Question

a first order reaction 50% complets in 20 min.how much time will be required for 90% completion?

Answer 1

Use the two equations below.

k = 0.693/t_1/2

Then substitute k into the below equation.
ln(No/N) = kt
No = 100
N = 10
k from above.
solve for t in min. Something like 66 minutes.