Question

potassium - 40 has a half - life of 1.25 billion years. if a rock has 60 g of potassium - 40 how much would be left after 2 half - lives?
30 g
20 g
60 g
15 g

Explanation:

Step1: Calculate after first half - life

After one half - life, the amount of potassium - 40 is halved.
If the initial amount \(N_0 = 60\space g\), after the first half - life \(N_1=\frac{N_0}{2}\)
\(N_1=\frac{60}{2}=30\space g\)

Step2: Calculate after second half - life

After the second half - life, the amount is halved again.
\(N_2=\frac{N_1}{2}\)
Since \(N_1 = 30\space g\), then \(N_2=\frac{30}{2}=15\space g\)

Answer:

15 g