WKST 8: Interpreting Coefficients
in Balanced Chemical Equations
Name_______________________________
Date__________
Block____
Remember that in any chemical reaction, the atoms and ions that make up the reactants are
simply rearranged to form the products. Once the electrically balanced chemical formulas of the
reactants and products are written, we have to use coefficients in front of each reactant and
product in order to balance the number of atoms or ions of each element on both sides of the
reaction arrow.
The coefficients in a balanced chemical equation give us information about the relative amounts
of reactants and products involved in the reaction. The interpretation of what the coefficients
mean will vary depending on the nature of each reactant and product.
Coefficients may be interpreted as the number of:
a) atoms of an element
b) ions of an element
c) molecules of a molecular compound
d) formula units of an ionic compound - or of a molecular compound
e) volumes of gas at STP
(standard temperature and standard pressure: 0.0 oC and 1 atmosphere of
pressure (or 760 mmHg), the pressure at sea level)
Coefficients are NEVER interpreted as the MASS of a substance.
Examples:
1. Given the balanced chemical equation:
2 Na(s) + Cl2(g) → 2 NaCl(s)
Remember that if there is no number written in front of a chemical formula, the coefficient is
understood to be “1”.
The “2 Na(s)” is read as “2 atoms of sodium metal” since sodium is written uncombined.
The “ Cl2(g)” is read as “1 molecule of chlorine” since chlorine exists as a diatomic molecule
when not combined with any other element in a compound (BrINClHOF elements!).
The “2 NaCl(s)” is read as “2 formula units of solid sodium chloride” since sodium chloride is a
compound and is a solid at room temperature.
2. Given the balanced chemical equation:
2 H2(g) + O2(g) → 2 H2O(g)
The “2 H2(g)” is read as “2 molecules of hydrogen gas.” The “ O2(g)” is read as “1 molecule of
oxygen gas.” The “2 H2O(g)” is read as “2 molecules of water vapor.” The entire equation may
then be read as: “2 molecules of hydrogen gas react with 1 molecule of oxygen gas to form 2
molecules of water vapor.”
Since all reactants are gases, we assume STP conditions for the reaction and then we read:
“2 volumes of hydrogen gas react with 1 volume of oxygen gas.”
Interpret each of the following balanced chemical reactions using “molecule”, “atoms”, or
formula units”:
1.
2 Mg(s) + O2(g) → 2 MgO(s)
2.
Pb(NO3)2(aq) + 2 KI(aq) → PbI2(s)
3.
Zn(s) + 2 HCl(aq) → ZnCl2(aq) + H2(g)
4.
Na2SO4(aq) + BaCl2(aq) → BaSO4(s) + 2 NaCl(aq)
5.
+
CuSO4 • 5 H2O(s) → CuSO4(s) + 5 H2O(g)
6.
K2O(s) + H2O(l) → 2 KOH(aq)
7.
K3PO4 →
3 K+1
+ PO4 –3
2 KNO3(aq)
Write the balanced chemical equations for each of the following interpretations:
8. 2 formula units of solid sodium hydrogen carbonate decompose when heated to form 1
formula unit of solid sodium carbonate, 1 molecule of water vapor, and 1 molecule of carbon
dioxide gas.
9. 2 formula units of aqueous sodium hydroxide react with 1 formula unit of sulfuric acid to
form 1 formula unit of aqueous sodium sulfate and 2 molecules of liquid water.
10. 1 formula unit of solid iron(II) sulfide reacts with 2 formula units of hydrochloric acid to
form 1 molecule of hydrogen sulfide gas and 1 formula unit of aqeuous iron(II) chloride.
11. 2 atoms of potassium metal react with 2 molecules of liquid water to produce 2 formula
units of aqueous potassium hydroxide and 1 molecule of hydrogen gas.
12. 1 molecule of chlorine gas reacts with 2 formula units of aqueous sodium bromide to form
2 formula units of aqueous sodium chloride and 1 molecule of liquid bromine.
13. 4 atoms of aluminum metal react with 3 molecules of oxygen gas to form 2 formula units
of solid aluminum oxide.
14.
2 atoms of copper metal react with 1 atom of solid sulfur to form 1 formula unit of solid
copper(I) sulfide.
15. 2 atoms of solid sulfur react with 3 molecules of oxygen gas to form 2 molecules of sulfur
trioxide gas.