APES: Half Life Calculations

Front

What is half-life?

Back

Half-life is the time required for half of the radioactive atoms in a sample to decay.

Front

If the half-life of an isotope is 10 years, how much of a 100 g sample remains after 20 years?

Back

25 g (after 2 half-lives, 100g -> 50g -> 25g).

Front

How do you calculate the number of half-lives that have passed?

Back

Divide the total time elapsed by the half-life of the substance.

Front

What is the remaining mass of a 200 g sample after 3 half-lives if the half-life is 5 years?

Back

25 g (200g -> 100g -> 50g -> 25g).

Front

What percentage of a radioactive substance remains after one half-life?

Back

50%.

Front

If a substance has a half-life of 4 years, how long will it take for a 64 g sample to decay to 8 g?

Back

12 years (3 half-lives: 64g -> 32g -> 16g -> 8g).

Front

What is the formula to calculate remaining mass after a certain number of half-lives?

Back

Remaining mass = Initial mass × (1/2)^(number of half-lives).

Front

If the half-life of carbon-14 is 5730 years, how much of a 200 g sample remains after 11460 years?

Back

50 g (after 2 half-lives).

Front

What is the remaining percentage of a substance after 4 half-lives?

Back

6.25%.

Front

How many half-lives are needed for a substance to decay from 80 g to 10 g?

Back

3 half-lives (80g -> 40g -> 20g -> 10g).