1. Section 4 Determining Chemical Formulas
Chapter 7
Objectives
Define empirical formula, and explain how the term applies to ionic and molecular compounds.
Determine an empirical formula from either a percentage or a mass composition.
Explain the relationship between the empirical formula and the molecular formula of a given compound.
Determine a molecular formula from an empirical formula.

2. Empirical and Actual Formulas
Section 4 Determining Chemical Formulas
Chapter 7
Empirical and Actual Formulas

3. Section 4 Determining Chemical Formulas
Chapter 7
An empirical formula consists of the symbols for the elements combined in a compound, with subscripts showing the smallest whole-number mole ratio of the different atoms in the compound.
For an ionic compound, the formula unit is usually the compound’s empirical formula.
For a molecular compound, however, the empirical formula does not necessarily indicate the actual numbers of atoms present in each molecule.
example: the empirical formula of the gas diborane is BH3,
but the molecular formula is B2H6.

4. Calculation of Empirical Formulas
Section 4 Determining Chemical Formulas
Chapter 7
Calculation of Empirical Formulas
To determine a compound’s empirical formula from its percentage composition, begin by converting percentage composition to a mass composition.
Assume that you have a g sample of the compound.
Then calculate the amount of each element in the sample.
example: diborane
The percentage composition is 78.1% B and 21.9% H.
Therefore, g of diborane contains 78.1 g of B and 21.9 g of H.

5. Calculation of Empirical Formulas, continued
Section 4 Determining Chemical Formulas
Chapter 7
Calculation of Empirical Formulas, continued
Next, the mass composition of each element is converted to a composition in moles by dividing by the appropriate molar mass.
These values give a mole ratio of 7.22 mol B to 21.7 mol H.

6. Calculation of Empirical Formulas, continued
Section 4 Determining Chemical Formulas
Chapter 7
Calculation of Empirical Formulas, continued
To find the smallest whole number ratio, divide each number of moles by the smallest number in the existing ratio.
Because of rounding or experimental error, a compound’s mole ratio sometimes consists of numbers close to whole numbers instead of exact whole numbers.
In this case, the differences from whole numbers may be ignored and the nearest whole number taken.

7. Calculation of Empirical Formulas, continued
Section 4 Determining Chemical Formulas
Chapter 7
Calculation of Empirical Formulas, continued
Sample Problem L
Quantitative analysis shows that a compound contains 32.38% sodium, 22.65% sulfur, and 44.99% oxygen. Find the empirical formula of this compound.

8. Calculation of Empirical Formulas, continued
Section 4 Determining Chemical Formulas
Chapter 7
Calculation of Empirical Formulas, continued
Sample Problem L Solution
Given: percentage composition: 32.38% Na, 22.65% S, and 44.99% O
Unknown: empirical formula
Solution:
percentage composition mass composition
composition in moles smallest whole-number mole ratio of atoms

9. Calculation of Empirical Formulas, continued
Section 4 Determining Chemical Formulas
Chapter 7
Calculation of Empirical Formulas, continued
Sample Problem L Solution, continued

10. Calculation of Empirical Formulas, continued
Section 4 Determining Chemical Formulas
Chapter 7
Calculation of Empirical Formulas, continued
Sample Problem L Solution, continued
Smallest whole-number mole ratio of atoms: The compound contains atoms in the ratio mol Na: mol S:2.812 mol O.
Rounding yields a mole ratio of 2 mol Na:1 mol S:4 mol O.
The empirical formula of the compound is Na2SO4.

11. Calculation of Molecular Formulas
Section 4 Determining Chemical Formulas
Chapter 7
Calculation of Molecular Formulas
The empirical formula contains the smallest possible whole numbers that describe the atomic ratio.
The molecular formula is the actual formula of a molecular compound.
An empirical formula may or may not be a correct molecular formula.
The relationship between a compound’s empirical formula and its molecular formula can be written as follows.
x(empirical formula) = molecular formula

12. Calculation of Molecular Formulas, continued
Section 4 Determining Chemical Formulas
Chapter 7
Calculation of Molecular Formulas, continued
The formula masses have a similar relationship.
x(empirical formula mass) = molecular formula mass
To determine the molecular formula of a compound, you must know the compound’s formula mass.
Dividing the experimental formula mass by the empirical formula mass gives the value of x.
A compound’s molecular formula mass is numerically equal to its molar mass, so a compound’s molecular formula can also be found given the compound’s empirical formula and its molar mass.

