Question

The half-life of carbon-14 is 5,730 years. How many grams of parent material will remain from a 100-gram sample of carbon-14 after 2 half-lives?

75

6.25

12.5

25

Answer 1

The answer is: 25 grams.

Half-life is the time required for a quantity (in this example number of radioactive nuclei of carbon-14) to reduce to half its initial value.

Initial mass of sample: 100 grams.

After first half-life: 50% · 1000 g ÷ 100% = 50 g.

After second half-life: 0.5 · 50 g = 25 g.

Living things have carbon-14 in their organism, but when they died, the amount of C-14 they contain begins to decrease as the C-14 undergoes radioactive decay.

Measuring the amount of C-14 in a sample from a dead plant or animal when the animal or plant died.

Answer 2

The question is quite misleading at first because we are given the half life of the material. But the question is basically a simple geometric progression in which we need to multiply twice by 1/2 the initial amount since it is said that it has undergone 2 half lives.

therefore, after 2 half lives = 50 * 0.5 * 0.5 = 25