Nuclear Chemistry Answer Key
Half-life Practice Problems
1. Complete the chart for the decay of a 900-gram sample of thulium-165 which has a half-life
of 30 hours.
½ lives
time (hours)
percent
fraction
mass
0
0
100
1
900
1
30
50
1/2
450
2
60
25
1/4
225
3
90
12.5
1/8
112.5
4
120
6.25
1/16
56.25
5
150
3.125
1/32
28.125
6
180
1.56
1/64
14.0625
2. Consider a 750.0-gram sample of phosphorus-32, which has a half-life of 14.3 days.
a. How many grams of the sample will remain after 71.5 days?
b. What percent of the original sample is left after 85.8 days?
c. What percent has decayed?
a. 23.4375 grams
b. 1.5625%
c. 98.4375%
3. Technetium-99 has a half-life of 6.0 hours. After how many half-lives will 20% of the sample
remain?
Sometime AFTER 2 half-lives
4. Iodine-131 has a half-life of 8 days. It is used medicinally to treat thyroid disorders. If a
hospital has 44.00 grams on hand and a normal dose for treatment is 1.50 grams, what is the
longest period of time they can wait before ordering more? Why might they have to order
more I-131 sooner?
32 days; they would have to order more earlier if they used some of it
to treat a patient.
5. What fraction of a 347-gram sample of iron-61 has decayed after 1 hour if its half-life is 6
minutes? What percent remains?
0.3662 grams; 0.09766%
6. 17.5 grams of a 280-gram sample of element A remain.
a. How many half-lives have passed?
b. What is the half-life if the time that has passed is 64 hours?
a. 4 half-lives
b. 6 hours