KCET 2025 Chemistry Paper

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:
Thus, the concentration of the reactant after 80 seconds is 0.0125 M .