1. Chemical Formulas and Composition Stoichiometry2
Chemical Formulas and Composition Stoichiometry 1

2. Chapter Goals Chemical Formulas Ions and Ionic Compounds
Names and Formulas of Some Ionic Compounds
Atomic Weights
The Mole

3. Chapter Goals Formula Weights, Molecular Weights, and Moles
Percent Composition and Formulas of Compounds
Derivation of Formulas from Elemental Composition
Determination of Molecular Formulas
Some Other Interpretations of Chemical Formulas
Purity of Samples

4. Chemical Formulas
Chemical formula shows the chemical composition of the substance.
ratio of the elements present in the molecule or compound
He, Au, Na – monatomic elements
O2, H2, Cl2 – diatomic elements
O3, S4, P8 - more complex elements
H2O, C12H22O11 – compounds
Substance consists of two or more elements 4

5. Chemical Formulas Compound 1 Molecule Contains
HCl 1 H atom & 1 Cl atom
H2O 2 H atoms & 1 O atom
NH3 1 N atom & 3 H atoms
C3H8 3 C atoms & 8 H atoms 8

6. Ions and Ionic Compounds
Ions are atoms or groups of atoms that possess an electric charge.
Two basic types of ions:
Positive ions or cations
one or more electrons less than neutral
Na+, Ca2+, Al3+
NH4+ - polyatomic cation
Negative ions or anions
one or more electrons more than neutral
F-, O2-, N3-
SO42-, PO43- - polyatomic anions 9

7. Ions and Ionic Compounds
Sodium chloride
table salt is an ionic compound 9

8. Names and Formulas of Some Ionic Compounds
Table 2-3 displays the formulas, charges, and names of some common ions.
You must know the names, formulas, and charges of the common ions in table 2-3.
Some examples are:
Anions - Cl1-, OH1-, SO42-, PO43-
Cations - Na1+, NH41+, Ca2+, Al3+ 10

9. Names and Formulas of Some Ionic Compounds
Formulas of ionic compounds are determined by the charges of the ions.
Charge on the cations must equal the charge on the anions.
The compound must be neutral.
NaCl sodium chloride (Na1+ & Cl1-)
KOH potassium hydroxide(K1+ & OH1-)
CaSO4 calcium sulfate (Ca2+ & SO42-)
Al(OH)3 aluminum hydroxide (Al3+ & 3 OH1-) 11

10. Names and Formulas of Some Ionic Compounds
Table 2-2 gives names of several molecular compounds.
You must know all of the molecular compounds from Table 2-2.
Some examples are:
H2SO4 - sulfuric acid
FeBr2 - iron(II) bromide
C2H5OH - ethanol 12

11. Names and Formulas of Some Ionic Compounds
You do it!
What is the formula of nitric acid?
HNO3
What is the formula of sulfur trioxide?
SO3
What is the name of FeBr3?
iron(III) bromide 13

12. Names and Formulas of Some Ionic Compounds
You do it!
What is the name of K2SO3?
potassium sulfite
What is charge on sulfite ion?
SO32- is sulfite ion
What is the formula of ammonium sulfide?
(NH4)2S 14

13. Names and Formulas of Some Ionic Compounds
You do it!
What is charge on ammonium ion?
NH41+
What is the formula of aluminum sulfate?
Al2(SO4)3
What is charge on both ions?
Al3+ and SO42- 14

14. Atomic Weights
Weighted average of the masses of the constituent isotopes if an element.
Tells us the atomic masses of every known element.
Lower number on periodic chart.
How do we know what the values of these numbers are? 15

15. The Mole
A number of atoms, ions, or molecules that is large enough to see and handle.
A mole = number of things
Just like a dozen = 12 things
One mole = x 1023 things
Avogadro’s number = x 1023
Symbol for Avogadro’s number is NA. 16

16. The Mole How do we know when we have a mole?
count it out
weigh it out
Molar mass - mass in grams numerically equal to the atomic weight of the element in grams.
H has an atomic weight of g
g of H atoms = x 1023 H atoms
Mg has an atomic weight of g
g of Mg atoms = x 1023 Mg atoms 17

