Class 12th Chemistry 2017 Set2 Outside Delhi Board Paper Solution
A first order reaction takes 20 minutes for 25% decomposition. Calculate the time when 75% of the reaction will be completed?
(Given: Log 2 = 0.3010, Log 3 = 0.4771, Log 4 = 0.6021)
For the first order reaction
k = (2.303 / t) log (a / a – x)
When x = (25 / 100) a = 0.25 a
t = 20 minutes (given)
Therefore, k = (2.303 / 20) log (a / a – 0.25 a)
k = (2.303 / 20) log (1 / 0.75)
= 0.013120 min-1
Hence the value of the rate constant is 0.013120 min-1
Now we need to find Time for rest 75% reaction t= ?, when x = 0.75 a
From above, k = 0.013120 min-1
Therefore, t = (2.303 / 0.013120) log (a / a – 0.75 a)
= (2.303 / 0.013120 ) log (1 / 0.25 )
= 105.6 min.
The time at which the reaction will be 75% complete is 105.6 min.
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