13. Comparing Empirical and Molecular Formulas
Section 4 Determining Chemical Formulas
Chapter 7
Comparing Empirical and Molecular Formulas

14. Calculation of Molecular Formulas, continued
Section 4 Determining Chemical Formulas
Chapter 7
Calculation of Molecular Formulas, continued
Sample Problem N
In Sample Problem M, the empirical formula of a compound of phosphorus and oxygen was found to be P2O5. Experimentation shows that the molar mass of this compound is g/mol. What is the compound’s molecular formula?

15. Calculation of Molecular Formulas, continued
Section 4 Determining Chemical Formulas
Chapter 7
Calculation of Molecular Formulas, continued
Sample Problem N Solution
Given: empirical formula
Unknown: molecular formula
Solution:
x(empirical formula) = molecular formula

16. Calculation of Molecular Formulas, continued
Section 4 Determining Chemical Formulas
Chapter 7
Calculation of Molecular Formulas, continued
Sample Problem N Solution, continued
Molecular formula mass is numerically equal to molar mass.
molecular molar mass = g/mol
molecular formula mass = amu
empirical formula mass
mass of phosphorus atom = amu
mass of oxygen atom = amu
empirical formula mass of P2O5 =
2  amu + 5 × amu = amu

17. Calculation of Molecular Formulas, continued
Section 4 Determining Chemical Formulas
Chapter 7
Calculation of Molecular Formulas, continued
Sample Problem N Solution, continued
Dividing the experimental formula mass by the empirical formula mass gives the value of x.
(P2O5) = P4O10
The compound’s molecular formula is therefore P4O10.

18. Chapter 7 Multiple Choice
Standardized Test Preparation
Multiple Choice
1. Which of the following compounds does not contain a polyatomic ion?
A. sodium carbonate
B. sodium sulfate
C. sodium sulfite
D. sodium sulfide

19. Chapter 7 Multiple Choice
Standardized Test Preparation
Multiple Choice
1. Which of the following compounds does not contain a polyatomic ion?
A. sodium carbonate
B. sodium sulfate
C. sodium sulfite
D. sodium sulfide

20. Chapter 7 Multiple Choice
Standardized Test Preparation
Multiple Choice
2. The correct formula for ammonium phosphate is
A. (NH4)3PO4.
B. (NH4)2PO4.
C. NH4PO4.
D. NH4(PO4)2.

21. Chapter 7 Multiple Choice
Standardized Test Preparation
Multiple Choice
2. The correct formula for ammonium phosphate is
A. (NH4)3PO4.
B. (NH4)2PO4.
C. NH4PO4.
D. NH4(PO4)2.

22. Chapter 7 Multiple Choice
Standardized Test Preparation
Multiple Choice
3. When writing the formula for a compound that contains a polyatomic ion,
A. write the anion’s formula first.
B. use superscripts to show the number of polyatomic ions present.
C. use parentheses if the number of polyatomic ions is greater than 1.
D. always place the polyatomic ion in parentheses.

23. Chapter 7 Multiple Choice
Standardized Test Preparation
Multiple Choice
3. When writing the formula for a compound that contains a polyatomic ion,
A. write the anion’s formula first.
B. use superscripts to show the number of polyatomic ions present.
C. use parentheses if the number of polyatomic ions is greater than 1.
D. always place the polyatomic ion in parentheses.

24. Chapter 7 Multiple Choice 4. The correct name for NH4CH3COO is
Standardized Test Preparation
Multiple Choice
4. The correct name for NH4CH3COO is
A. ammonium carbonate.
B. ammonium hydroxide.
C. ammonium acetate.
D. ammonium nitrate.

25. Chapter 7 Multiple Choice 4. The correct name for NH4CH3COO is
Standardized Test Preparation
Multiple Choice
4. The correct name for NH4CH3COO is
A. ammonium carbonate.
B. ammonium hydroxide.
C. ammonium acetate.
D. ammonium nitrate.

