The rate constant for a first order reaction was found to be `0.082 min^(-1)`. If initial concentration of reactant is `0.15M`, how long would it take:
(a) to reduce the concentration of `A` to `0.03 M`?
(b) to reduce the concentration of A by `0.03 M`?

Explanation

Case (a): Given `K=0.082 min^(-1)`,
`[A]_(0)=0.015 M, [A]_(t)=0.03 M`
`t=2.303/K "log" [A]_(0)/[A]_(t)`
`=2.303/0.082 "log" 0.15/0.03=19.63 min`
Case (b): Given `K =0.082 min^(-1)`,
`[A]_(0)=0.15 M`,
`[A]_(t)=0.15-0.03=0.12M`
`t=2.303/0.082 "log" 0.15/0.12=2.72 min`