STOICHIOMETRY
Chemistry 11
Ms. McGrath
STOICHIOMETRY
The study of the quantities of reactants and products in a
chemical reactions.
ex.
NaHCO3(s) + CH3COOH(aq)  NaCH3COO(aq) + H2O(l) +
CO2(g)
According to this reaction, one formula unit of baking soda
reacts with one formula unit of acetic acid to form a salt,
water and carbon dioxide. If we wanted to carry out this
reaction, how would we know the amount of baking soda and
vinegar to use? The particles are too small and numerous to
be counted.
STOICHIOMETRY
LET’S RECALL A FEW IMPORTANT CONCEPTS
Isotopes: An element’s atom have different
number of neutrons.
ex. Carbon can exist with 6, 7, and 8 neutrons,
therefore they have varying atomic masses (12, 13
and 14). These carbon atoms are called Carbon-12,
Carbon-13 and Carbon-14.
STOICHIOMETRY
LET’S RECALL A FEW IMPORTANT CONCEPTS
Average Atomic Mass (u): is the average of all
the masses of all the element’s isotopes.
We have to take into account:
 the abundance of each isotope
 the mass of each isotope
This is the mass that is seen on the periodic table.
STOICHIOMETRY
LET’S RECALL A FEW IMPORTANT CONCEPTS
Calculating Average Atomic Mass (u)
ex. Carbon
Carbon-12
Carbon-13
Carbon-14
98.9% abundance
1.1% abundance
1 x 10-10 abundance
We can determine that the average atomic mass is
very close to 12.
AVERAGE ATOMIC MASS (U)
PRACTICE
Page 45 #1, 2, 3, and 4
SECTION REVIEW
Page 46 #1, 2, 3 and 4
THE AVOGADRO CONSTANT AND THE MOLE
We will learn how chemists group large numbers of
atoms into amounts that are easily measured.
Certain items, because of their size, are often
handled in bulk.
Some common quantities:
Item
Quantity
gloves
pair
soft drinks
six-pack
eggs
dozen
pens
gross (12 dozen)
paper
ream
Amount
2
6
12
144
500
THE AVOGADRO CONSTANT AND THE
MOLE
The Chemist`s Dozen
Egg farmers and grocers use the dozen (a unit of
12) to count eggs, chemists use the mole (a much
larger number) to represent 6.02 x 1023 atoms,
molecules or formula units. When farmers think
of two dozen eggs, they are also thinking of 24
eggs.
2 dozen x 12 eggs = 24 eggs
1 dozen
THE AVOGADRO CONSTANT AND THE
MOLE

We can convert between moles and particles in a
similar way. For example, 1 mol of aluminum
has 6.02 x 1023 atoms of Al. Thus 2 mol of
aluminum atoms contain 12.0 x 1023 atoms of Al.
2 mol x (6.02 x 1023 atoms) = 1.20 x 1024 atoms
1 mol
THE AVOGADRO CONSTANT AND THE MOLE
Easily measurable amounts of elements contain
huge numbers of atoms. Chemists use a quantity
that is much larger than a dozen or ream to group
atoms or molecules together.
This quantity is the mole (symbol mol).
A mole = 6.02 x 1023 (Avogadro’s number)
THE AVOGADRO CONSTANT AND THE MOLE

THE AVOGADRO CONSTANT AND THE MOLE
Consider the following balanced chemical reaction:
Fe(s) + S(s)  FeS(s)
The above reactants combine in a 1 : 1 ratio. One
56 u atom of Iron reacts with one 32 u atom of
Sulfur to yield one 88 u formula unit of
Iron Sulfide.
THE AVOGADRO CONSTANT AND THE MOLE
How big is a mole?
A mole of marbles would spread over the surface
of the earth, and produce a layer about 50 miles
thick.
A mole of sand, spread over the United States,
would produce a layer 3 inches deep.
A mole of dollars could not be spent at the rate of
a billion dollars a day over a trillion years.
These examples shows you just how big a mole is.
Complete
page 50.
questions 2 and 4 of the ThoughtLab on