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Science Grade 9-12 Answer Key

Science: Balancing Chemical Equations

Using coefficients to conserve atoms in reactions

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Science: Balancing Chemical Equations

Using coefficients to conserve atoms in reactions

Science - Grade 9-12

Instructions: Balance each chemical equation by adding whole-number coefficients. Do not change subscripts. Check that the number of each kind of atom is equal on both sides.
  1. 1

    Balance the equation: H2 + O2 -> H2O

    Start by balancing oxygen, then adjust hydrogen.

    The balanced equation is 2H2 + O2 -> 2H2O. There are 4 hydrogen atoms and 2 oxygen atoms on each side.
  2. 2

    Balance the equation: N2 + H2 -> NH3

    Balance nitrogen first, then match the hydrogen atoms.

    The balanced equation is N2 + 3H2 -> 2NH3. There are 2 nitrogen atoms and 6 hydrogen atoms on each side.
  3. 3

    Balance the equation: Na + Cl2 -> NaCl

    The balanced equation is 2Na + Cl2 -> 2NaCl. There are 2 sodium atoms and 2 chlorine atoms on each side.
  4. 4

    Balance the equation: Mg + O2 -> MgO

    Because oxygen appears as O2, you will need an even number of oxygen atoms in the products.

    The balanced equation is 2Mg + O2 -> 2MgO. There are 2 magnesium atoms and 2 oxygen atoms on each side.
  5. 5

    Balance the equation: Al + O2 -> Al2O3

    Balance aluminum and oxygen using the smallest whole numbers.

    The balanced equation is 4Al + 3O2 -> 2Al2O3. There are 4 aluminum atoms and 6 oxygen atoms on each side.
  6. 6

    Balance the equation: Fe + O2 -> Fe2O3

    The balanced equation is 4Fe + 3O2 -> 2Fe2O3. There are 4 iron atoms and 6 oxygen atoms on each side.
  7. 7

    Balance the equation: C3H8 + O2 -> CO2 + H2O

    Balance carbon first, then hydrogen, and oxygen last.

    The balanced equation is C3H8 + 5O2 -> 3CO2 + 4H2O. There are 3 carbon atoms, 8 hydrogen atoms, and 10 oxygen atoms on each side.
  8. 8

    Balance the equation: CH4 + O2 -> CO2 + H2O

    The balanced equation is CH4 + 2O2 -> CO2 + 2H2O. There are 1 carbon atom, 4 hydrogen atoms, and 4 oxygen atoms on each side.
  9. 9

    Balance the equation: Ca + H2O -> Ca(OH)2 + H2

    Treat the hydroxide group in Ca(OH)2 carefully when counting atoms.

    The balanced equation is Ca + 2H2O -> Ca(OH)2 + H2. There are 1 calcium atom, 4 hydrogen atoms, and 2 oxygen atoms on each side.
  10. 10

    Balance the equation: KClO3 -> KCl + O2

    Balance potassium and chlorine together, then oxygen.

    The balanced equation is 2KClO3 -> 2KCl + 3O2. There are 2 potassium atoms, 2 chlorine atoms, and 6 oxygen atoms on each side.
  11. 11

    Balance the equation: NaOH + H2SO4 -> Na2SO4 + H2O

    The balanced equation is 2NaOH + H2SO4 -> Na2SO4 + 2H2O. There are 2 sodium atoms, 1 sulfur atom, 4 oxygen atoms from sulfate plus 2 more in water, and 4 hydrogen atoms on each side.
  12. 12

    Balance the equation: AgNO3 + NaCl -> AgCl + NaNO3

    Check each atom count before changing anything.

    The balanced equation is AgNO3 + NaCl -> AgCl + NaNO3. The equation is already balanced with coefficients of 1 for all substances.
  13. 13

    Balance the equation: CaCO3 -> CaO + CO2

    The balanced equation is CaCO3 -> CaO + CO2. The equation is already balanced with coefficients of 1 for all substances.
  14. 14

    Balance the equation: NH3 + O2 -> NO + H2O

    Balance nitrogen and hydrogen first, then oxygen.

    The balanced equation is 4NH3 + 5O2 -> 4NO + 6H2O. There are 4 nitrogen atoms, 12 hydrogen atoms, and 10 oxygen atoms on each side.
  15. 15

    Balance the equation: C2H5OH + O2 -> CO2 + H2O

    The balanced equation is C2H5OH + 3O2 -> 2CO2 + 3H2O. There are 2 carbon atoms, 6 hydrogen atoms, and 7 oxygen atoms on each side.
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