Download - Section 3.6—Counting Molecules
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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 1023 water molecules in 18mL of water
This means a 12 ounce bottle of water (355 mL) would have 1.19 1025 molecules of water.
That would be 9.89 1023 “dozen” water molecules.
These huge numbers are impractical!
mL355
mL
molecules H2O6.021023
18= _________ molecules H2O1.19 1025
molecules1.19 1025
molecules
dozen1
12= _________ dozen9.89 1023
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What does a “mole” count in?
A mole = 6.02 1023 (called Avogadro’s number)
“mole” 6.02 1023
1 mole of doughnuts 6.02 1023 doughnuts
1 mole of atoms
1 mole of molecules
6.02 1023 atoms
6.02 1023 molecules
6.02 1023 = 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 H2O
Example: Molecules & Moles
1.25 mol H2O
mol H2O
Molecules H2O
6.02 1023
17.521023
1 mol = 6.021023 moleculesExample: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 × 1022 molecules
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= _______ moles
Let’s Practice #1
2.8 × 1022 molecules
molecules
mole1
6.02 1023
0.047
1 mol = 6.021023 moleculesExample:How many moles are equal to 2.8 × 1022 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 WeightMolecular MassFormula WeightFormula Mass
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Mass for 1 mole of atoms
The average atomic mass = grams for 1 mole
Element Mass
1 mole of carbon atoms 12.01 g
1 mole of oxygen atoms
1 mole of hydrogen atoms
16.00 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
1
2
3
4
To find the molar mass of a molecule:
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Example: Molar Mass
Example:Find the
molar mass for
CaBr2
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Example: Molar Mass
Count the number of each type of atom1
Ca
Br
1
2
Example:Find the
molar mass for
CaBr2
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Example: Molar Mass
Find the molar mass of each atom on the periodic table2
Ca
Br
1
2
40.08 g/mole
79.91 g/mole
Example:Find the
molar mass for
CaBr2
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Example: Molar Mass
Multiple the # of atoms molar mass for each atom3
Ca
Br
1
2
40.08 g/mole
79.91 g/mole
Example:Find the
molar mass for
CaBr2
= 40.08 g/mole
= 159.82 g/mole
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Example: Molar Mass
Find the sum of all the masses4
Ca
Br
1
2
40.08 g/mole
79.91 g/mole
= 40.08 g/mole
= 159.82 g/mole+
199.90 g/mole
1 mole of CaBr2 molecules would have a mass of 199.90 g
Example:Find the
molar mass for
CaBr2
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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(NO3)2
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Example: Molar Mass & Parenthesis
Be sure to distribute the subscript outside the parenthesis to each element inside the parenthesis.
1
6
87.62 g/mole
16.00 g/mole
= 87.62 g/mole
= 96.00 g/mole+
211.64 g/mole
1 mole of Sr(NO3)2 molecules would have a mass of 211.64 g
2 14.01 g/mole = 28.02 g/mole
Sr
N
O
Example:Find the
molar mass for Sr(NO3)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.
1
3
26.98 g/mole
1.01 g/mole
= 26.98 g/mole
= 3.03 g/mole+
78.01 g/mole
1 mole of Al(OH)3 molecules would have a mass of 78.01 g
3 16.00 g/mole = 48.00 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 H2O = _______ g H2Omol H2O
g H2O18.02
1
22.53
When converting between grams and moles, the molar mass is needed
1 mole H2O molecules = 18.02 g
HO
21
1.01 g/mole16.00 g/mole
= 2.02 g/mole= 16.00 g/mole+
18.02 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 NaCl1
58.44
2.63 1023
1 mol = 6.021023 molecules
1 moles NaCl molecules = 58.44 g
NaCl
11
22.99 g/mole35.45 g/mole
= 22.99 g/mole= 35.45 g/mole+
58.44 g/mole
mol NaCl
molecules NaCl6.021023
1
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 #3
25.5 g NaCl = _______ mole NaClg NaCl
mole NaCl1
58.44
0.44
1 mole NaCl molecules = 58.44 g
NaCl
11
22.99 g/mole35.45 g/mole
= 22.99 g/mole= 35.45 g/mole+
58.44 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 × 1024
molecules of SrCl2?
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2.75 × 1024 molecules SrCl2
Let’s Practice #4
= _________ g SrCl2
molecules SrCl2
mol SrCl21
6.02 × 1023
7.24
1 mol = 6.021023 molecules
1 moles SrCl2 molecules = 158.52 g
SrCl
12
87.62 g/mole35.45 g/mole
= 87.62 g/mole= 70.90 g/mole+
158.52 g/mole
mol SrCl2
g SrCl2158.52
1
Example:How many grams is a sample of 2.75 × 1024
molecules of SrCl2?