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

Question

Quizplus · Accepted Answer

After 25 minutes, half of the bromine-74 will have decayed. After another 25 minutes (50 minutes total), half of that remaining amount will have decayed again. After another 25 minutes (75 minutes total), half of that smaller amount will have decayed again.

To find how much of the sample is still active after 75 minutes, we can use the formula: 
Amount remaining = initial amount x (1/2)^(time/half-life)

Plugging in the values, we get: 
Amount remaining = 4.0 mg x (1/2)^(75/25) 
Amount remaining = 4.0 mg x (1/2)^3 
Amount remaining = 4.0 mg x (1/8) 
Amount remaining = 0.5 mg

Therefore, the best choice is B) 0.50 mg.

The Half-Life of Bromine-74 Is 25 Min