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A certain radioactive element disintegrates with a decay constant of $7.9 \times 10^{-10} / \mathrm{s}$. At a given instant of time, if the activity of the sample is equal to $55.3 \times 10^{11}$ disintegration/second, then number of nuclei at that instant of time is
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$7.0 \times 10^{21}$
Decay constant, $\lambda=7.9 \times 10^{-10} /$ second Activity, $\quad A=55.3 \times 10^{11} \frac{\text { disintegration }}{\text { second }}$ Let the number of nuclei at that instant be $N$.
We know activity, $A=\lambda N$
$\therefore \quad \quad N=\frac{A}{\lambda}=\frac{55.3 \times 10^{11}}{7.9 \times 10^{-10}}=7.0 \times 10^{21}$
We know activity, $A=\lambda N$
$\therefore \quad \quad N=\frac{A}{\lambda}=\frac{55.3 \times 10^{11}}{7.9 \times 10^{-10}}=7.0 \times 10^{21}$
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