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Uranium-235 undergoes alpha decay. (U)
Carbon-14 undergoes beta decay. (C)
Radium-226 decays into Radon-222. (Ra  Rn)
Thorium-234 is produced after a nucleus underwent
alpha decay. (Th)
Problem #10: The half-life of thorium-227 is 18.72 days. How many days are
required for three-fourths of a given amount to decay?
Problem #11: The half life of iodine-131 is 8.040 days. What percentage of an
iodine-131 sample will remain after 40.20 days?
Problem #12: Rn-222 has a half-life of 3.82 days. How long before only 1/16 of the
original sample remains?
Problem #13: The half-life of Palladium-100 is 4 days. After 12 days a sample of Pd100 has been reduced to a mass of 4.00 mg. (a) Determine the starting mass. (b)
What is the mass 8 weeks after the start?
Answers:
1. 23592U  42He + 23190Th
2. 146C  0-1e + 147N
3. 22688Ra  22286Rn + 42He
4. 23892U  23490Th + 42He
#10 Solution:
3/4 = 0.75 <--- amount decayed
1 - 0.75 = 0.25 <--- amount remaining
(1/2)n = 0.25
n=2
18.72 d times 2 = 37.44 d
#11 Solution:
40.20 d / 8.040 d = 5
(1/2)5 = 0.03125
percent remaining = 3.125%
#12 Solution:
recognize 1/16 as a fraction associated with 4 half-lives (from 1/24 = 1/16)
3.82 days x 4 = 15.3 days
#13 Solution:
12 day / 4 day = 3
(1/2)3 = 0.125
4.00 mg / 0.125 = 32.0 mg
8 weeks = 56 days
56 d / 4 = 14 half-lives
(1/2)14 = 0.000061035
32.0 mg times 0.000061035 = 0.00195 mg (rounded to three figs)