1. Nuclear Decay
Half-Life Practice

2. Number of half-lives passed
Radioactive Half-Life Practice Problems
1. How many grams of iodine 131 (half life- 5 days) would be left after 20 days if you start with 25 grams?
Answer: 1.56 g
The half life is
Number of half-lives passed
Amount of Matter
Time 1 2 3 4 5
5 days
25 g
0 { days}
1/2 X
12.5g
5 days
1/2
6.25 g
10 days X
1/2
3.12 g
15 days X
1/2
1.56 g
20 days
1/16 amount left over

3. Number of half-lives passed
2. How long will it take 600 grams of Plutonium 239 (half life 24,000 years) to decay to grams?
120,000 yrs
The half life is
Number of half-lives passed
Amount of Matter
Time 1 2 3 4 5
24,000 yrs
0 yrs
600 g
300 g
24,000 yrs
150 g
48,000 yrs
75 g
72,000 yrs
37.5 g
96,000 yrs
18.75 g
120,000 yrs

4. Number of half-lives passed
3.K-42 has a half-life of 15.5 hrs. If g of K-42 remains undecayed after 62.0 hours, what was the original sample size?
210 g
The half life is
Number of half-lives passed
Amount of Matter
Time 1 2 3 4 5
15.5 hrs
210 g
0 hrs
105 g
15.5 hrs
52.5 g
31 hrs
26.25 g
46.5 hrs g
62 hrs

5. Number of half-lives passed
4. An isotope of cesium (cesium-137 has a half -life of 30 years. If 20 mg of cesium-137 disintegrates over a period of 90 years, how many mg of cesium-137 would remain?
2.5 mg
The half life is
Number of half-lives passed
Amount of Matter
Time 1 2 3 4 5
30 years
20 mg
0 Years
10 mg
30 Years
5 mg
60 Years
2.5 mg
90 Years

6. Number of half-lives passed
5. Thallium-208 has a half-life of 3 min. How long will it take for g to decay to 7.50 g?
12 minutes
The half life is
Number of half-lives passed
Amount of Matter
Time 1 2 3 4 5
3 minutes
120 g
0 minutes
60 g
3 minutes
30 g
6 minutes
15 g
9 minutes
7.5 g
12 minutes

7. Number of half-lives passed
6. If 60 g of Lithium-9 has a half-life of 100 years, how long will it take for lithium-9 to decay to 15 g?
300 years
The half life is
Number of half-lives passed
Amount of Matter
Time 1 2 3 4 5
100 years
60 g
100 years
200 years
30 g
15 g
300 years