Percent composition of a compound

Answer Key - Grade 10 Science Worksheet - Building Confidence

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

1. 1.

   Water (H₂O) has 2 H atoms at 1.01 g/mol each, and its molar mass is 18.02 g/mol. What percent of its mass is hydrogen, to 0.1%, Ari?

   11.2 %
2. 2.

   Carbon dioxide (CO₂) has 2 O atoms at 16 g/mol each, and its molar mass is 44.01 g/mol. What percent of its mass is oxygen, to 0.1%, Bo?

   72.7 %
3. 3.

   Baking soda (NaHCO₃) has 3 O atoms at 16 g/mol each, and its molar mass is 84.01 g/mol. What percent of its mass is oxygen, to 0.1%, Kira?

   57.1 %
4. 4.

   Water's percent bar shows hydrogen at 11.2%. What share is oxygen, Jun?

   • 11.2%
   • 88.8%
   • 33.3%
5. 5.

   Water is 11.2% hydrogen by mass. What does that mean, Noah?

   • 11.2 out of each 100 atoms in water are hydrogen atoms
   • Each 100 g of water holds 11.2 g of hydrogen
   • Large samples alone are 11.2% hydrogen
6. 6.

   A 5 g drop of water and a 500 g jug of water: which is the higher percent hydrogen, Pia?

   • The jug, since it holds more water
   • The drop, since small samples have more hydrogen
   • Neither; both are 11.2% hydrogen
7. 7.

   In water, hydrogen is 2 of the 3 atoms. Why is it just 11.2% of the mass, Ren?

   • Each H atom is far lighter than the O atom
   • Because hydrogen is lost when water is weighed
   • Because 2 of 3 atoms works out to 11.2%
8. 8.

   Which setup gives the percent hydrogen in water, Tala?

   • (18.02 g ÷ 2.02 g) × 100
   • (2.02 g ÷ 18.02 g) × 100
   • (1.01 g ÷ 18.02 g) × 100
9. 9.

   Which setup gives the percent carbon in carbon dioxide, Zee?

   • (12.01 g ÷ 32.00 g) × 100
   • (12.01 g ÷ 44.01 g) × 100
   • (44.01 g ÷ 12.01 g) × 100
10. 10.

    Which setup gives the percent oxygen in baking soda (NaHCO₃), Esme?

    • (84.01 g ÷ 48.00 g) × 100
    • (16.00 g ÷ 84.01 g) × 100
    • (48.00 g ÷ 84.01 g) × 100