Calculating Half Life of Radioactive Isotopes: Solved Examples, Lecture notes of Chemistry

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Calculating Half Life

Half Life Practice

1. If 20.0 g of a radioactive isotope are present at 1:00 PM and 5.0 g remain at 2:00 PM, what is the half life of the isotope?
2. If we start with 400 atoms of a radioactive substance, how many would remain after one half-life? 2 half- lives, 3 half- lives, 4 half lives?
3. A sample of carbon-14 has been decaying for 22,920 years and is now 35.0 grams. What was the size of the original sample? (The half-life of carbon-14 is 5,730 years)

1. If we start with 400 atoms of a radioactive substance, how many would remain after one half-life? 2 half- lives, 3 half- lives, 4 half lives?

• After 1 half- life 200
• After 2 half-lives? 100
• After 3 half-lives? 50
• After 4 half-lives? 25

A sample of carbon-14 has been decaying for 22,920 years and

is now 35.0 grams. What was the size of the original sample?

(The half-life of carbon-14 is 5,730 years)

• 22920/5730 = 4 half lives
• You will need to work backwards 35  70  140  280  560. grams