k = 1/([A]o*half life)
= 1/(0.75*20)
= 6.667*10^-2 M-1.hrs-1
we have:
[A]o = 0.75 M
[A] = 0.25 M
k = 6.667*10^-2 M-1.hrs-1
use integrated rate law for 2nd order reaction
1/[A] = 1/[A]o + k*t
1/(0.25) = 1/(0.75) + 6.667*10^-2*t
4 = 1.333 +6.667*10^-2*t
6.667*10^-2*t = 2.667
t = 40 hrs
Answer: 40 hrs
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