Question

Ten grams of $C{o}^{57}$ kept in an open container beta-decays with a half-life of $270$ days. The weight of the material inside the container after $540$ days will be nearly

• A. $10$ $g$
• B. $5$ $g$
• C. $2.5$ $g$
• D. $1.25$ $g$

Answer 1

Answer: (C)

$2.5$ $g$

The half life period of the substance is $270$ days,
Hence after 540 days, we are done with two half lives.
Hence only a fourth of initial mass is left over.
This corresponds to $\frac{1}{4}\times 10=2.5g$