For a first-order reaction, the integrated rate equation is given by:
k=t2.303log[R][R]0
Given:
The substance decomposes to 40%, meaning the remaining concentration is [R]=40% of the initial concentration [R]0.
So, [R][R]0=40100=2.5
Time, t=560 s
Substituting the values into the rate equation:
k=5602.303log(2.5)
Given log2.5=0.3979,
k=5602.303×0.3979 s−1=5600.91636 s−1=1.636×10−3 s−1
To find the rate constant in min−1, we multiply by 60 s/min:
k=1.636×10−3 s−1×60 s/min=0.09816 min−1=9.816×10−2 min−1
Since none of the options match the correct calculated value, 'None of the above' is the correct choice.