Moles and Mass
LG: I can use molar mass to convert
between mass and moles
Relative Atomic Mass
• The mass of all elements on the periodic table
relative to carbon-12.
– ie. An atom of hydrogen is assigned a mass of
~ 1µ, because it is roughly 1/12 the mass of an
atom of carbon-12
• The ratio of masses hold true as long as the
units are the same
– Example: A mole of hydrogen weighs 1g, a mole
or carbon weighs 12g
Molar Mass
• The mass of one mole of a substance is the
“molar mass”
– Molar Mass Units: g/mol
Symbol: M
– Recall: Chemists decided to take the atomic
weight of an element and define the number of
particles in that many grams to be one mole.
– Example: What is the molar mass of:
Argon, Ar
Oxygen, O2
Sulfur, S8
Magnesium chloride, MgCl2
Calculations with Molar Mass
• Molar mass can be used to convert between mass and
moles
– Mass Units: g
Symbol: m
• Think about the units involved to answer the following:
– What is the molar
mass of lithium?
– What is the mass of 4.5
moles of lithium?
– How many moles of lithium
are contained in a 28g sample?
m
n
M
Practice
• What is the mass of 4.2 moles of sodium
hydroxide?
• A can of coke contains 40.0g of sucrose,
C12H22O11 how many moles of sucrose are in a
can of coke?
“The Mole Bridge”
• By combining two formulas, moles can be
used to convert between mass and number of
entities
m
N
M
n
N
A
• Example: how many atoms are in a 7.2g
sample of carbon?

Homework
• New Text: Pg 277 # 1, 3-12
• Old Text: Pg 172-177 # 13, 5, 8-13 (“practice”, not
“section” questions)
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