South Pasadena • Chemistry
Name
Period
Date
8 · Nuclear Chemistry
HALF
LIFE
WORKSHEET
1. Plot a graph for a group of atoms that have a half-life of 2 tosses.
0
2.
1
2
3
4
5
6
Toss Number
7
8
9
10
24
Na has a half-life of 15 hours.
(a) If we start with 64 grams of 24Na, fill in the chart for how much 24Na we will have after each
time interval.
Time
0h
15 h
30 h
45 h
60 h
75 h
90 h
105 h
Mass of 24Na
64 g
32 g
16 g
8g
4g
2g
1g
0.5 g
half-lives
1
2
3
4
5
6
7
(b) What percentage of a sample will remain after 60 hours?
6.25%
4g
× 100% = 6.25%
64 g
3. 257Lr has a half-life of 8 seconds. If we begin with 80 g of 257Lr, how long will it take
before only 5 g remains?
Time
Mass of
257
Lr
0s
8s
16 s
24 s
32 s
80 g
40 g
20 g
10 g
5g
half-lives
1
2
3
4
32 s
4. 24Na has a half-life of 15 hours. If we start with 90 grams of 24Na, how much will we
have after 75 hours?
Time
24
Mass of Na
0h
15 h
30 h
45 h
60 h
75 h
90 g
45 g
22.5 g
11.3 g
5.6 g
2.8 g
2.8 g
half-lives
1
2
3
4
5
95
5. A 64 grams sample of Tc (technetium) is reduced to 4 grams in 80 hours. What is
the half-life of Tc-95?
Time
95
Mass of Tc
0h
80 h
64 g
32 g
16 g
8g
20 h
4g
half-lives
1
2
3
4 · t1/2 = 80 hrs
t1/2 = 20 hrs
4
6. A sample of 253Fm decays to 6.25% of its activity in 18 days. What is its half-life?
Time
0h
Mass of 253Fm
100%
18 d
50%
25%
12.5% 6.25%
4.5 d
half-lives
1
2
3
4 · t1/2 = 18 days
t1/2 = 4.5 days
4
7. 24Na has a half-life of 15 hours. If we need to have 2 grams of 24Na after 45 hours,
how much must we begin with?
Time
0h
15 h
30 h
45 h
Mass of 24Na
16 g
8g
4g
2g
16 g
half-lives
1
2
3
8. 257Lr has a half-life of 8 seconds. How long will it take for 75% of the sample to
decay?
When 75% decays, 25% remains.
Time
0s
8s
16 s
Mass of 257Lr
100%
50%
25%
16 s
half-lives
1
9.
2
14
C has a half-life of 5000 years and is used to date fossils. A sample is only 3% as
radioactive as it should be compared to carbon from a living organism. How old is
the fossil?
Time
0y
5000 y 10,000 y 15,000 y 20,000 y 25,000 y
Mass of 24Na
100%
50%
25%
12.5%
6.3%
3.1%
half-lives
1
2
3
4
5
25,000 years