Answer:
[tex]28.037\ \text{s}[/tex]
Explanation:
[tex][A]_0[/tex] = Initial concentration = 1.28 M
[tex][A][/tex] = Final concentration = [tex]0.17[A]_0[/tex]
k = Rate constant = 0.0632 s
t = Time taken
For first order reaction we have the relation
[tex]kt=\ln\dfrac{[A]_0}{[A]}\\\Rightarrow t=\dfrac{\ln\dfrac{[A]_0}{[A]}}{k}\\\Rightarrow t=\dfrac{\ln\dfrac{[A]_0}{0.17[A]_0}}{0.0632}\\\Rightarrow t=28.037\ \text{s}[/tex]
Time taken to reach the required concentration would be [tex]28.037\ \text{s}[/tex].