# 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?

What is a mole?

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

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

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 mL 355 molecules H2O 6.021023 18 = _________ molecules H2O 1.19  1025 This means a 12 ounce bottle of water (355 mL) would have 1.19  1025 molecules of water. molecules 1.19  1025 dozen 1 12 = _________ dozen 9.89  1023 That would be 9.89  1023 “dozen” water molecules. These huge numbers are impractical!

What does a “mole” count in?
A mole = 6.02  1023 (called Avogadro’s number) 6.02  1023 = 602,000,000,000,000,000,000,000 “mole” 6.02  1023 1 mole of doughnuts 6.02  1023 doughnuts 1 mole of atoms 6.02  1023 atoms 1 mole of molecules 6.02  1023 molecules This means a 12 ounce bottle of water would have 19.7 “moles” of water…a much easier-to-work-with number!

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

Example: Molecules & Moles
How many molecules of water are in 1.25 moles? 1 mol = 6.021023 molecules 1.25 mol H2O Molecules H2O 6.02  1023 = _______ molecules H2O 7.521023 1 mol H2O

How many moles are equal to 2.8 × 1022 molecules
Let’s Practice #1 Example: How many moles are equal to 2.8 × 1022 molecules

How many moles are equal to 2.8 × 1022 molecules
Let’s Practice #1 Example: How many moles are equal to 2.8 × 1022 molecules 1 mol = 6.021023 molecules 2.8 × 1022 molecules 1 mole = _______ moles 0.047 6.02  1023 molecules

Molar Mass What is molar mass?

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

Mass for 1 mole of atoms The average atomic mass = grams for 1 mole
Average atomic mass is found on the periodic table Element Mass 1 mole of carbon atoms 12.01 g 1 mole of oxygen atoms 16.00 g 1 mole of hydrogen atoms 1.01 g Unit for molar mass: g/mole or g/mol

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

Calculating a Molecule’s Mass
To find the molar mass of a molecule: 1 Count the number of each type of atom 2 Find the molar mass of each atom on the periodic table 3 Multiple the # of atoms  molar mass for each atom 4 Find the sum of all the masses

Find the molar mass for CaBr2
Example: Molar Mass Example: Find the molar mass for CaBr2

Find the molar mass for CaBr2
Example: Molar Mass 1 Count the number of each type of atom Example: Find the molar mass for CaBr2 Ca 1 Br 2

Find the molar mass for CaBr2
Example: Molar Mass 2 Find the molar mass of each atom on the periodic table Example: Find the molar mass for CaBr2 Ca 1 40.08 g/mole Br 2 79.91 g/mole

Find the molar mass for CaBr2
Example: Molar Mass 3 Multiple the # of atoms  molar mass for each atom Example: Find the molar mass for CaBr2 Ca 1 40.08 g/mole = 40.08 g/mole Br 2 79.91 g/mole = g/mole

Find the molar mass for CaBr2
Example: Molar Mass 4 Find the sum of all the masses Example: Find the molar mass for CaBr2 Ca 1 40.08 g/mole = 40.08 g/mole Br 2 79.91 g/mole = + g/mole g/mole 1 mole of CaBr2 molecules would have a mass of g

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

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 Sr 1 87.62 g/mole = 87.62 g/mole N 2 14.01 g/mole = 28.02 g/mole O 6 16.00 g/mole = + 96.00 g/mole g/mole 1 mole of Sr(NO3)2 molecules would have a mass of g

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

Find the molar mass for Al(OH)3
Let’s Practice #2 Be sure to distribute the subscript outside the parenthesis to each element inside the parenthesis. Example: Find the molar mass for Al(OH)3 Al 1 26.98 g/mole = 26.98 g/mole O 3 16.00 g/mole = 48.00 g/mole H 3 1.01 g/mole = + 3.03 g/mole 78.01 g/mole 1 mole of Al(OH)3 molecules would have a mass of g

Using Molar Mass in Conversions
What is molar mass?

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

Example: Moles to Grams
When converting between grams and moles, the molar mass is needed Example: How many grams are in 1.25 moles of water? H O 2 1 1.01 g/mole 16.00 g/mole = 2.02 g/mole + 18.02 g/mole 1 mole H2O molecules = g 1.25 mol H2O 18.02 g H2O = _______ g H2O 22.53 1 mol H2O

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

Example: Grams to Molecules
How many molecules are in 25.5 g NaCl? Na Cl 1 22.99 g/mole 35.45 g/mole = + 58.44 g/mole 1 moles NaCl molecules = g 1 mol = 6.021023 molecules 25.5 g NaCl 1 mol NaCl 6.021023 molecules NaCl 58.44 g NaCl 1 mol NaCl = _________ molecules NaCl 2.63  1023

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

How many moles are in 25.5 g NaCl?
Let’s Practice #3 Example: How many moles are in 25.5 g NaCl? Na Cl 1 22.99 g/mole 35.45 g/mole = + 58.44 g/mole 1 mole NaCl molecules = g 25.5 g NaCl 1 mole NaCl = _______ mole NaCl 0.44 58.44 g NaCl

How many grams is a sample of 2.75 × 1024 molecules of SrCl2?
Let’s Practice #4 Example: How many grams is a sample of 2.75 × 1024 molecules of SrCl2?

How many grams is a sample of 2.75 × 1024 molecules of SrCl2?
Let’s Practice #4 Example: How many grams is a sample of 2.75 × 1024 molecules of SrCl2? Sr Cl 1 2 87.62 g/mole 35.45 g/mole = 70.90 g/mole + g/mole 1 moles SrCl2 molecules = g 1 mol = 6.021023 molecules 2.75 × 1024 molecules SrCl2 1 mol SrCl2 158.52 g SrCl2 6.02 × 1023 molecules SrCl2 1 mol SrCl2 = _________ g SrCl2 7.24