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The half-life of a radioactive isotope is three hours. If the initial mass of the isotope were $256 \mathrm{~g}$, the mass of it remaining undecayed after 18 hours would be
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$4.0 \mathrm{~g}$
$4.0 \mathrm{~g}$
$\mathrm{t}_{1 / 2}=3 \mathrm{hrs}$. Initial mass $\left(\mathrm{C}_0\right)=256 \mathrm{~g}$
$\therefore \mathrm{C}_{\mathrm{n}}=\frac{\mathrm{C}_0}{2^{\mathrm{n}}}=\frac{256}{(2)^6}=\frac{256}{64}=4 \mathrm{~g}$
$\therefore \mathrm{C}_{\mathrm{n}}=\frac{\mathrm{C}_0}{2^{\mathrm{n}}}=\frac{256}{(2)^6}=\frac{256}{64}=4 \mathrm{~g}$
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