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

2. What is a mole?

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

4. 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

5. 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!

6. 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!

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

8. 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

9. 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

10. 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

11. Molar Mass
What is molar mass?

12. 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

13. 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

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

15. 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

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

17. 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

18. 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

19. 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

20. 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

21. 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

22. 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

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

24. 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

25. Using Molar Mass in Conversions
What is molar mass?

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

27. 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

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

29. 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

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

31. 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

32. 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?

33. 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