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.

   Oxygen is 99.757% O-16 (15.995 u), 0.038% O-17 (16.999 u) and 0.205% O-18 (17.999 u) (NIST). What is its average atomic mass, to 0.01 u, Esme?

   16 u
4. 4.

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

   • Near 26 u
   • Exactly at 25 u
   • A little above 24 u
5. 5.

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

   • Magnesium has no isotopes
   • A plain average was used
   • The masses were rounded
6. 6.

   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
7. 7.

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

   • Disagree: it is an average
   • Agree: the table says so
   • Agree: chlorine atoms match exactly
8. 8.

   Noah says carbon-13 has 7 protons because its mass is higher. Agree or disagree?

   • Agree: heavier atoms gain protons
   • Disagree: it has 6 protons
   • Agree: more mass, more protons
9. 9.

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

   • Agree: two halves make a whole
   • Agree: it drops to zero
   • Disagree: a quarter is left
10. 10.

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

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