Introduction to the Mole and Molar Mass
Revised 2/21/10
PurposeAt the completion of this unit students willHave a conceptual understanding of the mole as
the method of “counting” items and finding the mass of items that can’t be seen.
Be able to calculate the number of items (molecules, atoms, ions, and formula units) if given the number of moles.
Be able to calculate the number of moles if given the number of items.
Be able to calculate the mass of a sample if given the number of moles in a sample.
Be able to calculate the number of moles in a sample if given the mass of a sample.
Be able to prepare a sample containing a given number of moles.
Be able to determine the molar mass of a compound.
BackgroundWhen you buy eggs you usually ask for a
_______ eggs.
You know that one dozen of any item is ______.
Dozen
12
Paper
Paper is packaged by a ream.
A ream of paper has 500 sheets.
Why is it useful to use units like a dozen or a ream?
ThinkWhat determines how many items should make
up a particular unit?
If you were asked to design a new unit to count
something, what would you consider when choosing how many items should be included in your new counting unit?
ANALYSIS AND INTERPRETATIONAs you know, a dozen represents 12 items.
I decided to make a new counting unit. I called this unit a DART. A DART has
_____ items.4
1. A DART of oranges will have _____ oranges.
4
2. A DART of pretzels has ____ pretzels.
4
3. A DART of molecules of water has ____ molecules of water.
4
4. A DART of particles has ___ particles.4
5. A DART of atoms of iron has _____ atoms of iron.
4
26Fe
55.85
6. A DART of formula units of salt has _______ formula units of salt.
4
7. How many Hershey’s Kisses are in 2 DARTS? _______8
8. How many caramels are in 10 DARTS? _______40
9. How many Dum Dum Pops are in 400 DARTS? _______
1600
10. How many Starbursts are in 1/2 DART? _____2
11. Write the directions for finding the number of items if given the number of DARTS
12. How many oranges are in 5 DARTS? _______ 20
13. How many apples are in 0.5 DARTS? _______ 2
14. How many pencils are in 0.25 (1/4) DARTS? ___
1
15. How many atoms of silver are in 20 DARTS? _______
80
47Ag
107.9
16. How many molecules of water are in 5 DARTS? _______
20
17. How many DARTS are 16 Hershey’s Kisses? ______4
18. How many DARTS are 100 pretzels? ____
25
19. How many DARTS are 400 Starbursts? ____
100
20. How many DARTS is 1 orange? ________
(Write a fraction or a decimal.)
¼ 0.25
DARTS
DART of molecules of water would be too small to see.
Scientists had to select a bigger unit for counting molecules of substances.
The unit scientists use is called a MOLE.
One MOLE of anything has 602,200,000,000,000,000,000,000 items.
AVOGADRO’S NUMBER
6.022 x 1023 One MOLE of anything has 6.022 x 1023items.
28. How many Hershey’s Kisses make up 1 MOLE?
6.022 x 1023
29. How many caramels make up 10 MOLES?
10 [6.022 x 1023]60.22 x 1023
Or
6.022 x 1024
30. Find the number of Jolly Ranchers in 4 MOLES.
4 [6.022 x 1023]
24.088 x 1023
2.41 x 1024
31. Find the number of atoms of sodium in 2 MOLES.
2[6.022 x 1023 ]
12.044 x 1023
1.20 x 1024
11Na
22.99
32. Find the number of molecules of water in 6 MOLES.
6[6.022 x 1023]
36.113 x 1023
3.61 x 1024
33. Find the number of caramels in 0.5 MOLES.
.5[6.022 x 1023 ]
3.011 x 1023
34. How many moles of caramels is 6.022x1023 of caramels? ___
1
35. How many moles of Starbursts is 6.022x1023 of Starbursts? ___
1
36. How many moles of Gobstoppers is 12.04x1023 of
Gobstoppers? ___ 2
12.04 x 1023 = 26.022 x 1023
37. How many atoms of potassium make up one MOLE?
6.022 x 1023
19K
39.10
38. How many atoms of potassium make up 2 MOLES?
2[6.022 x 1023 ]
12.044 x 1023 1.20 x 1024
19K
39.10
39. How many molecules of water make up 1 MOLE?
6.022 x 1023
40. How many molecules of water make up 5 MOLES?
5[6.022 x 1023 ]
30.110 x 1023
3.01 x 1024
41. How many moles are 6.022 x 1023 atoms of sodium? _____
1
42. How many moles are 12.04 x 1023 atoms of carbon? _____
2
12.04 x 1023 = 26.022 x 1023
43. How many moles are 18.06 x 1023 atoms of sodium? _____
3
18.06 x 1023 = 36.022 x 1023
44. How many moles are 60.22 x 1023 atoms of sodium?__ 10
60.22 x 1023 = 106.022 x 1023
45. How many moles are 6.022 x 1023 molecules of water? _____1
46. How many moles are 12.04 x 1023 molecules of water? _____2
12.04 x 1023 = 26.022 x 1023
47. How many moles are 30.10 x 1023 molecules of water? _____5
30.10 x 1023 = 56.022 x 1023
Part 2
Molar Mass
Scientists use the Periodic Table to determine the mass of a mole of atom of an element.
