Question

How much radioactive potassium-40 will there be after 2 halflives pass (one halflife is 1.25 billion years)?

1) 3 micrograms
2) 2 micrograms
3) 6 micrograms
4) 24 micrograms

Answer 1

After two half-lives of potassium-40, which is a total of 2.5 billion years, only one fourth of the original amount will remain, making it 3 micrograms if we started with 12 micrograms.

Step-by-step explanation:

After two half-lives have passed, the amount of radioactive potassium-40 (40 K) will be one quarter of the original amount. Starting with an unspecified original quantity, after the first half-life has elapsed (1.25 billion years), half of it will remain. After the second half-life, which is another 1.25 billion years (making it 2.5 billion years in total), half of the remainder will be left. Therefore, if we start with an initial amount of 12 micrograms of 40 K, after one half-life, this amount would reduce to 6 micrograms, and after the second half-life, it would again halve to 3 micrograms.