4.3 
APPLICATION EXAMPLE
FOR MOLES Gas Behavior
How does the behavior of gases affect airbags?
What is pressure?
Pressure – Force of gas particles
running into a surface
Pressure and Number of Molecules
If pressure is molecular collisions with the
container…
As number of
molecules
increases,
there are more
molecules to
collide with
the wall
Collisions
between
molecules and
the wall
increase
As # of molecules increases, pressure increases
Pressure
increases
Pressure and Volume
If pressure is molecular collisions with the
container…
As volume
increases,
molecules can
travel farther
before hitting
the wall
Collisions
between
molecules and
the wall
decrease
As volume increases, pressure decreases
Pressure
decreases
Section 4.3—Counting
Molecules
So the number of molecules affects pressure of an
airbag…how do we “count” molecules?
How do auto makers utilize enough atoms
of gas to create pressure that inflates the
bag,
But not so many that the pressure is too
great when it hits you and your face that it
causes permanent damage
Count the atoms
What is a mole?
Definition
Mole – SI unit for counting
The only acceptable abbreviation for “mole” is
“mol”…not “m”!!
What is a counting unit?
You’re already familiar with one counting unit…a
“dozen”
A dozen = 12
“Dozen”
12
A dozen doughnuts
12 doughnuts
A dozen books
12 books
A dozen cars
12 cars
A dozen people
12 people
What does a “mole” count in?
A mole = 6.02  1023 (called Avogadro’s number)
6.02  1023 = 602,000,000,000,000,000,000,000
“mole”
6.02  1023
1 mole of doughnuts
6.02  1023 doughnuts
1 mole of atoms
6.02  1023 atoms
1 mole of molecules
6.02  1023 molecules
This means a 12 ounce bottle of water would have
19.7 “moles” of water…a much easier-to-work-with
number!
Molar Mass
Definition
Molar Mass – The mass for one mole
of an atom or molecule.
Other terms commonly used for the same meaning:
Molecular Weight
Molecular Mass
Formula Weight
Formula Mass
Mass for 1 mole of atoms
The average atomic mass = grams for 1 mole
Average atomic mass is found on the periodic table
Element
Mass
1 mole of carbon atoms
12.01 g
1 mole of oxygen atoms
16.00 g
1 mole of hydrogen
atoms
1.01 g
Unit for molar mass: g/mole or g/mol
Molar mass for molecules
The molar mass for a molecule = the
sum of the molar masses of all the
atoms
Calculating a Molecule’s Mass
To find the molar mass of a molecule:
1
Count the number of each type of atom
2
Find the molar mass of each atom on the periodic
table
3
Multiple the # of atoms  molar mass for each atom
4
Find the sum of all the masses
Example: Molar Mass
Example:
Find the
molar
mass for
CaBr2
Example: Molar Mass
1
Example:
Find the
molar
mass for
CaBr2
Count the number of each type of atom
Ca
1
Br
2
Example: Molar Mass
2
Example:
Find the
molar
mass for
CaBr2
Find the molar mass of each atom on the periodic table
Ca
1
40.08 g/mole
Br
2
79.91 g/mole
Example: Molar Mass
3
Example:
Find the
molar
mass for
CaBr2
Multiple the # of atoms  molar mass for each atom
Ca
1  40.08 g/mole =
40.08 g/mole
Br
2  79.91 g/mole =
159.82 g/mole
Example: Molar Mass
4
Example:
Find the
molar
mass for
CaBr2
Find the sum of all the masses
Ca
1  40.08 g/mole =
40.08 g/mole
Br
2  79.91 g/mole =
+ 159.82 g/mole
199.90 g/mole
1 mole of CaBr2 molecules would have a mass of
199.90 g
Example: Molar Mass & Parenthesis
Be sure to distribute the subscript outside the
parenthesis to each element inside the parenthesis.
Example:
Find the
molar
mass for
Sr(NO3)2
Example: Molar Mass & Parenthesis
Be sure to distribute the subscript outside the
parenthesis to each element inside the parenthesis.
Example:
Find the
molar
mass for
Sr(NO3)2
Sr
1  87.62 g/mole =
87.62 g/mole
N
2  14.01 g/mole =
28.02 g/mole
O
6  16.00 g/mole =
+ 96.00 g/mole
211.64 g/mole
1 mole of Sr(NO3)2 molecules would have a mass of 211.64 g
Let’s Practice #2
Example:
Find the
molar
mass for
Al(OH)3
Let’s Practice #2
Be sure to distribute the subscript outside the
parenthesis to each element inside the parenthesis.
Example:
Find the
molar
mass for
Al(OH)3
Al
1  26.98 g/mole =
26.98 g/mole
O
3  16.00 g/mole =
48.00 g/mole
H
3  1.01 g/mole
=
+ 3.03 g/mole
78.01 g/mole
1 mole of Al(OH)3 molecules would have a mass of 78.01 g
Using Molar Mass in
Conversions
Example: Moles to Grams
Example:
How many
grams are
in 1.25
moles of
water?
Example: Moles to Grams
When converting between grams and moles, the
molar mass is needed
Example:
How many
grams are
in 1.25
moles of
water?
1.25 mol H2O
H 2  1.01 g/mole =
2.02 g/mole
O 1  16.00 g/mole = + 16.00 g/mole
18.02 g/mole
1 mole H2O molecules = 18.02 g
18.02 g H2O
1
mol H2O
22.53 g H2O
= _______
Let’s Practice #3
Example:
How many
moles are
in 25.5 g
NaCl?
Let’s Practice #3
Na 1  22.99 g/mole = 22.99 g/mole
Cl 1  35.45 g/mole = + 35.45 g/mole
58.44 g/mole
Example:
How many
moles are
in 25.5 g
NaCl?
25.5 g NaCl
1 mole NaCl molecules = 58.44 g
1
mole NaCl
58.44 g NaCl
0.44
= _______
mole NaCl
Let’s Practice #4
Example:
How many
grams is a
sample of
2.75 × 1024
molecules of
SrCl2?
Let’s Practice #4
Example:
How many
grams is a
sample of
2.75 × 1024
molecules of
SrCl2?
2.75 × 1024 molecules
SrCl2
Sr 1  87.62 g/mole = 87.62 g/mole
Cl 2  35.45 g/mole = + 70.90 g/mole
158.52 g/mole
1 moles SrCl2 molecules = 158.52 g
1 mol = 6.021023 molecules
1
mol SrCl2
6.02 × 1023 molecules
SrCl2
158.52 g SrCl2
1
mol SrCl2
7.24
= _________
g SrCl2
Example: Grams to Molecules
Example:
How many
molecules
are in
25.5 g
NaCl?
Example: Grams to Molecules
Example:
How many
molecules
are in
25.5 g
NaCl?
Na 1  22.99 g/mole = 22.99 g/mole
Cl 1  35.45 g/mole = + 35.45 g/mole
58.44 g/mole
1 moles NaCl molecules = 58.44 g
1 mol = 6.021023 molecules
25.5 g NaCl
1
mol NaCl
58.44 g NaCl
6.021023 molecules NaCl
1
mol NaCl
2.63  1023 molecules NaCl
= _________