introduction to the mole and molar mass

Introduction to the Mole and Molar

Mass

PurposeAt the completion of this unit students will• Have 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.

Background

• When 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?

• What 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 INTERPRETATION• As you know, a dozen represents 12 items. • What if I decided to make a new counting

unit? I called this unit a DART. Each dart is

• _____ items.

1 Dart = 4 items, just like1 Dozen = 12 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

15. How many atoms of silver are in 20 DARTS? _______80

47Ag

107.9

Write the directions for finding the number of items if given the number of DARTS

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

21. How many DARTS are 2 caramels? _____0.5

23. How many DARTS are 48 Hershey’s Kisses? ____12

25. How many DARTS are 24 Jolly Ranchers? ______6

27. How many DARTS are 20 caramels? ______5

Write your own directions for finding the number of DARTS given the number of pieces

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

•6.02 x 1023 • One MOLE of anything

has 6.02 x 1023 items.

A. What is the Mole?• A counting number (like a dozen)

• Avogadro’s number (NA)

• 1 mol = 6.02 1023 items

A large amount!!!!

1 mole of hockey pucks would equal the mass of the moon!

A. What is the Mole?

• 1 mole of pennies would cover the Earth 1/4 mile deep!

1 mole of basketballs would fill a bag the size of the earth!

28. How many Hershey’s Kisses make up 1 MOLE? 6.02 x 1023

A mole of Caramels is _____ caramels.6.02 x 1023

A mole of Jolly Ranchers is ______Jolly Ranchers

6.02 x 1023

A mole of silver atoms is _______ atoms of silver.

47Ag

107.9

6.02 x 1023

29. How many caramels make up 10 MOLES?

10 [6.02 x 1023]

60.2 x 1023

Which is

6.02 x 1024

30. Find the number of Jolly Ranchers in 4 MOLES.

4 [6.02 x 1023] =

2.408 x 1024

31. Find the number of atoms of sodium in 2 MOLES.

2[6.02 x 1023]=

1.20 x 1024

11Na

22.99

32. Find the number of molecules of water in 6 MOLES.

6[6.02 x 1023]=

3.61 x 1024

33. Find the number of caramels in 0.5 MOLES.

.5[6.02 x 1023 ]=

3.011 x 1023

34. How many moles of caramels is 6.02x1023 of caramels? ___1

35. How many moles of Starbursts is 6.02x1023 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? 6.02 x 1023

19K

39.10

38. How many atoms of potassium make up 2 MOLES?

2[6.02 x 1023 ]

12.044 x 1023

1.20 x 1024

19K

39.10

39. How many molecules of water make up 1 MOLE?

6.02 x 1023

40. How many molecules of water make up 5 MOLES?

5[6.02 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


18.06 x 1023 = 36.022 x 1023


60.22 x 1023 = 106.022 x 1023

45. How many moles are 6.02 x 1023 molecules of water? _____1


12.04 x 1023 = 26.022 x 1023


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 carbon (C) atoms would have a mass of 12.01g.

Periodic Table

1 mole carbon atoms = __________ atoms = __________ g..

6C

12.01

12.01

6.02 x 1023

1 mole helium atoms = __________ atoms = _______ g..

2He

4.00

4.00

6.02 x 1023

1 mole iron atoms = __________ atoms = _______ g..

26Fe

55.85

55.85

6.02 x 1023

1 mole boron atoms = __________ atoms = _______ g..

5B

10.81

10.81

6.02 x 1023

1 mole He atoms is 4.00 g, so

2 mole He atoms = ______ g.

2He

4.00

8.00

1 mole Fe atoms is 55.85 g, so

0.5 mole Fe atoms = ______ g.

26Fe

55.85

27.93

1 mole C atoms is 12.01 g, so

12 mole C atoms = ______ g

6C

12.01

144.12

1 mole B atoms is 10.81 g, so

3 mole B atoms = ______ g

5B

10.81

32.43

120.1 g C atoms = ______ mol

6C

12.01

10

120.1 ÷ 12.01 = 10

8.00 g He atoms = ______ mol

2He

4.00

2

8.00 ÷ 4.00 = 2

167.55 g Fe atoms = ______ mol

26Fe

55.85

3

167.55 ÷ 55.85 = 3

5.405 g B atoms = ______ mol

5B

10.81

0.5

5.405 ÷ 10.81 = 0.5

66. The mass of 1 mole of Nickel (Ni) is ____ g.

28Ni

58.69

58.69

67. The mass of 4 moles of Strontium (Sr) is _____g.

38Sr

87.62

350.48

68. The mass of 0.5 moles of Lead (Pb) is ____ g.

82Pb

207.2

103.6

69. 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 Chloride

• The molar mass of Potassium Chloride is:

Potassium (K) 39.10 g/mole

Chloride (Cl) +34.45 g/mole

73.55 g/mole

Potassium Permanganate

• The 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.

70. The molar mass of 1 mole of Sodium Chloride (NaCl) is____ g.58.44

71. The molar mass of 1 mole of Lead Iodide (PbI2) is _____ g.461.0

72. The mass of 100 moles of Nickel Sulfide (NiS) is ________ g.9076.00

73. The mass of 1 mole of is Glucose (C6H12O6) is ______ g.180.156

74. The mass of 1 mole of is Sucrose (C12H24O12 ) _______ g.306.31

75. The mass of 4 moles of Glucose (C6H12 O6) is _________ g.720.624

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 gManganese (Mn) = 54.94 x 1 = 54.94 gOxygen (O) = 16.00 x 4 = 64.00 g --------------- 158.04 gStep 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 gN (Nitrogen) = 14 x 2= 28 gO (Oxygen) = 16 x 6= 96 gStep 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


75.57 grams of KBr

.635 moles KBr


100.0 grams of KClO4

.72 moles KClO4


225.5 grams of Sucrose (C12H24O12)

.63 moles Sucrose


350.0 grams of Glucose (C6H12O6)

1.943 moles Glucose

Homework


3.00 grams of NaCl

.05 moles


25.0 grams of NH3

1.47 moles NH3


16.0 grams of KCl

.21 moles KCl


30.25 grams of BeCl2

.38 moles KCl


175.25 grams of Li2S

3.81 moles Li2S


75.62 grams of CO2

1.72 moles CO2


56.5 grams of H2O

3.14 moles H2O


22.6 grams of CH3COOH

.38 moles CH3COOH


18.4 grams of AgCl

.13 moles AgCl

10. Calculate the moles present

in: 68.3 grams of HCN

2.53 moles HCN

introduction to the mole and molar mass

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