Balance chemical equations

Answer Key - Grade 10 Science Worksheet - Building Confidence

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

1. 1.

   In 3 H₂O, the coefficient is 3 and each molecule has 2 hydrogen atoms. How many hydrogen atoms in all, Ari?

   6 atoms
2. 2.

   In 2 CH₄, the coefficient is 2 and each molecule has 4 hydrogen atoms. How many hydrogen atoms in all, Bo?

   8 atoms
3. 3.

   In 4 NH₃, the coefficient is 4 and each molecule has 3 hydrogen atoms. How many hydrogen atoms in all, Esme?

   12 atoms

4. Read this first

   Atom tally for CH₄ + 2 O₂ → CO₂ + 2 H₂O (methane burning) (balanced equation)

   element	atoms on the left	atoms on the right
   C	1	1
   H	4	4
   O	4	4

5. 4.

   Using the tally, is the methane equation balanced, Jun?

   • Yes: each element matches
   • No: carbon is missing
   • No: oxygen is off by two atoms
6. 5.

   Using the tally, where do the 4 oxygen atoms on the right come from, Kira?

   • 2 in CO₂ and 2 in 2 H₂O
   • Four in the 2 H₂O
   • Four in the CO₂ molecule
7. 6.

   Using the tally, why is there a 2 in front of H₂O, Luz?

   • To match the two carbon atoms there
   • Because water is heavy
   • To match methane's 4 hydrogens
8. 7.

   When balancing, which numbers may be changed, Milo?

   • Both
   • Coefficients
   • Subscripts
9. 8.

   In 2 H₂O, what does the 2 in front show, Noah?

   • Two oxygen atoms each
   • Two water molecules
   • Two bonds
10. 9.

    Is 2 H₂ + O₂ → 2 H₂O balanced, Tala?

    • No: oxygen is off
    • Yes
    • No: hydrogen is off
11. 10.

    What makes an equation balanced, Zee?

    • The arrow points right
    • The same number of molecules on each side
    • Each element's atoms match on both sides