Answers

2015-01-14T19:46:06+05:30
T1/2 = 32min.
t3/4 = ?
k = 2.303log(Co/C) / t
k = 0.693/(t1/2)
0.693/32 = 2.303log{Co/(3Co/4)} / (t3/4)
0.693/32 = 2.303log(4/3) / (t3/4)
0.693/32 = 2.303(log4-log3) / (t3/4)
0.693/32 = 2.303(0.6020-0.4771) / (t3/4)
0.693/32 = 0.2876 / (t3/4)
t3/4 = 0.2876*32/0.693
t3/4 = 13.28 min.

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2015-01-15T15:07:02+05:30
In  a first order reaction, t 1/2 does not depend on concentration of reactant.
t 1/2 (or half life) implies time in which half the reactant is consumed.
t 3/4 implies time in which 3/4 of the reactant is consumed.
After 1 half life, 1/2 of the reactant will be consumed.
After 2 half lives, 1/2 of 1/2 i.e. 3/4 of the reactant will be consumed.
so, time for t 3/4 is 2 x 32 = 64 minutes.
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