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Section 3.6—Counting Molecules So the number of molecules affects pressure of an airbag…how do we “count” molecules?

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What is a mole?

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Definition Mole – SI unit for counting The only acceptable abbreviation for “mole” is “mol”…not “m”!!

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What is a counting unit? You’re already familiar with one counting unit…a “dozen” “Dozen”12 A dozen doughnuts 12 doughnuts A dozen books A dozen cars A dozen people 12 books 12 cars 12 people A dozen = 12

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Why can’t we count atoms in “dozens”? Atoms and molecules are extremely small There are 6.02 water molecules in 18mL of water This means a 12 ounce bottle of water (355 mL) would have 1.19 molecules of water. That would be 9.89 “dozen” water molecules. These huge numbers are impractical! mL 355 mL molecules H 2 O 6.02 = _________ molecules H 2 O 1.19 molecules 1.19 molecules dozen 1 12 = _________ dozen 9.89 10 23

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What does a “mole” count in? A mole = 6.02 (called Avogadro’s number) “mole” 6.02 mole of doughnuts 6.02 doughnuts 1 mole of atoms 1 mole of molecules 6.02 atoms 6.02 molecules 6.02 = 602,000,000,000,000,000,000,000 This means a 12 ounce bottle of water would have 19.7 “moles” of water…a much easier-to-work-with number!

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Example: Molecules & Moles Example: How many molecules of water are in 1.25 moles?

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= _______ molecules H 2 O Example: Molecules & Moles 1.25 mol H 2 O mol H 2 O Molecules H 2 O 6.02 mol = 6.02 molecules Example: How many molecules of water are in 1.25 moles?

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Let’s Practice #1 Example: How many moles are equal to 2.8 × molecules

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= _______ moles Let’s Practice #1 2.8 × molecules molecules mole mol = 6.02 molecules Example: How many moles are equal to 2.8 × molecules

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Molar Mass

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Definition Molar Mass – The mass for one mole of an atom or molecule. Other terms commonly used for the same meaning: Molecular Weight Molecular Mass Formula Weight Formula Mass

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Mass for 1 mole of atoms The average atomic mass = grams for 1 mole ElementMass 1 mole of carbon atoms g 1 mole of oxygen atoms 1 mole of hydrogen atoms g 1.01 g Unit for molar mass: g/mole or g/mol Average atomic mass is found on the periodic table

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Molar mass for molecules The molar mass for a molecule = the sum of the molar masses of all the atoms

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Calculating a Molecule’s Mass Count the number of each type of atom Find the molar mass of each atom on the periodic table Multiple the # of atoms molar mass for each atom Find the sum of all the masses To find the molar mass of a molecule:

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Example: Molar Mass Example: Find the molar mass for CaBr 2

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Example: Molar Mass Count the number of each type of atom 1 Ca Br 1 2 Example: Find the molar mass for CaBr 2

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Example: Molar Mass Find the molar mass of each atom on the periodic table 2 Ca Br g/mole g/mole Example: Find the molar mass for CaBr 2

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Example: Molar Mass Multiple the # of atoms molar mass for each atom 3 Ca Br g/mole g/mole Example: Find the molar mass for CaBr 2 = g/mole = g/mole

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Example: Molar Mass Find the sum of all the masses 4 Ca Br g/mole g/mole = g/mole = g/mole g/mole 1 mole of CaBr 2 molecules would have a mass of g Example: Find the molar mass for CaBr 2

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Example: Molar Mass & Parenthesis Be sure to distribute the subscript outside the parenthesis to each element inside the parenthesis. Example: Find the molar mass for Sr(NO 3 ) 2

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Example: Molar Mass & Parenthesis Be sure to distribute the subscript outside the parenthesis to each element inside the parenthesis g/mole g/mole = g/mole = g/mole g/mole 1 mole of Sr(NO 3 ) 2 molecules would have a mass of g g/mole = g/mole Sr N O Example: Find the molar mass for Sr(NO 3 ) 2

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Let’s Practice #2 Example: Find the molar mass for Al(OH) 3

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Let’s Practice #2 Be sure to distribute the subscript outside the parenthesis to each element inside the parenthesis g/mole 1.01 g/mole = g/mole = 3.03 g/mole g/mole 1 mole of Al(OH) 3 molecules would have a mass of g g/mole = g/mole Al O H Example: Find the molar mass for Al(OH) 3

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Using Molar Mass in Conversions

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Example: Moles to Grams Example: How many grams are in 1.25 moles of water?

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Example: Moles to Grams 1.25 mol H 2 O = _______ g H 2 O mol H 2 O g H 2 O When converting between grams and moles, the molar mass is needed 1 mole H 2 O molecules = g H O g/mole g/mole = 2.02 g/mole = g/mole g/mole Example: How many grams are in 1.25 moles of water?

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Example: Grams to Molecules Example: How many molecules are in 25.5 g NaCl?

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25.5 g NaCl Example: Grams to Molecules = _________ molecules NaCl g NaCl mol NaCl mol = 6.02 molecules 1 moles NaCl molecules = g Na Cl g/mole g/mole = g/mole = g/mole g/mole mol NaCl molecules NaCl 6.02 Example: How many molecules are in 25.5 g NaCl?

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Let’s Practice #3 Example: How many moles are in 25.5 g NaCl?

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Let’s Practice # g NaCl = _______ mole NaCl g NaCl mole NaCl mole NaCl molecules = g Na Cl g/mole g/mole = g/mole = g/mole g/mole Example: How many moles are in 25.5 g NaCl?

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Let’s Practice #4 Example: How many grams is a sample of 2.75 × molecules of SrCl 2 ?

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2.75 × molecules SrCl 2 Let’s Practice #4 = _________ g SrCl 2 molecules SrCl 2 mol SrCl × mol = 6.02 molecules 1 moles SrCl 2 molecules = g Sr Cl g/mole g/mole = g/mole = g/mole g/mole mol SrCl 2 g SrCl Example: How many grams is a sample of 2.75 × molecules of SrCl 2 ?

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