Average atomic mass from isotopes

Answer Key - Grade 10 Science Worksheet - Challenge Me

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

1. 1.

   Magnesium is 78.99% Mg-24 (23.985 u), 10% Mg-25 (24.986 u) and 11.01% Mg-26 (25.983 u) (NIST). What is its average atomic mass, to 0.01 u, Ari?

   24.31 u
2. 2.

   Silicon is 92.223% Si-28 (27.977 u), 4.685% Si-29 (28.976 u) and 3.092% Si-30 (29.974 u) (NIST). What is its average atomic mass, to 0.01 u, Bo?

   28.09 u
3. 3.

   Before calculating, about where should magnesium's average mass fall, Jun?

   • Exactly at 25 u
   • Near 26 u
   • A little above 24 u
4. 4.

   A student gets 24.98 u for magnesium. What likely went wrong, Kira?

   • The masses were rounded
   • Magnesium has no isotopes
   • A plain average was used
5. 5.

   Oxygen is 99.757% oxygen-16. Why is its average so close to 16, Luz?

   • Oxygen-16 is almost all of it
   • The masses are equal
   • Oxygen has one isotope
6. 6.

   Milo says every chlorine atom has a mass of 35.45 u. Agree or disagree?

   • Agree: chlorine atoms match exactly
   • Disagree: it is an average
   • Agree: the table says so
7. 7.

   Pia says a sample is gone after two half-lives. Agree or disagree?

   • Disagree: a quarter is left
   • Agree: two halves make a whole
   • Agree: it drops to zero
8. 8.

   An ancient leaf buried in mud has half the carbon-14 of a living leaf. About how old is it, Ren?

   • About 11,400 years
   • About 2,850 years
   • About 5,700 years
9. 9.

   A piece of old wood has one quarter of its carbon-14 left. About how old is it, Tala?

   • About 11,400 years
   • About 5,700 years
   • About 22,800 years
10. 10.

    Why does carbon-14 dating work only on things that were once alive, Zee?

    • Rocks have no atoms
    • Carbon-14 is made in fossils
    • They took in carbon while living