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r = k[A] for a reaction, 50% of A is decomposed in 120 minutes. The time taken for 90% decomposition of A is minutes.
399
r = k[A] ⇒ first order ⇒ t1/2 = 120 min (independent of concentration)
k = 0.693t1/2 = 2.303t90\% 10010
t90\% = 2.303 1200.693
t90\% 398.8 399 min
Answer: 399
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