Mass and the Mole
Chemistry
Section 11.2
Objectives
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Relate the mass of an atom to the mass of
a mole of atoms.
Calculate the number of moles in a given
mass of an element and the mass of a given
number of moles of an element.
Calculate the number of moles of an
element when given the number of atoms of
the element.
Calculate the number of atoms of an
element when given the number of moles of
the element.
The Mass of the Mole
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Do a dozen limes have the same mass
as a dozen eggs?
No.
The same is true for moles.
One mol of Fe does not have the
same mass as one mol of Ag.
Molar Mass
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This is IMPORTANT!
Mass in grams of 1 mole of any pure
substance is termed the molar mass.
Molar mass of any element equals its
atomic mass
Units are g/mol
Molar Mass
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For Example:
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Manganese
What is Manganese’s molar mass?
54.94 amu OR g/mol
What does one mole of Manganese weigh?
54.94 g
How many atoms of Manganese are in one mol?
6.022 x 1023
Molar Mass
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We can use the molar mass of every
element as a conversion factor.
Example:
# of mol x # of grams = mass
1 mol
Mole to Mass Conversions
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Chromium (Cr) is a transition element.
Calculate the mass in grams of 0.0450
moles of Cr.
Always start with ?
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What is given in the problem!
0.0450 mol Cr
Mole to Mass Conversions
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Molar mass is the conversion factor
52.00 g/1 mol Cr
Put what you want to end up with on
the top of the conversion factor.
Mole to Mass Conversions
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0.0450 mol Cr x
52 g
1 mol
= 2.34 g Cr
Practice Problems
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Pg. 316
12
Mass to Atoms Conversions
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Let’s take the conversion factor further.
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2 step conversion factor problems
Mass
Moles
Representative
Particles
Mass to Atoms Conversions
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If given the mass of an element, you
can find the number of atoms in that
mass.
Example: How many atoms of gold
are in a pure gold nugget having a
mass of 25.0 g?
Mass to Atoms Conversions
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Always start with?
What is given in the problem.
25.0 g Au
 Mass
 Moles  Particles
Mass  Moles
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Use molar mass of Au conversion factor:
196.97 g
1 mol Au
Moles  Particles
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Avogadro’s conversion:
6.022 x 1023 atoms
1 mol Au
MassMoles Atoms
1st conversion
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2nd conversion
25.0 g Au x 1 mol Au x 6.022 x 1023 atoms =
196.97g Au
1 mol Au
7.65 x 1022 atoms Au
Practice Problems
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Pg 318
14
Homework
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Molar mass worksheet