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