`90%` of a first order reaction was completed in `100 min`. How much time it will take for `80%` completion of a reaction

Options

A. `90 min`
B. `80 min`
C. `70 min`
D. `60 min`

Answer

C

Explanation

`t_(90%) = log.(100)/(100-90) = log 10 = 1`
`t_(80%) = log.(100)/(100-80) = log 5 = 0.7`
`(t_(80%))/(t_(90%)) = (0.7)/(1)`
`:. t_(80%) = 100 xx 0.7 = 70 min`