17. The Mole
Example 2-1: Calculate the mass of a single Mg atom in grams to 3 significant figures. 18

18. The Mole
Example 2-1: Calculate the mass of a single Mg atom in grams to 3 significant figures. 19

19. The Mole
Example 2-1: Calculate the mass of a single Mg atom in grams to 3 significant figures. 20

20. The Mole
Example 2-1: Calculate the mass of a single Mg atom, in grams, to 3 significant figures. 21

21. The Mole
Example 2-2: Calculate the number of atoms in one-millionth of a gram of Mg to 3 significant figures. 22

22. The Mole
Example 2-2: Calculate the number of atoms in one-millionth of a gram of Mg to 3 significant figures. 23

23. The Mole
Example 2-2: Calculate the number of atoms in one-millionth of a gram of Mg to 3 significant figures. 24

24. The Mole
Example 2-2: Calculate the number of atoms in one-millionth of a gram of Mg to 3 significant figures. 25

25. The Mole
Example 2-3. How many atoms are contained in 1.67 moles of Mg? 28

26. The Mole
Example 2-3. How many atoms are contained in 1.67 moles of Mg? 28

27. The Mole
Example 2-3. How many atoms are contained in 1.67 moles of Mg? 28

28. The Mole
Example 2-3. How many atoms are contained in 1.67 moles of Mg? 28

29. The Mole
Example 2-3. How many atoms are contained in 1.67 moles of Mg? 28

30. The Mole
Example 2-4: How many moles of Mg atoms are present in 73.4 g of Mg?
You do it! 29

31. The Mole
Example 2-4: How many moles of Mg atoms are present in 73.4 g of Mg? 31

32. The Mole
Example 2-4: How many moles of Mg atoms are present in 73.4 g of Mg? 31

33. IT IS IMPERATIVE THAT YOU KNOW HOW TO DO THESE PROBLEMS
The Mole
Example 2-4: How many moles of Mg atoms are present in 73.4 g of Mg?
IT IS IMPERATIVE THAT YOU KNOW
HOW TO DO THESE PROBLEMS 31

34. Formula Weights, Molecular Weights, and Moles
How do we calculate the molar mass of a compound?
add atomic weights of each atom
The molar mass of propane, C3H8, is: 32

35. Formula Weights, Molecular Weights, and Moles
The molar mass of calcium nitrate, Ca(NO3)2 , is:
You do it! 32

36. Formula Weights, Molecular Weights, and Moles33

37. Formula Weights, Molecular Weights, and Moles
One Mole of Contains
Cl2 or 70.90g x 1023 Cl2 molecules
2(6.022 x 1023 ) Cl atoms
C3H8 You do it! 34

38. Formula Weights, Molecular Weights, and Moles
One Mole of Contains
Cl2 or 70.90g x 1023 Cl2 molecules
2(6.022 x 1023 ) Cl atoms
C3H8 or g x 1023 C3H8 molecules
3 (6.022 x 1023 ) C atoms
8 (6.022 x 1023 ) H atoms 34

39. Formula Weights, Molecular Weights, and Moles
Example 2-5: Calculate the number of C3H8 molecules in 74.6 g of propane. 38

40. Formula Weights, Molecular Weights, and Moles
Example 2-5: Calculate the number of C3H8 molecules in 74.6 g of propane. 38

41. Formula Weights, Molecular Weights, and Moles
Example 2-5: Calculate the number of C3H8 molecules in 74.6 g of propane. 38

42. Formula Weights, Molecular Weights, and Moles
Example 2-5: Calculate the number of C3H8 molecules in 74.6 g of propane. 38

43. Formula Weights, Molecular Weights, and Moles
Example 2-7. How many (a) moles, (b) molecules, and (c) oxygen atoms are contained in 60.0 g of ozone, O3? The layer of ozone in the stratosphere is very beneficial to life on earth.
You do it! 40

44. Formula Weights, Molecular Weights, and Moles
Example 2-7. How many (a) moles, (b) molecules, and (c) oxygen atoms are contained in 60.0 g of ozone, O3? The layer of ozone in the stratosphere is very beneficial to life on earth.
(a) 40

