1. Half-Life

2. What is a half-life?
A half-life is the amount of time that it takes half of an amount of a substance to undergo radioactive decay.

3. How to solve half-life problems:
Find the half-life in the problem.
Determine the total number of half-lives that has occurred.
Amount of time elapsed ÷half-life = # of half-lives
Divide the amount of chemical by 2 for each half life.

4. Sample Problem One:
Phosphorous-32 has a half life of 14.3 days. How much of a 50g sample of P-32 will be left after 42.9 days?

5. Answer: 6.25g Half-life = 14.3days # of half-lives = 42.9 ÷ 14.3
Divide 50g by 2 three times
50g ÷ 2 = 25g
25g ÷2 = 12.5g
12.5g ÷2 = 6.25g

6. Sample Problem Two:
Rosium has a half-life of 25 seconds. How much of a 17g sample will be left after 50 seconds?

7. Answer: 4.25g Half-Life = 25 seconds
# of half-lives = 50seconds ÷ 25 seconds
= 2 half lives
Divide 17g by 2 twice
17g ÷ 2 = 8.5g
8.5g ÷ 2 = 4.25g