Half-life: how much is left

Answer Key - Grade 10 Science Worksheet

Circle the best answer, or write your answer on the line.

1. 1.

   A sample holds 80 mg of iodine-131, which has a half-life of 8.02 days. How much is left after 24.06 days, Ari?

   10 mg
2. 2.

   A sample holds 64 mg of fluorine-18, which has a half-life of 110 minutes. How much is left after 440 minutes, Bo?

   4 mg
3. 3.

   A sample holds 100 mg of technetium-99m, which has a half-life of 6.01 hours. How much is left after 18.03 hours, Esme?

   12.5 mg

4. Read this first

   Decay of a tracer sample (example data)

   time (days)	mass left (mg)
   0	80
   8	40
   16	20
   24	10

5. 4.

   Using the table, what is the half-life of the tracer, Jun?

   • 24 days
   • 40 days
   • 8 days
6. 5.

   Using the table, how much tracer is left at 32 days, Kira?

   • 5 mg
   • 2.5 mg
   • 0 mg
7. 6.

   Using the table, which isotope's half-life fits the tracer best, Luz?

   • Technetium-99m, 6.01 hours
   • Carbon-14, 5,730 years
   • Iodine-131, 8.02 days
8. 7.

   Technetium-99m has a half-life of 6.01 hours. About how long until 1/4 of a sample is left, Milo?

   • About 12 hours
   • About 1.5 hours
   • About 24 hours
9. 8.

   A wooden tool has half the carbon-14 of a living tree. About how old is it, Pia?

   • About 11,460 years
   • About 2,865 years
   • About 5,730 years
10. 9.

    Why can't carbon-14 date a fossil that is 66 million years old, Ren?

    • Fossils have too much carbon-14
    • Too little carbon-14 would be left
    • Carbon-14 decays too slowly
11. 10.

    Where does new carbon-14 keep forming, Zee?

    • In the ocean floor
    • In the upper atmosphere
    • Deep inside rocks