Introduction to the Mole and Molar Mass Revised 2/21/10

Purpose At 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?

Think  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.  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 Fe 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? _______ Ag 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 602,200,000,000,000,000,000,0 00 items.

AVOGADRO’S NUMBER  x  One MOLE of anything has  x items.

28. How many Hershey’s Kisses make up 1 MOLE ? x 10 23

29. How many caramels make up 10 MOLES ? 10 [ x ] x Or x 10 24

30. Find the number of Jolly Ranchers in 4 MOLES. 4 [ x ] x x 10 24

31. Find the number of atoms of sodium in 2 MOLES. 2[6.022 x ] x x Na 22.99

32. Find the number of molecules of water in 6 MOLES. 6[6.022 x ] x x 10 24

33. Find the number of caramels in 0.5 MOLES..5[6.022 x ] x 10 23

34. How many moles of caramels is 6.022x10 23 of caramels? ___ 1

35. How many moles of Starbursts is 6.022x10 23 of Starbursts? ___ 1

36. How many moles of Gobstoppers is 12.04x10 23 of Gobstoppers? ___ x = x 10 23

37. How many atoms of potassium make up one MOLE? x K 39.10

38. How many atoms of potassium make up 2 MOLES? 2[6.022 x ] x x K 39.10

39. How many molecules of water make up 1 MOLE? x 10 23

40. How many molecules of water make up 5 MOLES? 5[6.022 x ] x x 10 24

41. How many moles are x atoms of sodium? _____ 1

42. How many moles are x atoms of carbon? _____ x = x 10 23

43. How many moles are x atoms of sodium? _____ x = x 10 23

44. How many moles are x atoms of sodium?__ x = x 10 23

45. How many moles are x molecules of water? _____ 1

46. How many moles are x molecules of water? _____ x = x 10 23

47. How many moles are x molecules of water? _____ x = x 10 23

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. 38 Sr 87.62

The mass of 1 mole of Lead (Pb) is ____ g. 82 Pb 207.2

#3. The mass of 1 mole of Nickel (Ni) is ____ g. 28 Ni 58.69

The mass of 4 moles of Strontium (Sr) is _____g. 38 Sr

The mass of 0.5 moles of Lead (Pb) is ____ g. 82 Pb

#4. The mass of 100 moles of (Ni) is _____ g. 28 Ni

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 Permanganate  The molar mass of KMnO 4 is the sum of all the molar masses times the number of atoms of each element for the molecule (subscript).

Potassium Permanganate KMnO 4  Potassium (K) g/mole  Manganese (Mn) 54.94g/mole  Oxygen (O) 4[16.00 g/mole] g/mole.

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

The molar mass of 1 mole of Lead Iodide (PbI 2 ) is _____ g

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

The mass of 1 mole of Sucrose (C 12 H 24 O 12 ) _______ g

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 KMnO 4 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 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 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(NO 3 ) 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 H 2 O.11 moles water

2. Calculate the moles present in: grams of KBr.635 moles KBr

3. Calculate the moles present in: grams of KClO 4.72 moles KClO 4

4. Calculate the moles present in: grams of Sucrose (C 12 H 24 O 12 ).63 moles Sucrose

5. Calculate the moles present in: grams of Glucose (C 6 H 12 O 6 ) 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 NH moles NH 3

3. Calculate the moles present in: 16.0 grams of KCl.21 moles KCl

4. Calculate the moles present in: grams of BeCl 2.38 moles KCl

5. Calculate the moles present in: grams of Li 2 S 3.81 moles Li 2 S

6. Calculate the moles present in: grams of CO moles CO 2

7. Calculate the moles present in: 56.5 grams of H 2 O 3.14 moles H 2 O

8. Calculate the moles present in: 22.6 grams of CH 3 COOH.38 moles CH 3 COOH

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