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Half-Life Question – Type 1
Half-Life Question – Type 1: What mass remains from the original element?
The mass of a sample of 42K is 100 grams. How many grams remain after 49.6 hours?
• Reffy, Table N (p. 6) shows that the half-life of 42K is 12.36 hours.
• One half-life = 12.36 hours
• 49.6 hours = 4 half-lives ( 49.6 = 4) 12.36
• Let’s figure out the mass of 42K after 4 half-lives. Take the original mass and divide it by two, four times.
After 4 half-lives, 100 grams is reduced to 6.25 grams. Only 6.25 grams remain from the original mass.
There are a few different types of half-life questions. We will learn about each of them. Have fun!
1 half-life
2 half-lives
3 half-lives
4 half-lives
100 = 50 2
50 = 25 2
25 =12.5 2
12.5 = 6.25 2
491 © UNIT 12: NUCLEAR CHEMISTRY
