Balancing Chemical Equations
and its Relationship to the
Mole
Just follow these steps…….
First you need an equation with the correct “formula”.
You’ll probably be given this in the question
Just like this one:
Mg + O2  MgO
Then all you do is list the atoms that are
involved on each side of the arrow
Mg + O2  MgO
Then start balancing:
[1] Just count up the atoms on each side
Mg + O2 
MgO
1
Mg
1
2
O
1
[2] The numbers aren’t balanced so then add
more molecules to make up for any shortages
Mg + O2  2
MgO
1
Mg
1
2
2
O
1
2
And then adjust totals:
But the numbers still aren’t equal, so
add more reactants:
2 Mg + O2 2 MgO
2 1
2
Mg
2
O
2
And adjust totals again:
NOW BOTH SIDES HAVE EQUAL
NUMBERS OF ATOMS
Try to balance these equations
using the same method:
[1]
Na + Cl2  NaCl
[2] CH4 + O2  CO2 + H2O
[3] Li + HNO3  LiNO3 + H2
[4] Al + O2  Al2O3
Here are the answers:
[1]
2 Na + Cl2  2 NaCl
[2] CH4 + 2 O2  CO2 + 2 H2O
[3] 2 Li + 2 HNO3  2 LiNO3 + H2
[4] 4 Al + 3 O2  2 Al2O3
Counting Atoms
• Chemistry is a quantitative science - we
need a “counting unit.”
• The MOLE
• 1 mole is the amount of substance that
contains as many particles (atoms or
molecules) as there are in 12.0 g of C-12.
Avogadro
Paper
The Mole is Developed
Carbon Atoms
Number
Hydrogen Atoms
Mass (amu)
12
(6.02 x
Mass (amu)
1
Mass carbon / Mass hydrogen
12 amu =
1 amu
12
1
12
1
24
[2 x 12]
2
[2 x 1]
24 amu =
2 amu
120
[10 x 12]
10
[10 x 1]
120 amu =
10 amu
12
1
600 amu =
50 amu
12
1
600
[50 x 12]
Avogadro’s
number
Number
Mass Ratio
1023)
x (12)
50
[50 x 1]
Avogadro’s
number (6.02 x 1023) x (1)
(6.02 x 1023) x (12) =
(6.02 x 1023) x (1)
12
1
The More Important Lesson About
Avogadro’s Number!
 A MOLE of any substance contains as many elementary units
(atoms and molecules) as the number of atoms in 12 g of the
isotope of carbon-12.
 This number is called AVOGADRO’s number NA = 6.022 x 1023
particles/mol
 The mass of one mole of a substance is called MOLAR MASS
symbolized by MM
 Units of MM are g/mol
 Examples
H2
hydrogen
2.02
g/mol
He
helium
4.0
g/mol
N2
nitrogen
28.0
g/mol
O2
oxygen
32.0
g/mol
CO2
carbon dioxide
44.0
g/mol
1 Mole of Particles
Looks Like…?