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CBSE Class 12 Chemistry 2017 Outside Delhi Set 1 Solved Paper

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Question : 15
Total: 26
A first order reaction takes 20 minutes for 25% decomposition. Calculate the time when 75% of the reaction will be completed.
(Given : log‌2=0.3010,log‌3=0.4771,log‌4= 0.6021)
Solution:  
t‌=‌
2.303
k
‌log‌
[A]0
[A]

20‌min‌=‌
2.303
k
‌log‌
100
75

t‌=‌
2.303
k
‌log‌
100
25

Divide (i) equation by (ii)
‌
20
t
‌=‌
‌
2.303
k
‌log‌
100
75
2.303
k
‌log‌
100
25

‌=‌
log‌4∕3
log‌4

‌
20
t
‌=0.1250∕0.6021
t‌=96.3‌min
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