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Q.
The half life of polonium is $140$ days. In what time will $15 \,g$ of polonium be disintegrated out of its initial mass of $16 \,g$ ?
Nuclei
Solution:
Here, $m_{0}=16\,g, m=16-15=1\,g$
As $\frac{m}{m_{0}}=\left(\frac{1}{2}\right)^{n}$ or $\frac{1}{16}=\left(\frac{1}{2}\right)^{4}=\left(\frac{1}{2}\right)^{n}$
$\Rightarrow n=4$
Time taken to disintegrate $= T_{1 /2}\times n=140\times4$
$=560$ days