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