1. Half – Life
Problems

2. The half-life of Zn-71 is 2. 4 minutes. If one had 100
The half-life of Zn-71 is 2.4 minutes. If one had g at the beginning, how many grams would be left after 7.2 minutes has elapsed?
100 g g g
100 g 1
2.4
50 g 2
4.8
25 g 3
7.2
12.5 g g

3. Os-182 has a half-life of 21. 5 hours. How many grams of a 10
Os-182 has a half-life of 21.5 hours. How many grams of a 10.0 gram sample would have decayed after exactly three half-lives?
10 g g g 10 1
21.5
5 g 2 43
2.5 g 3
64.5
1.25 g g

4. After 24. 0 days, 2. 00 milligrams of an original 128
After 24.0 days, 2.00 milligrams of an original milligram sample remain. What is the half-life of the sample? 4
128 mg 1
64 mg 2
32 mg 3
16 mg 4
8 mg 5
4 mg 6
24.0
2 mg
2. 2
3. 5
4. 6

5. 1 second 2. 2 seconds 3. 3 seconds 4. 3.34 seconds
Fermium-253 has a half-life of seconds. A radioactive sample is considered to be completely decayed after 10 half-lives. How much time will elapse for this sample to be considered gone?
1 second 1
.334 2
.668 3
1.002 4
1.336 5
1.67 6
2.004 7
2.338 8
2.672 9
3.006 10
3.34 seconds
2. 2 seconds
3. 3 seconds
seconds

6. 100.0 grams of an isotope with half-life of 26 hours is present at time zero. How much time will elapse before 50 grams remains?
100 1
26 hrs
50 g
100 g g g g

7. 15.28 days 2. 11.46 day 3. 7.64 days 4 3.82 days Original sample 1
Rn-222 has a half-life of 3.82 days. How long before only 1/16 of the original sample remains?
15.28 days
Original sample 1
3.82
1/2 2
7.64
1/4 3
11.46
1/8 4
15.28
1/16
day
days
days