Answer :
Given:
Time for 25% decomposition = 20 minutes
First we need to calculate the value of rate constant. To calculate rate constant, we apply the formula for the first order reaction:
K = log
Where t is time,
a is the no. of moles per litre,
(a-x) is the concentration of a after time t
As, t = 20 minutes (given)
a = 100, x = 25
∴ k = log
⇒ k = log
⇒ k = 0.0143 min-1
Now, t= ?, when reaction is 75% complete, i.e., x= 75
To calculate the time, we apply the same formula
K = log
Using formula, we write
t = log
As, k = 0.0143 min-1 (calculated above)
∴ t = log
⇒ t = log
⇒ t = 96.96 minute
Thus, the time when 75% of the reaction will be completed is 96.96 minutes.
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