It takes 33.4 s for the concentration of A to fall to one-fourth of its original value.
A half-life is the time it takes for the concentration to fall to half its original value.
Assume the initial concentration is 1.00 mol/L. Then,
[tex]\text{1.00 mol/L }\stackrel{\text{1st half-life }}{\longrightarrow}\text{ 0.50 mol/L } \stackrel{\text{ 2nd half-life }}{\longrightarrow}\text{0.25 mol/L}\\[/tex]
The concentration drops to one-fourth of its initial value in two half-lives.
∴ Time = 2 × 16.7 s = 33.4 s
