Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Chapter 7 Table of Contents Chapter 7 Section 4 Determining Chemical Formulas Chemical Formulas and Chemical Compounds

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Chapter 7 Section 4 Determining Chemical Formulas 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.

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Chapter 7 Empirical and Actual Formulas Chapter 7 Section 4 Determining Chemical Formulas

End Show © Copyright Pearson Prentice Hall 4 Percent Composition and Chemical Formulas > Slide of 40 Empirical Formulas What does the empirical formula of a compound show? 7.4

End Show Slide of 40 © Copyright Pearson Prentice Hall 5 Percent Composition and Chemical Formulas > Empirical Formulas The empirical formula gives the lowest whole- number ratio of the atoms of the elements in a compound. The empirical formula of a compound shows the smallest whole-number ratio of the atoms in the compound. 7.4

End Show Slide of 40 © Copyright Pearson Prentice Hall 6 Percent Composition and Chemical Formulas > Empirical Formulas Ethyne (C 2 H 2 ) is a gas used in welder’s torches. Styrene (C 8 H 8 ) is used in making polystyrene. These two compounds of carbon have the same empirical formula (CH) but different molecular formulas. 7.4

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Chapter 7 Section 4 Determining Chemical Formulas 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 BH 3, but the molecular formula is B 2 H 6.

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Chapter 7 Section 4 Determining Chemical Formulas 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.

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Chapter 7 Section 4 Determining Chemical Formulas 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.

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Chapter 7 Section 4 Determining Chemical Formulas 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.

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Chapter 7 Section 4 Determining Chemical Formulas 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.

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Chapter 7 Section 4 Determining Chemical Formulas 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

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Chapter 7 Section 4 Determining Chemical Formulas Calculation of Empirical Formulas, continued Sample Problem L Solution, continued

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu 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. Section 4 Determining Chemical Formulas Rounding yields a mole ratio of 2 mol Na:1 mol S:4 mol O. The empirical formula of the compound is Na 2 SO 4.

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Chapter 7 Section 4 Determining Chemical Formulas 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

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Chapter 7 Section 4 Determining Chemical Formulas 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.

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Chapter 7 Comparing Empirical and Molecular Formulas Chapter 7 Section 4 Determining Chemical Formulas

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Chapter 7 Visual Concepts Click below to watch the Visual Concept. Visual Concept Comparing Molecular and Empirical Formulas /student/ch07/sec04/vc00/hc607_04_v 00fs.htm

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Chapter 7 Visual Concepts Click below to watch the PROBLEM ACTIVITY. PROBLEM ACTIVITY Empirical Formula Problem Activity

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Chapter 7 Visual Concepts Click below to watch the PROBLEM ACTIVITY. PROBLEM ACTIVITY Molecular Formula Problem Activity

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Chapter 7 Calculation of Molecular Formulas, continued Sample Problem N In Sample Problem M in textbook, the empirical formula of a compound of phosphorus and oxygen was found to be P 2 O 5. Experimentation shows that the molar mass of this compound is g/mol. What is the compound’s molecular formula? Section 4 Determining Chemical Formulas

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Chapter 7 Calculation of Molecular Formulas, continued Sample Problem N Solution Given: empirical formula Unknown: molecular formula Solution: x(empirical formula) = molecular formula Section 4 Determining Chemical Formulas

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu 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 P 2 O 5 = 2  amu + 5 × amu = amu Section 4 Determining Chemical Formulas

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu 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. Section 4 Determining Chemical Formulas (P 2 O 5 ) = P 4 O 10 The compound’s molecular formula is therefore P 4 O 10.

End Show © Copyright Pearson Prentice Hall Slide of 40 Section Quiz -or- Continue to: Launch: Assess students’ understanding of the concepts in Section 7.4 Section Quiz. 7.4.

© Copyright Pearson Prentice Hall Slide of 40 End Show 1. Calculate the percent by mass of carbon in cadaverine, C 5 H 14 N 2, a compound present in rotting meat. a. 67.4% C b. 58.8% C c. 51.7% C d. 68.2% C 7.4 Section Quiz.

© Copyright Pearson Prentice Hall Slide of 40 End Show 2. Which of the following is NOT an empirical formula? a. NO 2 b. H 2 N c. CH d. C 3 H Section Quiz.

© Copyright Pearson Prentice Hall Slide of 40 End Show 7.4 Section Quiz. 3. Determine the molecular formula of a compound that contains 40.0 percent C, 6.71 percent H, and percent O and has a molar mass of g. a. C 2 H 4 O 2 b. CH 2 O c. C 2 H 3 O d. C 2 H 4 O

© Copyright Pearson Prentice Hall Slide of 25 End Show Online Self-Check Quiz Complete the online Quiz and record answers. Ask if you have any questions about your answers. click here for online Quiz 7.4 (7 questions) You must be in the “Play mode” for the slideshow for hyperlink to work.

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Chapter 7 Practice Problems pg. 249 Complete Section Review 7.4 pg. 249 #1-4 as homework. You may record in your notes and separate notebook paper as needed. #5 is bonus.

© Copyright Pearson Prentice Hall Slide of 25 End Show Answers for HW pg. 249 #1-4 (#5 bonus) FOR ME ONLY.

© Copyright Pearson Prentice Hall Slide of 28 End Show VIDEOS FOR ADDITIONAL INSTRUCTION Additional Videos for Section 7.4: Determining Chemical Formulas Empirical Formula - Molecular Formula

© Copyright Pearson Prentice Hall Slide of 28 End Show SCI LINKS FOR CHAPTER Additional Student SCI LINKS for CHAPTER 7 The NSTA-sponsored SciLinks Web site contains links to accurate and up- to-date science information on the Internet. Just click on the button below to go to the SciLinks site at and log in. Then, type in the SciLinks code for the topic you want to research. The following is a list of the SciLinks codes for this chapter. Chapter 7: Chemical Bonding Topic: Chemical Formulas SciLinks code: HC60271 Topic: Acids SciLinks code: HC60012 Topic: Photochemical Reaction SciLinks code: HC61137

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