Calculating Half-Life of Radioactive Isotopes Worksheet:
1) How much of a 100.0 gram sample of 198Au is left over after 8.10 days if its half-life is
2.70 days?
2) A 50.0 gram sample of 16N decays to 12.5 grams in 14.4 seconds. What is its half-life?
3) The half-life of 42K is 12.4 hours. How much of a 750.0 gram sample is left after 62.0
hours?
4) What is the half-life of 99Tc if a 500.0 gram sample decays to 62.5 grams in 639,000
years?
5) The half-life of 232Th is 1.4 x 1010 years. If there are 25.0 grams of the sample left after
2.8 x 1010 years, how many grams were in the original sample?
6) There are 5.0 grams of 131I left after 40.35 days. How many grams were in the original
sample if its half-life is 8.07 days?
7) An isotope of 137Cs has a half-life of 30 years. If 1.0 grams of Cesium-137 disintegrates
over a period of 90 years, how many grams of Cesium-137 would remain?
8) A 2.5 gram sample of an isotope of Strontium-90 was formed in a 1960 explosion of an
atomic bomb at Johnson Island in the Pacific Test Site. The half-life of Strontium-90 is 28
years. In what year will only 0.625 grams of this Strontium-90 remain?
9) Actinium-226 has a half-life of 29 hours. If 100.0 grams of Actinium-226 disintegrates
over a period of 58 hours, how many grams of Actinium-226 will remain?
10) Thallium-201 has a half-life of 73 hours. If 4.0 grams of Thallium-201 disintegrates over a
period of 6.0 days and 2 hours, how many grams of Thallium-201 will remain?
11) Sodium-25 was to be used in an experiment, but it took 3.0 minutes to get the Sodium
from the reactor to the laboratory. If 5.0 grams of Sodium-25 was removed from the
reactor, how many grams of Sodium-25 were placed in the reaction vessel 3.0 minutes
later if the half-life of Sodium-25 is 60 seconds?
12) The half-life of Isotope X is 2.0 years. How many years would it take for a 4.0 gram
sample of Isotope X to decay and have only 0.50 grams of it remain?
13) Selenium-83 has a half-life of 25.0 minutes. How many minutes would it take for a 10.0
gram sample to decay and have only 1.25 grams of it remain?
14) Element-106 has a half-life of 0.90 seconds. If 250.0 grams of it were prepared, how
many grams would remain after 4.5 seconds?