Question

$10$ grams of ${}^{57}Co$ 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 very nearly

• A. $10\text{g}$
• B. $7.5\text{g}$
• C. $5\text{g}$
• D. $2.5\text{g}$

Answer 1

The correct option is A $10\text{g}$

As ${}^{57}Co$ undergoing beta decay, electron is being produced.
But mass of electron is much smaller compared to mass of ${}^{57}Co$ atom.
Therefore, after 570 days eventhough cobalt undergoes large decay mass of container will remain to be $10\text{g}$