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)

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