MTEL Chemistry Practice Exam

Correct Response: C. The time interval of 144 years represents 5 half-lives for strontium-90, so the starting amount of 50.0 g will be halved 5 times. After 1 half-life, 25.0 g remain. After 2 half-lives, 12.5 g remain. After 3 half-lives, 6.25 g remain. After 4 half-lives, 3.125 g remain. After 5 half-lives, 1.56 g remain. A is incorrect because 25.0 g is the mass that remains after 1 half-life. B is incorrect because 6.25 g is the amount that remains after 3 half-lives. D is incorrect because 0.391 g is the amount that would remain after 7 half-lives.