Question

A sample of Bismuth-212 has a mass of 2.64 grams (g) after 121 seconds (s). What was the initial mass of the sample if Bismuth-212 has a half-life of 60.5 s?
A.

0.66 g

B.

1.32 g

C.

5.28 g

D.

10.56 g

Answer 1

Use exponential decay: m = m0(1/2)^(t/t1/2). Here t/t1/2 = 121/60.5 = 2, so m = m0(1/2)^2 = m0/4. Thus m0 = 4m = 4(2.64 g) = 10.56 g.

Answer: D.