26. Chapter 7 Multiple Choice
Standardized Test Preparation
Multiple Choice
5. Which of the following is the correct formula for iron(III) sulfate?
A. Fe3SO4
B. Fe3(SO4)2
C. Fe2(SO4)3
D. 3FeSO4

27. Chapter 7 Multiple Choice
Standardized Test Preparation
Multiple Choice
5. Which of the following is the correct formula for iron(III) sulfate?
A. Fe3SO4
B. Fe3(SO4)2
C. Fe2(SO4)3
D. 3FeSO4

28. Chapter 7 Multiple Choice
Standardized Test Preparation
Multiple Choice
6. The molecular formula for acetylene is C2H2. The molecular formula for benzene is C6H6. The empirical formula for both is
A. CH.
B. C2H2.
C. C6H6.
D. (CH)2.

29. Chapter 7 Multiple Choice
Standardized Test Preparation
Multiple Choice
6. The molecular formula for acetylene is C2H2. The molecular formula for benzene is C6H6. The empirical formula for both is
A. CH.
B. C2H2.
C. C6H6.
D. (CH)2.

30. Chapter 7 Multiple Choice
Standardized Test Preparation
Multiple Choice
7. Which of the following shows the percentage composition of H2SO4?
A. 2.5% H, 39.1% S, 58.5% O
B. 2.1% H, 32.7% S, 65.2% O
C. 28.6% H, 14.3% S, 57.1% O
D. 33.3% H, 16.7% S, 50% O

31. Chapter 7 Multiple Choice
Standardized Test Preparation
Multiple Choice
7. Which of the following shows the percentage composition of H2SO4?
A. 2.5% H, 39.1% S, 58.5% O
B. 2.1% H, 32.7% S, 65.2% O
C. 28.6% H, 14.3% S, 57.1% O
D. 33.3% H, 16.7% S, 50% O

32. Chapter 7 Multiple Choice
Standardized Test Preparation
Multiple Choice
8. Which of the following compounds has the highest percentage of oxygen?
A. CH4O
B. CO2
C. H2O
D. Na2CO3

33. Chapter 7 Multiple Choice
Standardized Test Preparation
Multiple Choice
8. Which of the following compounds has the highest percentage of oxygen?
A. CH4O
B. CO2
C. H2O
D. Na2CO3

34. Chapter 7 Multiple Choice
Standardized Test Preparation
Multiple Choice
9. The empirical formula for a compound that is 1.2% H, 42.0% Cl, and 56.8% O is
A. HClO.
B. HClO2.
C. HClO3.
D. HClO4.

35. Chapter 7 Multiple Choice
Standardized Test Preparation
Multiple Choice
9. The empirical formula for a compound that is 1.2% H, 42.0% Cl, and 56.8% O is
A. HClO.
B. HClO2.
C. HClO3.
D. HClO4.

36. Chapter 7
Standardized Test Preparation
Short Answer
10. When a new substance is synthesized or is discovered experimentally, the substance is analyzed quantitatively. What information is obtained from this typical analysis, and how is this information used?

37. Chapter 7
Standardized Test Preparation
Short Answer
10. When a new substance is synthesized or is discovered experimentally, the substance is analyzed quantitatively. What information is obtained from this typical analysis, and how is this information used?
Answer: The percentage composition is obtained and used to determine the substance’s empirical formula.

38. Chapter 7
Standardized Test Preparation
Short Answer
11. An oxide of selenium is 28.8% O. Find the empirical formula. Assuming that the empirical formula is also the molecular formula, name the oxide.

39. Chapter 7
Standardized Test Preparation
Short Answer
11. An oxide of selenium is 28.8% O. Find the empirical formula. Assuming that the empirical formula is also the molecular formula, name the oxide.
Answer: SeO2; selenium dioxide

40. Chapter 7 Extended Response
Standardized Test Preparation
Extended Response
12. What is an empirical formula, and how does it differ from a molecular formula?

41. Chapter 7 Extended Response
Standardized Test Preparation
Extended Response
12. What is an empirical formula, and how does it differ from a molecular formula?
Answer: The empirical formula is the simplest whole-number ratio of the atoms in a chemical formula. The molecular formula is the actual number of atoms of each element in a chemical formula. The two formulas differ by an integer factor.

42. Chapter 7 Extended Response 13. What are Stock system names based on?
Standardized Test Preparation
Extended Response
13. What are Stock system names based on?

43. Chapter 7 Extended Response 13. What are Stock system names based on?
Standardized Test Preparation
Extended Response
13. What are Stock system names based on?
Answer: The Stock system is based on the oxidation number of the cation.