Question

The half-life of bromine-74 is 25 min. How much of a 6.0 mg sample is still active after 75 min?

Answer 1

`0.75 \ mg`

Explanation:

Find the number of periods during which the disintegration of the radioiosotope took place.

`"number of periods " = ("time")/ ("half-life")`

`"number of periods " = (75 \ min)/(25 \ min) = 3`

During each period, half the original amount disintegrates.

`6.0 \ mg -> 6.0/2 -> 6.0/4 -> 6.0/8 = 0.75 \ mg`

Note that each arrow represents 1 period.