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The Mole
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Molecular and Formula Mass
What is the average atomic mass of the following atoms?
What unit is used?
Oxygen
16.00amu
Zinc
65.38amu
Silver
107.87 amu
How about oxygen gas, O2
16.00amu x 2
= 32.00 amu
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Molecular Mass (used for Covalent compounds)
This is the mass of one molecule expressed in
unified atomic mass units (amu).
It is calculated by adding up the average atomic
mass of each atom found in a compound
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Calculate the molecular mass of H2O.
mass of H2O = 2 H + 1 O
= (2 x 1.01 amu) + (1 x 16.00 amu)
= 18.02 amu
Practice 1:
Calculate the molecular mass of C3H8.
mass of C3H8 = (3 x 12.01 amu) + (8 x 1.01 amu)
= 44.11 amu
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Formula Mass
(Used for Ionic Compounds)
Since ionic compounds do not actually consist of
individual molecules, the mass is referred to
as formula mass. The smallest unit of an ionic
compound is called a formula unit.
This is the total mass of each atom written in
the formula for an ionic compound expressed
in unified atomic mass units (amu)
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Calculate the formula mass of MgO.
mass of MgO = 1 Mg + 1 O
= (1 x 24.31amu) + (1 x 16.00amu)
= 40.31 amu
Practice 2:
Calculate the formula mass of CaCl2.
mass of CaCl2 = (1 x 40.08 amu) + (2 x 35.45 amu)
= 110.98 amu
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We have a problem!
How can you measure 44.01 amu in the lab?
We do not have an electronic scale which can
measure in amu, only g, kg or lb.
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Introducing the
MOLE and Molar Mass
In 1811, a chemist named
Amedeo Avogadro realized
that any convenient
quantity of matter must
contain an enormous
number of chemical
entities.
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Josef Loschmidt latter showed that approx.
602 000 000 000 000 000 000 000, or
6.02 x 1023 atoms was a convenient and
measurable number of entities to work with.
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How Big is a Mole?
• Mole Video
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Grouping Items
Item
Unit
socks
eggs
pencils
pair
dozen
gross (12 dozen)
paper
entities (atoms,
molecules, ions,
formula units)
ream
mole
Number
2
12
144
500
6.02 x 1023
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6.02 x
23
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This number is called Avogadro’s constant (NA)
One mole of an entity has 6.02 x 1023 entities in it.
1 mole = 6.02 x 1023 entities
The symbol for mole is ‘n’ not ‘m’ like you would
think… because???
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Why 6.02 x 1023 ?
Many experiments have shown that 6.02 x 1023
atoms of an element has approx. the same
mass in grams, g, as the relative atomic mass
in unified atomic mass units, u.
Element
Atomic Mass
(amu)
Molar Mass
(g/mol)
Argon
39.95 amu
39.95 g/mol
Uranium
238.03 amu
238.03 g/mol
Carbon
12.01 amu
12.01 g/mol
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Molar Mass is the mass, in grams per mole, of one
mole of a substance.
The SI unit for molar mass is g/mol.
The SI symbol for molar mass is “M” .
Example:
The mass of an ammonia molecule is 17.04 amu
The mass of one mole of ammonia molecules is 17.04 g
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How large is 6.02 x 1023 ?
One mole of pennies would make at least seven
stacks that would reach the moon.
One mole of TimBits would cover the Earth and
be 8 kilometers deep.
Astronomers estimate that there is a mole of
stars in the Universe.
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Practice 3
What is the molecular mass of glucose?
180.18 amu
What is the molar mass of glucose?
180.18 g/mol
What is the mass of 1 mole of table salt?
58.44 g
What is the mass of 3 moles of table salt?
175.32 g
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Textbook Reference pp. 167 - 171
Assigned Questions
p. 168 # 2, 4, 5, 6
p. 170 # 8, 11, 12
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