The half-life of bromine-74 is 25 min. How much of a 4.0mg sample is still active after 75 min?

Question

a) 0.50mg b)1.0mg c)2.0mg d)0.25mg e)4.0mg

Answer 1

a) 0.5mg

3 x 25 mins means half three times.
4mg > 2mg > 1mg > 0.5mg :o)

Answer 2

75/25 = 3 and that means 3 half-lives have passed. So activitiy will be (1/2)^3 = 1/8th of the original. Since original quantity is given as 4 mg, 1/8th will be 0.5 mg.

Answer a is right.

Half life is that time by when half the original radioactive substance decays.

Answer 3

It's halved every 25 minutes, so ...
after 25 mins 2mg
after 50 mins 1 mg
after 75 mins 0.5 mg

Hope this helps