Section 3.3
The Mole: A Measurement of
Matter
 OBJECTIVES:
–Describe how Avogadro’s
number is related to a mole of
any substance.
1
Section 3.3
The Mole: A Measurement of
Matter
 OBJECTIVES:
–Calculate the mass of a mole of
any substance.
2
What is a Mole?
 You
can measure mass,
 or volume,
 or you can count pieces.
 We measure mass in grams.
 We measure volume in liters.
 We
3
count pieces in MOLES.
Moles (abbreviated: mol)
 Defined
as the number of carbon
atoms in exactly 12 grams of
carbon-12.
 1 mole is 6.02 x 1023 particles.
 Treat it like a very large dozen
 6.02
x 1023 is called Avogadro’s
number.
4
Representative particles
 The
smallest pieces of a substance.
–For a molecular compound: it is
the molecule.
–For an ionic compound: it is the
formula unit.
–For an element: it is the atom.
5
Types of questions
 How
many molecules of CO2 are
there in 4.56 moles of CO2 ?
 How many moles of water is 5.87 x
1022 molecules?
 How many atoms of carbon are
there in 1.23 moles of C6H12O6 ?
 How many moles is 7.78 x 1024
formula units of MgCl2?
6
Measuring Moles
 Remember
relative atomic mass?
 The amu was one twelfth the mass
of a carbon-12 atom.
 Since the mole is the number of
atoms in 12 grams of carbon-12,
 the decimal number on the periodic
table is also the mass of 1 mole of
those atoms in grams.
7
Gram Atomic Mass (gam)
 Equals
the mass of 1 mole of an
element in grams
 12 grams of C has the same number
of pieces as 1 gram of H and 56
grams of iron.
 We can write this as
12 g C = 1 mole C
 We can count things by weighing
8 them.
Examples
 How
much would 2.34 moles of
carbon weigh?
 How many moles of magnesium is
24 g of Mg?
 How many atoms of lithium is 1.00
g of Li?
 How much would 3.45 x 1022 atoms
of U weigh?
9
What about compounds?
in 1 mole of H2O molecules there are
two moles of H atoms and 1 mole of O
atoms
 To find the mass of one mole of a
compound
– determine the moles of the elements
they have
– Find out how much they would weigh
– add them up
10

What about compounds?
What is the mass of one mole of CH4?
1 mole of C = 12 g
4 mole of H x 1 g = 4 g
1 mole CH4 = 12 + 4 = 16 g
 The Gram Molecular Mass (gmm) of
CH4 is 16 g
– this is the mass of one mole of a
molecular compound.

11
Gram Formula Mass (gfm)
The mass of one mole of an ionic
compound.
 Calculated the same way as gmm.
 What is the GFM of Fe2O3?
2 moles of Fe x 56 g = 112 g
3 moles of O x 16 g = 48 g
The GFM = 112 g + 48 g = 160 g

12
Section 3.3
Mole-Mass and Mole-Volume
Relationships
 OBJECTIVES:
–Use the molar mass to convert
between mass and moles of a
substance.
13
Section 3.3
Mole-Mass and Mole-Volume
Relationships
 OBJECTIVES:
–Use the mole to convert among
measurements of mass,
volume, and number of
particles.
14
Molar Mass
 Molar
mass is the generic term for
the mass of one mole of any
substance (in grams)
 The same as: 1) gram molecular
mass, 2) gram formula mass, and 3)
gram atomic mass- just a much
broader term.
15
Examples
Calculate the molar mass of the
following and tell what type it is:
 Na2S
 N2O4
C
 Ca(NO3)2
 C6H12O6
 (NH4)3PO4

16
Molar Mass
 The
number of grams of 1 mole
of atoms, ions, or molecules.
 We can make conversion factors
from these.
–To change grams of a
compound to moles of a
compound.
17
For example

18
How many moles is 5.69 g of NaOH?
For example

How many moles is 5.69 g of NaOH?

5.69 g

19



For example

How many moles is 5.69 g of NaOH?
mole 

5.69 g


g 
 need to change grams to moles
20
For example

How many moles is 5.69 g of NaOH?
mole 

5.69 g


g 
 need to change grams to moles
 for NaOH
21
For example

How many moles is 5.69 g of NaOH?
mole 

5.69 g


g 
 need to change grams to moles
 for NaOH
 1mole Na = 23 g 1 mol O = 16 g
 1 mole of H = 1 g
22
For example

How many moles is 5.69 g of NaOH?
mole 

5.69 g


g 
 need to change grams to moles
 for NaOH
 1mole Na = 23 g 1 mol O = 16 g
 1 mole of H = 1 g
 1 mole NaOH = 40 g
23
For example

How many moles is 5.69 g of NaOH?
1 mole 

5.69 g


40.00 g 
 need to change grams to moles
 for NaOH
 1mole Na = 23 g 1 mol O = 16 g
 1 mole of H = 1 g
 1 mole NaOH = 40 g
24
For example

How many moles is 5.69 g of NaOH?

5.69 g

1 mole 
 = 0.142 mol NaOH
40.00 g 
need to change grams to moles
 for NaOH
 1mole Na = 23 g 1 mol O = 16 g
 1 mole of H = 1 g
 1 mole NaOH = 40 g
25

Examples
 How
many moles is 4.56 g of
CO2?
 How many grams is 9.87 moles of
H2O?
 How many molecules is 6.8 g of
CH4?
 49 molecules of C6H12O6 weighs
how much?
26
Gases
 Many
of the chemicals we deal with
are gases.
–They are difficult to weigh.
 Need to know how many moles of gas
we have.
 Two things effect the volume of a gas
–Temperature and pressure
 We need to compare them at the same
temperature and pressure.
27
Standard Temperature and
Pressure
 0ºC
and 1 atm pressure
 abbreviated STP
 At STP 1 mole of gas occupies
22.4 L
 Called the molar volume
 1 mole = 22.4 L of any gas at STP
28
Examples
 What
is the volume of 4.59 mole
of CO2 gas at STP?
 How many moles is 5.67 L of O2
at STP?
 What is the volume of 8.8 g of
CH4 gas at STP?
29
Density of a gas
D=m/V
–for a gas the units will be g / L
 We can determine the density of any
gas at STP if we know its formula.
 To find the density we need the mass
and the volume.
 If you assume you have 1 mole, then
the mass is the molar mass (from PT)
30 At STP the volume is 22.4 L.

Examples
 Find
the density of CO2 at STP.
 Find the density of CH4 at STP.
31
The other way
 Given
the density, we can find the
molar mass of the gas.
 Again, pretend you have 1 mole at
STP, so V = 22.4 L.
m = D x V
 m is the mass of 1 mole, since you
have 22.4 L of the stuff.
 What is the molar mass of a gas with a
density of 1.964 g/L?
32  2.86 g/L?
Summary
 These
33
four items are all equal:
a) 1 mole
b) molar mass (in grams)
c) 6.02 x 1023 representative
particles
d) 22.4 L at STP
Thus, we can make conversion
factors from them.