The halflife of a radioactive element is 20 min. The time interval between the stages of its $33 \%$ and $67…

The halflife of a radioactive element is 20 min. The time interval between the stages of its $33 \%$ and $67 \%$ decay is
  1. $40 \mathrm{~min}$
  2. $20 \mathrm{~min}$
  3. $30 \mathrm{~min}$
  4. $25 \mathrm{~min}$

Solution

Change in $67 \%$ to $33 \%$ is almost half the concentration change.
$67 \% \frac{\text { Half }}{\text { change }} 33.5(\approx 33 \%)$
So time interval between the stages of its $33 \%$ and $67 \%$ decay is same as $t_{1 / 2}=20 \mathrm{~min} .$ ^

Asked in: JEE-TOPICTESTS-CHEMISTRY

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