For example, a mole of Iron (Fe) atoms would have a mass of 55.85g.
Periodic Table
The mass of 1 mole of Strontium (Sr) is ___ g.
38Sr
87.62
87.62
The mass of 1 mole of Lead (Pb) is ____ g.
82Pb
207.2
207.2
#3. The mass of 1 mole of Nickel (Ni) is ____ g.
28Ni
58.69
58.69
The mass of 4 moles of Strontium (Sr) is _____g.
38Sr
87.62350.48
The mass of 0.5 moles of Lead (Pb) is ____ g.
82Pb
207.2
103.6
#4. The mass of 100 moles of (Ni) is _____ g.
28Ni
58.69
5869.00
Molar Mass Scientists also use the Periodic Table to determine the molar mass (formula weight) of compounds.
The molar mass of the compound is the sum of the molar masses of the elements that make up the compound.
Potassium PermanganateThe molar mass of KMnO4 is the
sum of all the molar masses times the number of atoms of each element for the molecule (subscript).
Potassium Permanganate
KMnO4
Potassium (K) 39.10 g/mole
Manganese (Mn) 54.94g/mole
Oxygen (O) 4[16.00 g/mole] 158.04 g/mole.
The molar mass of 1 mole of Sodium Chloride (NaCl) is____ g.
58.44
The molar mass of 1 mole of Lead Iodide (PbI2) is _____ g.461.0
The mass of 100 moles of Nickel Sulfide (NiS) is ________ g. 9076.00
The mass of 1 mole of Sucrose (C12H24O12 ) _______ g.306.31
Mole Conversions
In chemistry, the mole is the standard measurement of amount.
However, balances DO NOT give readings in moles. Balances give readings in grams.
So the problem is that, while we compare amounts of one substance to another using moles, we must also use grams, since this is the information we get from balances.
There are three steps to converting grams of a substance to moles.
1. Determine how many grams are given in the problem.
2. Calculate the molar mass of the substance.
3. Divide step one by step two.
The three steps above can be expressed in the following proportion:
Grams Moles
--------------- = --------------
Molar Mass 1 Mole
Example #1 - Convert 25.0 grams of KMnO4 to moles.Step One: The problem will tell you how many grams are present. Look
for the unit of grams. The problem gives us 25.0 grams.
Step Two: You need to know the molar mass of the substance. The
molar mass of KMnO4 is 158.034 grams/mole.
Potassium (K) = 39.10 x 1 = 39.10 g
Manganese (Mn) = 54.94 x 1 = 54.94 g
Oxygen (O) = 16.00 x 4 = 64.00 g
---------------
158.04 g
Step Three: You divide the grams given by the substance's molar mass (25.0/158.04)
The answer of 0.16 mole has been rounded off.
Example #2 - Calculate how many moles are in 57.0 grams of Mg(NO3)2
Step One: 57.0 grams are given in the text of the problem.
Step Two: The molar mass is 148 grams/mole.
Mg (Magnesium) = 24 x 1 = 24 g
N (Nitrogen) = 14 x 2 = 28 g
O (Oxygen) = 16 x 6 = 96 g
Step Three: Again you divide the grams by the substances molar mass (57.0 g/148 g).
This answer has been rounded to 0.39 moles.
Practice Problems
1. Calculate the moles present in:2.00 grams of H2O
.11 moles water
2. Calculate the moles present in:75.57 grams of KBr
.635 moles KBr
3. Calculate the moles present in:100.0 grams of KClO4
.72 moles KClO4
4. Calculate the moles present in:225.5 grams of Sucrose (C12H24O12)
.63 moles Sucrose
5. Calculate the moles present in:
350.0 grams of Glucose (C6H12O6)
1.943 moles Glucose
Homework
1. Calculate the moles present in:3.00 grams of NaCl
.05 moles
2. Calculate the moles present in:25.0 grams of NH3
1.47 moles NH3
3. Calculate the moles present in:16.0 grams of KCl
.21 moles KCl
4. Calculate the moles present in:30.25 grams of BeCl2
.38 moles KCl
5. Calculate the moles present in:175.25 grams of Li2S
3.81 moles Li2S
6. Calculate the moles present in:75.62 grams of CO2
1.72 moles CO2
7. Calculate the moles present in:56.5 grams of H2O
3.14 moles H2O
8. Calculate the moles present in:
22.6 grams of CH3COOH
.38 moles CH3COOH
9. Calculate the moles present in:18.4 grams of AgCl
.13 moles AgCl
10. Calculate the
moles present in: 68.3 grams of HCN
2.53 moles HCN