To find the concentration of a reactant after a certain time in a first-order reaction, we use the concept of half-life. Given: The half-life (t1∕2) of the reaction is 20 seconds. Initial concentration ([A0]) is 0.2 M . We want to find the concentration after 80 seconds. Calculation: Determine the number of half-lives: ‌ Number of half-lives ‌=‌
t
t1∕2
=‌
80
20
=4 Calculate the concentration after 4 half-lives: For a first-order reaction, the concentration after n half-lives is given by: [At]=[A0](‌
1
2
)n Substituting the values:
[At]=0.2×(‌
1
2
)4=0.2×‌
1
16
=0.0125M
Thus, the concentration of the reactant after 80 seconds is 0.0125 M .