Half-life: how much is left

Answer Key - Grade 10 Science Worksheet - Challenge Me

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

1. 1.

   A wooden object has 12.5% of the carbon-14 found in living wood. Using a half-life of 5,730 years, about how old is it, Bo?

   17,190 years
2. 2.

   A wooden object has 6.25% of the carbon-14 found in living wood. Using a half-life of 5,730 years, about how old is it, Esme?

   22,920 years
3. 3.

   Ancient scrolls found near Qumran give 10.8 counts per minute per gram of carbon; living carbon gives 13.6. About how old are they, Jun?

   • About 1,900 years
   • About 19,000 years
   • About 5,730 years
4. 4.

   A tracer's count falls from 80 to 10 per minute in 24 hours. What is its half-life, Kira?

   • 12 hours
   • 8 hours
   • 3 hours
5. 5.

   Hospitals make technetium-99m (6.01 h) on site from molybdenum-99 (66 h). Why, Luz?

   • It decays too fast to ship far
   • It lasts too long to store
   • Molybdenum-99 is stable
6. 6.

   Milo says nothing is left after two half-lives, since two halves make a whole. Agree or disagree?

   • Disagree: a quarter is left
   • Agree: half-lives add to 100%
   • Agree: two halves make the whole
7. 7.

   Noah says a sample loses the same mass in every half-life. Agree or disagree?

   • Agree: it drops by a fixed amount
   • Disagree: it loses half of what's left
   • Agree: mass leaves at a set speed
8. 8.

   Pia says each nucleus waits exactly one half-life and then decays. Agree or disagree?

   • Disagree: it describes a large group
   • Agree: each one has a timer
   • Agree: they decay together
9. 9.

   Fluorine-18, used in PET scans, has a half-life of 110 minutes. What fraction is left after 220 minutes, Tala?

   • 1/4
   • 1/2
   • 0
10. 10.

    Which isotope has the longest half-life, Zee?

    • Carbon-14
    • Fluorine-18
    • Iodine-131