Question

The half - time period of a radioactive element is 140 days. After 650 days, one gram of the element will reduce to (J 1986)

• A. $\frac{1}{2}g$
• B. $\frac{1}{4}g$
• C. $\frac{1}{8}g$
• D. $\frac{1}{16}g$

Answer 1

The correct option is D

$\frac{1}{16}g$

560 days

$=\frac{560}{140}$ = 4 half - lives.

Amount of reactant remaining after n half - lives.

$={\left(\frac{1}{2}\right)}^n\times intialamount$

$={\left(\frac{1}{2}\right)}^4\times 1.0g$

$=\frac{1}{16}g$