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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?
€
n =m
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
• Example: how many atoms are in a 7.2g sample of carbon?€
m
M= n =
N
AN
Homework• New Text: Pg 277 # 1, 3-12
• Old Text: Pg 172-177 # 1-3, 5, 8-13 (“practice”, not “section” questions)
Mark your Calendar!