1. How do we use the mole?
Chemistry Unit 9

2. Main Ideas
Chemists use the mole to count atoms, molecules, ions and formula units.
A mole always contains the same number of particles, however, moles of different substances have different masses.
The molar mass of a compound can be calculated from its chemical formula and can be used to convert from mass to moles of that compound.
A molecular formula of a compound is a whole- number multiple of its empirical formula

3. Measuring matter

4. 9.1 Measuring matter: Objectives
Explain how a mole is used to indirectly count the number of particles of matter.
Relate the mole to a common everyday counting unit.
Convert between moles and number of representative particles.

5. Mole
The mole is the SI base unit for measure of amount of a substance: x 1023
The number of carbon atoms in exactly 12 g of pure carbon-12.
Called Avogadro’s number – Italian physicist who in 1811, determined the volume of 1 mol of gas.
By mass, we can determine the number of particles (atoms, molecules) in a sample.
We typically round to 3 sig figs – 6.02 x 1023

6. The mole: A good comparison
The mole is a number. What other unit is used in a similar manner?
A dozen flowers, doughnuts or eggs.
A baker’s dozen of cookies or bagels.
A pair of socks or friends
If you have a dozen flowers and a dozen eggs, do they weigh the same?

7. The mole as a Conversion factor
In order to convert between moles and number of particles we need to use this ratio of equivalent values (conversion factor) to express the same quantity in different units.

8. Example Problems 1&2
How many particles are in 3.5 mols? How many moles of atoms are in 9.63 x 1026 atoms?

9. Question? What does the mole measure? A. mass of a substance
B. amount of a substance
C. volume of a gas
D. density of a gas

10. Question?
What is the conversion factor for determining the number of moles of a substance from a known number of particles? A. B.
C. 1 particle  6.02  1023
D. 1 mol  6.02  1023 particles

11. Practice problems
Page 322 #1-4; page 324 #5-14

12. mass and the mole

13. Mass and the Mole: objectives
Relate the mass of an atom to the mass of a mole of atoms.
Convert between number of moles and the mass of an element.
Convert between number of moles and number of atoms of an element.

14. Molar mass
Molar mass is the mass in grams of one mole of any pure substance.
Units are given in g/mol
Mass of the periodic table is given in amu, but also g/mol

15. Example Problems 3&4
If I need 3 mols of Cu, how do I measure the amount?
I measured 5.0g of Iron, how many atoms do I have?

16. Conversions

17. Example Problems 5&6
How many atoms of gold are in a U.S. Eagle bullion coin with a mass of 31.1g? How much does 5.8 x 1015 atoms of lead weigh?

18. question The mass in grams of 1 mol of any pure substance is:
A. molar mass
B. Avogadro’s number
C. atomic mass
D. 1 g/mol

19. question Molar mass is used to convert what? A. mass to moles
B. moles to mass
C. atomic weight
D. particles

20. Practice problems Page 328 #15-16; page 329 #17-18

21. moles of compounds

22. Moles of Compounds: Objectives
Recognize the mole relationships shown by a chemical formula
Calculate the molar mass of a compound.
Convert between number of moles and mass of a compound.
Apply conversion factors to determine the number of atoms or ions in a known mass of a compound

23. Calculate molar mass Steps to calculate molar mass:
Count the number of atoms in each molecule.
Find the molar mass of each atom.
Multiply the molar mass of each atom to the number of atoms in a compound.
Add the total molar masses together.

24. Example Problem 7
Find the molar mass of the following compounds/molecules.
H2O
NaCl
H2SO4

25. Example Problem 7 cont..
Al2O3
Fe2(SO4)3
CCl2F2

26. Number of Atoms
To determine the number of atoms or ions in a known mass of a compound
Find the molar mass of the compound.
Use molar mass and the mole as conversion factors to get the units needed.

27. Example Problem 8
What is the mass of 2.5 mols of (C3H5)2 S?

28. example problem 9
Calculate the number of moles of Ca(OH)2 in 325g of the compound?

29. example problem 10
How many atoms are in 212g of water?

