Question

Cobalt-57 is radioactive, emitting b particles. The half-life for this is 270 days. If 100 mg of this is kept in an open container, the mass of cobalt-57 after 540 days will be-

• A. 50 mg
• B. $50/\sqrt{2}$ mg
• C. 25 mg
• D. Zero

Answer 1

Answer: (C)

25 mg

Answer 2

$\frac { \mathrm { m } } { \mathrm { m } _ { 0 } } = \frac { \mathrm { N } } { \mathrm { N } _ { 0 } } = \left( \frac { 1 } { 2 } \right) ^ { \frac { 540 } { 270 } } = \frac { 1 } { 4 }$

m = $\frac { \mathrm { m } _ { 0 } } { 4 } = \frac { 100 \mathrm { mg } } { 4 } = 25 \mathrm { mg }$