1. Section 3.6—Counting Molecules
Objectives
Define a mole and Avogadro’s number
Define and calculate molar mass
Define molecular and atomic mass as mass for 1 mole of particles
Perform calculations between numbers of moles, particles, and masses
So the number of molecules affects pressure of an airbag…how do we “count” molecules?

2. What is a mole? Mole – SI unit for counting
The only acceptable abbreviation for “mole” is “mol”…not “m”!!
You have had experience with counting units before:
Dozen = counting unit that means “12”
A dozen eggs is 12 eggs.
Pair = counting unit that means “2”
A pair of mittens is 2 mittens.

3. The Mole
Since atoms and molecules are extremely small, even if we have a small sample, the number of molecules would be overwhelming.
For example, one drop of water contains about 5 x 1020 molecules.
500,000,000,000,000,000,000 molecules!!
These huge numbers are impractical!
Using the “mole” counting unit, it becomes more manageable. 1 drop of water is about 8 x 10-4 (0.0008) moles.

4. The Mole 1 mole of anything = 6.02  1023 of those things
(This number is called Avogadro’s number)
“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

5. Converting Between Moles & Molecules
Remember: 1 mole = 6.02 x 1023 particles
Use dimensional analysis to go from moles to particles:
3.24 mol Fe x x 1023 atoms =
1 mole
Use dimensional analysis to go from particles to moles:
9.56 atoms Fe x mole =
6.02 x 1023 atoms

6. Practice #1: Molecules & Moles
Example:
How many molecules of water are in 1.25 moles?

7. Practice #1: Molecules & Moles
Example:
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

8. How many moles are equal to 2.8 × 1022 molecules
Practice #2
Example:
How many moles are equal to 2.8 × 1022 molecules

9. How many moles are equal to 2.8 × 1022 molecules
Practice #2
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

10. How is a mole related to mass?
What is molar mass?

11. Molar Mass Molar Mass – The mass for one mole of atoms or molecules.
Unit for molar mass: g/mole or g/mol

12. For Elements Mass of 1 mole = the atomic mass of the element in grams
Recall: Atomic mass is found on the periodic table.
In THIS CLASS, we will round the atomic mass to the nearest 0.1 g

13. Examples Element Mass 1 mole of carbon atoms 12.0 g
1 mole of oxygen atoms
16.0 g
1 mole of hydrogen atoms
1.0 g

14. Practice 1. Determine the molar mass of helium.
Determine the molar mass of lead.
What is the mass of 1 mole of chlorine atoms?

15. Molar Mass for Compounds
Mass of 1 mole =
*SUM of the atomic masses of all
elements in the compound, in grams
*The sum of the atomic masses of all the elements in a compound is also called the formula mass or the gram formula mass.

16. Practice: Calculate the Molar Mass of CaCl21
Count the number of each type of atom
1 Ca and 2 Cl 2
Find the mass of each atom on the periodic table
Ca = and Cl = 35.5 3
Multiple the # of atoms  mass for each atom
1 x 40.1 = 40.1 for Ca x 35.5 = 71.0 for Cl 4
Find the sum of all the masses
= g/mol

17. Find the molar mass for CaBr2
Example 1: Molar Mass
Example:
Find the molar mass for CaBr2

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

19. Find the molar mass for CaBr2
Example 1: Molar Mass 2
Find the molar mass of each atom on the periodic table
Example:
Find the molar mass for CaBr2 Ca 1
40.1 g/mole Br 2
79.9 g/mole

20. Find the molar mass for CaBr2
Example 1: Molar Mass 3
Multiple the # of atoms  molar mass for each atom
Example:
Find the molar mass for CaBr2 Ca 1 
40.1 g/mole =
40.1 g/mole Br 2 
79.9 g/mole =
159.8 g/mole

21. Find the molar mass for CaBr2
Example 1: Molar Mass 4
Find the sum of all the masses
Example:
Find the molar mass for CaBr2 Ca 1 
40.1 g/mole =
40.1 g/mole Br 2 
79.9 g/mole = +
159.8 g/mole
199.9 g/mole
1 mole of CaBr2 molecules would have a mass of g

