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

   12 atoms

3. 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

4. 3.

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

   • Yes: each element matches
   • No: carbon is missing
   • No: oxygen is off by two atoms
5. 4.

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

   • Four in the CO₂ molecule
   • Four in the 2 H₂O
   • 2 in CO₂ and 2 in 2 H₂O
6. 5.

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

   • Because water is heavy
   • To match methane's 4 hydrogens
   • To match the two carbon atoms there
7. 6.

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

   • Two water molecules
   • Two bonds
   • Two oxygen atoms each
8. 7.

   Why can't H₂O be changed to H₂O₂ to balance oxygen, Pia?

   • Oxygen can't carry a subscript 2
   • It would weigh less
   • H₂O₂ is a different substance
9. 8.

   Is H₂ + O₂ → H₂O balanced, Ren?

   • Yes: the hydrogen atoms match
   • No: oxygen doesn't match
   • Yes: it has arrows
10. 9.

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

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

    What makes an equation balanced, Zee?

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