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3. Chemical kinetics — Chemistry STD 12 Science — Question

CBSE BoardEnglish MediumSTD 12 ScienceChemistry3. Chemical kinetics1 Mark
MCQ
A freshly prepared radioactive element has a half-life of $2\  hours$. It emits radiation whose intensity is $64$ times the permissible safe level The minimum time after which it would be possible to work with this sample is .......... $hr$
  • A
    $3$
  • B
    $9$
  • C
    $24$
  • ✓
    $12$
✓

Answer

Correct option: D.
$12$
d
$t_{1 / 2}=2$

$\lambda=\frac{\ln 2}{2} \,h r^{-1}$

First order decay:

$N(t)=N_{o} e^{-\lambda t}$

For permissible level:

$\frac{1}{64}=e^{-\lambda t}$

$6 \ln 2=\lambda t$

$t=12 \,h r$

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