Question

A radioactive sample of $60\mu g$ starts decaying.If the half life of the radioactive element is 24 days then calculate the approximate mass of radioactive element remains after 120 days.

• A. zero
• B. $2\mu g$
• C. $4\mu g$
• D. $8\mu g$
• E. $15\mu g$

Answer 1

The correct option is B $2\mu g$

Given : $T_{1/2}=24$ days $t=120$ days

Initial amount of the substance, $R_o=60\mu g$

Thus remaining amount, $R=\frac{R_o}{2^n}$ where $n=\frac{t}{T_{1/2}}$

$\therefore$ $n=\frac{120}{24}=5$

Hence remaining amount, $R=\frac{60\mu g}{2^5}\approx 2\mu g$