45. Formula Weights, Molecular Weights, and Moles
Example 2-7. How many (a) moles, (b) molecules, and (c) oxygen atoms are contained in 60.0 g of ozone, O3? The layer of ozone in the stratosphere is very beneficial to life on earth.
(b) 40

46. Formula Weights, Molecular Weights, and Moles
Example 2-7. How many (a) moles, (b) molecules, and (c) oxygen atoms are contained in 60.0 g of ozone, O3? The layer of ozone in the stratosphere is very beneficial to life on earth.
(c) 40

47. Formula Weights, Molecular Weights, and Moles
Example 2-9: Calculate the number of mmol in g of oxalic acid, (COOH)2.
You do it! 43

48. Percent Composition and Formulas of Compounds
% composition = mass of an individual element in a compound divided by the total mass of the compound x 100%
Determine the percent composition of C in C3H8. 45

49. Percent Composition and Formulas of Compounds
What is the percent composition of H in C3H8?
You do it! 46

50. Percent Composition and Formulas of Compounds
What is the percent composition of H in C3H8? 47

51. Percent Composition and Formulas of Compounds
Example 2-10: Calculate the percent composition of Fe2(SO4)3 to 3 significant figures.
You do it! 48

52. Percent Composition and Formulas of Compounds
Example 2-10: Calculate the percent composition of Fe2(SO4)3 to 3 sig. fig. 49

53. Derivation of Formulas from Elemental Composition
Empirical Formula - smallest whole-number ratio of atoms present in a compound
CH2 is the empirical formula for alkenes
No alkene exists that has 1 C and 2 H’s
Molecular Formula - actual numbers of atoms of each element present in a molecule of the compound
Ethene – C2H4
Pentene – C5H10
We determine the empirical and molecular formulas of a compound from the percent composition of the compound.
percent composition is determined experimentally 50

54. Derivation of Formulas from Elemental Composition
Example 2-11: A compound contains 24.74% K, 34.76% Mn, and 40.50% O by mass. What is its empirical formula?
Make the simplifying assumption that we have g of compound.
In g of compound there are:
24.74 g of K
34.76 g of Mn
40.50 g of O 51

55. Derivation of Formulas from Elemental Composition52

56. Derivation of Formulas from Elemental Composition53

57. Derivation of Formulas from Elemental Composition54

58. Derivation of Formulas from Elemental Composition55

59. Derivation of Formulas from Elemental Composition56

60. Determination of Molecular Formulas
Example 2-13: A compound is found to contain 85.63% C and 14.37% H by mass. In another experiment its molar mass is found to be 56.1 g/mol. What is its molecular formula?
short cut method 61

61. Determination of Molecular Formulas62

62. Some Other Interpretations of Chemical Formulas
Example 2-16: What mass of ammonium phosphate, (NH4)3PO4, would contain 15.0 g of N? 65

63. Some Other Interpretations of Chemical Formulas
Example 2-16: What mass of ammonium phosphate, (NH4)3PO4, would contain 15.0 g of N? 65

64. Some Other Interpretations of Chemical Formulas
Example 2-16: What mass of ammonium phosphate, (NH4)3PO4, would contain 15.0 g of N? 65

65. Purity of Samples
The percent purity of a sample of a substance is always represented as 66

66. Purity of Samples
Example 2-18: A bottle of sodium phosphate, Na3PO4, is 98.3% pure Na3PO4. What are the masses of Na3PO4 and impurities in g of this sample of Na3PO4?

67. Purity of Samples
Example 2-18: A bottle of sodium phosphate, Na3PO4, is 98.3% pure Na3PO4. What are the masses of Na3PO4 and impurities in g of this sample of Na3PO4?

68. Purity of Samples
Example 2-18: A bottle of sodium phosphate, Na3PO4, is 98.3% pure Na3PO4. What are the masses of Na3PO4 and impurities in g of this sample of Na3PO4?

69. Chemical Formulas and Composition Stoichiometry2
Chemical Formulas and Composition Stoichiometry 1