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Counting Atoms and Balancing
Chemical Equations
SubscriptsC12H22O11
There are 12 atoms of CarbonThere are 22 atoms of HydrogenThere are 11 atoms of Oxygen
If there is not a subscript listed, it is understood to be 1.
NaClThere is one atom of SodiumThere is one atom of Chlorine
Learning CheckNaHCO3
Sodium – 1Hydrogen – 1Carbon – 1Oxygen -3
HClHydrogen – 1Chlorine - 1
There are times you will see a compound with parenthesis.
Pb(NO3)2
The 2 after the parenthesis indicates there are two sets of the parenthesis.
Pb(NO3) (NO3)
So, in counting the atoms, you would have the following:
Lead – 1 Oxygen – 6 Nitrogen -2
(NH4)3PO4
Nitrogen – 3Hydrogen – 12Phosphorus – 1Oxygen – 4
Mg(OH)2
Magnesium – 1Oxygen – 2Hydrogen – 2
Learning Check
2H2SO4This means there are 2 compounds of sulfuric acid. Think:
H2SO4 + H2SO4
Counting the atoms:Hydrogen – 4 Sulfur – 2 Oxygen – 8
CoefficientCoefficient
3H3PO4
Hydrogen – 9Phosphorus – 3Oxygen - 12
2H2O
Hydrogen – 4Oxygen - 2
Learning Check
Reading Chemical Equations
The mass of all the reactants (the substances going into a reaction) must equal the mass of the products (the substances
produced by the reaction).
CH4 + 2O2 → CO2 + 2 H2O
Reactants Products
Law of Conservation of Mass
In a chemical reaction, matter cannot be created or destroyed. It
can only be changed.
The reactants MUST contain the same elements and the same
number of each element as the product. They CAN be in different
compounds.
Do the following equations follow the law of conservation of mass?
H2 + O2 H2O
Na + O2 Na2O
Mg + 2HCl MgCl2 + H2
HgO Hg + O2
N2 + 3H2 2NH3
2Na + 2H2O 2NaOH + H2
Zn + 2HCl ZnCl2 + H2
Learning Check
NONO
YESNO
YES
YESYES
Na + O2 → Na2O
For this equation to be balanced, there must be equal amounts of Na in the
reactants and products.
You must add coefficients to balance this equation:
2Na + O2 → Na2O
Balancing Equations
Balancing EquationsTwo ways…
M. I. N. O. H.or
Number charts
Balancing EquationsM. I. N. O. H.
Two ways…
Add coefficients
firstM - Metals
Balance metals such as Fe or Na first.
Add coefficients
firstM - Metals
Balance metals such as Fe or Na first.
Second I - Ions
Looks for polyatomic ions (such as PO4
-3 or SO4-2)
that cross from reactant to product unchanged.
Balance polyatomic ions as a group within its
parentheses.
Add coefficients
firstM - Metals
Balance metals such as Fe or Na first.
Second I - Ions
Looks for polyatomic ions (such as PO4
-3 or SO4-2)
that cross from reactant to product unchanged.
Balance polyatomic ions as a group within its
parentheses.
Third N - Non-metalsChlorine (Cl) or Sulfur (S) are common non-metals
to look for!
Add coefficients
firstM - Metals
Balance metals such as Fe or Na first.
Second I - Ions
Looks for polyatomic ions (such as PO4
-3 or SO4-2)
that cross from reactant to product unchanged.
Balance polyatomic ions as a group within its
parentheses.
Third N - Non-metalsChlorine (Cl) or Sulfur (S) are common non-metals
to look for!
Fourth O - OxygenRemember, oxygen by
itself is O2
Add coefficients
firstM - Metals
Balance metals such as Fe or Na first.
Second I - Ions
Looks for polyatomic ions (such as PO4
-3 or SO4-2)
that cross from reactant to product unchanged.
Balance polyatomic ions as a group within its
parentheses.
Third N - Non-metalsChlorine (Cl) or Sulfur (S) are common non-metals
to look for!
Fourth O - OxygenRemember, oxygen by
itself is O2
Fifth (last) H - HydrogenRemember, hydrogen by
itself is H2
Balancing EquationsTwo ways…
Number charts
Balancing Equations __H2 + __O2 __H2O
O -H -
O -H -
22
12
2
24
2
4
__H2SO4 + __NaOH __H2O +__Na2SO4
Na -SO4 –O –H -
Na -SO4 –O – H -
1113
2112
2
2
24
2
24
Balancing Equations
__C3H8 + __O2 __CO2 +__H2O
C -O –H –
C -O –H –
328
132
3
37
4
108
5
10
Balancing Equations