30. question How many moles of OH— ions are in 2.50 moles of Ca(OH)2?
B. 2.50
C. 4.00
D. 5.00

31. question How many particles of Mg are in 10 moles of MgBr2?
A x 1023
B x 1024
C x 1024
D x 1025

32. Practice problems
Page 335 #29-36; page 336 #37-41; page 339 #42-46

33. empirical and molecular formulas

34. Empirical and Molecular formulas: Objectives
Explain what is meant by the percent composition of a compound.
Determine the empirical and molecular formulas for a compound from percent and actual mass data.
Explain what a hydrate is and relate the name of the hydrate to its composition.
Determine the formula of a hydrate from laboratory data.

35. Percent composition
The percent composition is a percent by mass of each element in a compound.
Steps to determine percent composition of a compound:
Assume 1 mole of a compound.
Calculate molar mass of each element in the compound.
Use each element’s molar mass to calculate percent by mass.

36. Percent by mass
Percent by mass is a description of the amount of an element in a compound.
Percent by mass =

37. Example problem 11
What is the percent by mass of each element in NaHCO3?

38. Empirical formula
The empirical formula is the smallest whole number ratio of elements in a compound
This ratio provides the subscripts for the elements.
May or may not be the same as the actual molecular formula.
If they are different the molecular formula will be a simple multiple of the empirical formula.
Hydrogen peroxide: HO- empirical formula
H2O2 – actual formula (molecular formula)

39. Empirical formula
Steps to figure empirical formula from percent composition:
Assume an overall 100g sample of the compound.
Each element’s percentage can be used as mass in calculations.
Use this ‘mass’ to convert to moles. This provides a ‘mole ratio’ for the compound.

40. Empirical formula
Since these mole ratios are not whole numbers, we convert them to whole numbers what can be used as subscripts by dividing them all by the smallest ratio. (We assume the smallest mole ratio is a 1 in the compound)

41. example Problem 12
A compound has the following mass percentages: C – 48.64%, H – 8.16%, O – 43.20% What is the empirical formula for this molecule?

42. Molecular formula
The molecular formula specifies the actual number of atoms of each element in one molecule/formula unit of the substance.

43. Molecular formula Steps to determine the molecular formula:
Determine the molar mass of the empirical formula.
Determine the molar mass of the actual compound. (might be given to you)
Divide the molar mass of the actual compound by the molar mass of the empirical formula
Multiply all subscripts of the empirical formula by this molar mass ratio.

45. Example Problem 13
The mass of benzene has been experimentally determined to be g. We know that benzene is 92 % C by mass and 8 % H by mass. What is the molecular formula of benzene?

46. Practice Problems
Page 344 #54-57
Page 348 #58-61
Page 350 #62-66

47. hydrates
Hydrates are solid ionic compounds in which water molecules are trapped.
Hydrates are formed when water molecules adhere to the ions as the solid forms.
Water molecules become a part of the crystal solid structure.
The number of water molecules associated with each molecule is written following a dot after the molecular formula:
Na2CO310H2O

48. hydrates
Names of these compounds are named with a prefix representing the number of water molecules and the word hydrate.
Na2CO3  10H2O – sodium carbonate decahydrate
Prefixes are the same as the ones used in naming covalent molecules

49. naming hydrates

50. Anhydrous An anhydrous is a compound without water.
When a hydrate is heated, water molecules are driven off leaving the compound.

51. Hydrate formula Steps to determining the formula of a hydrate:
Determine the initial mass of the compound prior to heating.
Determine the final mass of the compound after heating.
The final mass is used to determine the number of moles of the anhydrous compound. Grams to moles

52. Hydrate formula
Calculate the difference of the initial mass and the final mass and use this mass to determine number of moles of water, grams to moles
Number of hydrates per compound molecule (molar ratio – hydrate: anhydrous) = moles of H2O/moles of compound

53. example Problem 14
A mass of 2.50 g of blue, hydrated copper sulfate (CuSO4) ?H2O) is place in a crucible and heated. After heating, 1.59g of white anhydrous copper sulfate (CuSO4) remains. What is the formula for the hydrate? Name the hydrate.