22. Find the molar mass for Sr(NO3)2
Example 2: Molar Mass
Be sure to distribute the subscript outside the parenthesis to each element inside the parenthesis.
Example:
Find the molar mass for Sr(NO3)2

23. 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 
87.6 g/mole =
87.6 g/mole N 2 
14.0 g/mole =
28.0 g/mole O 6 
16.0 g/mole = +
96.0 g/mole
211.6 g/mole
1 mole of Sr(NO3)2 molecules would have a mass of g

24. Find the molar mass for Al(OH)3
Let’s Practice #1
Example:
Find the molar mass for Al(OH)3

25. Find the molar mass for Al(OH)3
Let’s Practice #1
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 
27.0 g/mole =
27.0 g/mole O 3 
16.0 g/mole =
48.0 g/mole H 3 
1.0 g/mole = +
3.0 g/mole
78.0 g/mole
1 mole of Al(OH)3 molecules would have a mass of 78.0 g

26. Conversions Between Moles & Mass
What is molar mass?

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

28. Example: Moles to Grams
First determine the given quantity and the unknown one!
Next determine the molar mass for the compound.
Example:
How many grams are in 1.25 moles of water? H O 2 1
1.0 g/mole
16.0 g/mole  =
2.0 g/mole +
18.0 g/mole
Finally, use dimensional analysis to get the answer!
1.25 mol H2O
18.0
g H2O
= _______ g H2O
22.5 1
mol H2O

29. Example: Grams to Moles
Determine the number of moles in 58.6 g of Li(OH)2
58.6 g Li(OH) = ___ mol
Li x = 6.9 g/mol
O 2 x = 32.0 g/mol
H 2 x = 2.0 g/mol
40.9 g/mol
g Li(OH)2 x 1 mole = mol
40.9 g

30. Practice Determine the number of moles in 28.4 g of AgNO3.
2. What is the mass of 3.6 mol ZnO?
3. Determine the number of moles in 15.7 g of CO2.
4. What is the mass of 89.0 g of N2?

31. Converting From Particles to Mass
We have learned how to convert from the number of particles to moles and vice versa. We use 1 mol = 6.02 x 1023 as the conversion factor.
We have learned how to convert from number of moles to a mass and vice versa. We use
1 mol = molar mass (g) as the conversion factor.

32. Converting From Particles to Mass
Number of Particles
Mass
1 mol =
6.02 x 1023
particles
1 mol = molar mass (g/mol)
Mole
We can convert from number of particles
to mass in 2 separate steps.

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

34. Example: Grams to Molecules
How many molecules are in
25.5 g NaCl? Na Cl 1
23.0 g/mole
35.5 g/mole  = +
58.5 g/mole
1 mol = 6.021023 molecules
25.5 g NaCl 1
mol NaCl
58.5
g NaCl
= _________ molecules NaCl

35. Example: Grams to Molecules
How many molecules are in
25.5 g NaCl? Na Cl 1
23.0 g/mole
35.5 g/mole  = +
58.5 g/mole
1 mol = 6.021023 molecules
molecules NaCl
0.436 mol NaCl x
6.021023 1
mol NaCl
= _________ molecules NaCl
2.62  1023

36. Example: Molecules to Grams
How many grams is a sample of 2.75 × 1024 molecules of SrCl2?

37. 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
87.6 g/mole
35.5 g/mole  =
71.0 g/mole +
158.6 g/mole
1 mol = 6.021023 molecules
2.75 × 1024 molecules
SrCl2 1
mol SrCl2
6.02 × 1023
molecules
SrCl2
= _____ g SrCl2

38. 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
87.6 g/mole
35.5 g/mole  =
71.0 g/mole +
158.6 g/mole
1 mol = 6.021023 molecules
4.57
mol SrCl2
158.6
g SrCl2 1
mol SrCl2
= _________ g SrCl2
724