Suppose the half-life is 35.5 s for a first order reaction and the reactant concentration is 0.0700 M 28.6 s after the reaction starts. How many seconds after the start of the reaction does it take for the reactant concentration to decrease to 0.0063 M?

It takes 151.9 s for the reactant concentration to decrease to 0.0063 M

Explanation:

k = ln2 / t1/2

k = ln2 / 35.5 s = 0.0195 s^-1

ln [A]t = - kt + ln [A]o

ln (0.0063) = - 0.0195t + ln (0.0700)

t = 123.32 s

Since t is counted from 28.6 s as the initial time, the time after start of reaction to reach 0.0063 M is

28.6 + 123.32 s = 151.9 s