Only 2.50 mg of zirconium-86 remains from a 40.0-mg sample of zirconium-86 after 66 hours.?

Question

What is the half-life of zirconium-86?

a.) 4 hours
b.) 16.5 hours
c.) 22 hours
d.) 8.2 hours

Answer 1

b) 16.5 hours

66 hours/4 half lives = 16.5 hours

You start with 40 mg. After 16.5 hours, you're left with 20 mg. After another 16.5 hours, you have 10 mg. After another 16.5 hours, you have 5 mg. After yet another 16.5 hours, you have 2.5 mg.

Answer 2

2.5 mg is 1/16th of 40, so 66 hrs represents 4 half-lives.