Question

The half life of a radioactive isotope is $3$ hours. If the initial mass of the isotope were $256$ $g$, the mass of it remaining undecayed after $18$ hours would be:

• A. $12$ $g$
• B. $16$ $g$
• C. $4$ $g$
• D. $8$ $g$

Answer 1

Answer: (C)

$4$ $g$

$N=N_0{\left(\frac{1}{2}\right)}^n$ $n=$ number of half lives $=\frac{18}{3}=6$
$=256{\left(\frac{1}{2}\right)}^6=4$ $g$