54. question What is the empirical formula for the compound C6H12O6?
A. CHO
B. C2H3O2
C. CH2O
D. CH3O

55. question Which is the empirical formula for hydrogen peroxide? A. H2O2
B. H2O
C. HO
D. none of the above

56. question Heating a hydrate causes what to happen?
A. Water is driven from the hydrate.
B. The hydrate melts.
C. The hydrate conducts electricity.
D. There is no change in the hydrate.

57. question
A hydrate that has been heated and the water driven off is called:
A. dehydrated compound
B. antihydrated compound
C. anhydrous compound
D. hydrous compound

58. Practice problems
Page 353 #74-75; page 354 #76-82

59. Accumulating Content and Skills

60. Accumulating content
How does the mole apply to balanced equations?

61. example problem 15
How many grams of each reactant are needed to run the following reaction? How many grams of each product are produced? (Hint: complete, balance, convert)
CuSO4Ÿ5H2O(aq) + CaCl2(aq) 

62. example problem 16
From the reaction above, how much is needed of each reactant in the net ionic equation to produce the balanced amount of the precipitate? How much precipitate is produced?

63. key concepts

64. key concepts
The mole is a unit used to count particles of matter indirectly. One mole of a pure substance contains Avogadro’s number of particles.
Representative particles include atoms, ions, molecules, formula units, electrons, and other similar particles.

65. key concepts One mole of carbon-12 atoms has a mass of exactly 12 g.
Conversion factors written from Avogadro’s relationship can be used to convert between moles and number of representative particles.
The mass in grams of 1 mol of any pure substance is called its molar mass.

66. key concepts
The molar mass of an element is numerically equal to its atomic mass.
The molar mass of any substance is the mass in grams of Avogadro’s number of representative particles of the substance.
Molar mass is used to convert from moles to mass. The inverse of molar mass is used to convert from mass to moles.

67. key concepts
Subscripts in a chemical formula indicate how many moles of each element are present in 1 mol of the compound.
The molar mass of a compound is calculated from the molar masses of all of the elements in the compound.
Conversion factors based on a compound’s molar mass are used to convert between moles and mass of a compound.

68. key concepts
The percent by mass of an element in a compound gives the percentage of the compound’s total mass due to that element.
The subscripts in an empirical formula give the smallest whole-number ratio of moles of elements in the compound.
The molecular formula gives the actual number of atoms of each element in a molecule or formula unit of a substance.
The molecular formula is a whole-number multiple of the empirical formula.

69. key concepts
The formula of a hydrate consists of the formula of the ionic compound and the number of water molecules associated with one formula unit.
The name of a hydrate consists of the compound name and the word hydrate with a prefix indicating the number of water molecules in 1 mol of the compound.
Anhydrous compounds are formed when hydrates are heated.

70. question What does Avogadro’s number represent?
A. the number of atoms in 1 mol of an element
B. the number of molecules in 1 mol of a compound
C. the number of Na+ ions in 1 mol of NaCl (aq)
D. all of the above

71. question
The molar mass of an element is numerically equivalent to what?
A. 1 amu
B. 1 mole
C. its atomic mass
D. its atomic number

72. question How many moles of hydrogen atoms are in one mole of H2O2?
B. 2
C. 3
D. 0.5

73. question What is the empirical formula of Al2Br3? A. AlBr B. AlBr3
C. Al2Br
D. Al2Br3

74. question What is an ionic solid with trapped water molecules called?
A. aqueous solution
B. anhydrous compound
C. hydrate
D. solute

75. question
How many water molecules are associated with 3.0 mol of CoCl2 • 6H2O?
A. 18
B. 1.1  1025
C. 3.6  1024
D. 1.8  1024

76. question How many moles of Al are in 2.0 mol of Al2Br3? A. 2 B. 4 C. 6
D. 1

77. question How many atoms of hydrogen are in 3.5 mol of H2S?
B. 2.1  1023
C. 6.0  1023
D. 4.2  1024

78. question Which is not the correct formula for an ionic compound?
A. CO2
B. NaCl
C. Na2SO4
D. LiBr2