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Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Chapter 7 Section 3 Using Chemical Formulas Objectives Calculate the formula mass or molar mass of any given compound. Use molar mass to convert between mass in grams and amount in moles of a chemical compound. Calculate the number of molecules, formula units, or ions in a given molar amount of a chemical compound. Calculate the percentage composition of a given chemical compound.

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Chapter 7 A chemical formula indicates: the elements present in a compound the relative number of atoms or ions of each element present in a compound Chemical formulas also allow chemists to calculate a number of other characteristic values for a compound: formula mass molar mass percentage composition Section 3 Using Chemical Formulas

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Chapter 7 Formula Masses The formula mass of any molecule, formula unit, or ion is the sum of the average atomic masses of all atoms represented in its formula. example:formula mass of water, H 2 O average atomic mass of H: 1.01 amu average atomic mass of O: 16.00 amu Section 3 Using Chemical Formulas average mass of H 2 O molecule: 18.02 amu

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Chapter 7 Formula Masses The mass of a water molecule can be referred to as a molecular mass. The mass of one formula unit of an ionic compound, such as NaCl, is not a molecular mass. The mass of any unit represented by a chemical formula (H 2 O, NaCl) can be referred to as the formula mass. Section 3 Using Chemical Formulas

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Chapter 7 Formula Masses, continued Sample Problem F Find the formula mass of potassium chlorate, KClO 3. Section 3 Using Chemical Formulas

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Chapter 7 Formula Masses, continued Sample Problem F Solution The mass of a formula unit of KClO 3 is found by adding the masses of one K atom, one Cl atom, and three O atoms. Atomic masses can be found in the periodic table in the back of your book. In your calculations, round each atomic mass to two decimal places. Section 3 Using Chemical Formulas

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Chapter 7 Formula Masses, continued Sample Problem F Solution, continued Chapter 7 Section 3 Using Chemical Formulas formula mass of KClO 3 = 122.55 amu

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Chapter 7 Molar Masses The molar mass of a substance is equal to the mass in grams of one mole, or approximately 6.022  10 23 particles, of the substance. example: the molar mass of pure calcium, Ca, is 40.08 g/mol because one mole of calcium atoms has a mass of 40.08 g. The molar mass of a compound is calculated by adding the masses of the elements present in a mole of the molecules or formula units that make up the compound. Section 3 Using Chemical Formulas

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Chapter 7 Molar Masses, continued One mole of water molecules contains exactly two moles of H atoms and one mole of O atoms. The molar mass of water is calculated as follows. Section 3 Using Chemical Formulas molar mass of H 2 O molecule: 18.02 g/mol A compound’s molar mass is numerically equal to its formula mass.

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Chapter 7 Molar Masses, continued Sample Problem G What is the molar mass of barium nitrate, Ba(NO 3 ) 2 ? Section 3 Using Chemical Formulas

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Chapter 7 Molar Masses, continued Sample Problem G Solution One mole of barium nitrate, contains one mole of Ba, two moles of N (1  2), and six moles of O (3  2). Chapter 7 Section 3 Using Chemical Formulas molar mass of Ba(NO 3 ) 2 = 261.35 g/mol

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Chapter 7 Molar Mass as a Conversion Factor The molar mass of a compound can be used as a conversion factor to relate an amount in moles to a mass in grams for a given substance. To convert moles to grams, multiply the amount in moles by the molar mass: Amount in moles  molar mass (g/mol) = mass in grams Section 3 Using Chemical Formulas

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Chapter 7 Molar Mass as a Conversion Factor, continued Sample Problem H What is the mass in grams of 2.50 mol of oxygen gas? Section 3 Using Chemical Formulas

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Chapter 7 Molar Mass as a Conversion Factor, continued Given: 2.50 mol O 2 Section 3 Using Chemical Formulas Sample Problem H Solution Unknown: mass of O 2 in grams Solution: moles O 2 grams O 2 amount of O 2 (mol)  molar mass of O 2 (g/mol) = mass of O 2 (g)

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Chapter 7 Molar Mass as a Conversion Factor, continued Sample Problem H Solution, continued Calculate the molar mass of O 2. Section 3 Using Chemical Formulas Use the molar mass of O 2 to convert moles to mass.

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Chapter 7 Molar Mass as a Conversion Factor, continued Sample Problem I Ibuprofen, C 13 H 18 O 2, is the active ingredient in many nonprescription pain relievers. Its molar mass is 206.31 g/mol. a. If the tablets in a bottle contain a total of 33 g of ibuprofen, how many moles of ibuprofen are in the bottle? b. How many molecules of ibuprofen are in the bottle? c. What is the total mass in grams of carbon in 33 g of ibuprofen? Section 3 Using Chemical Formulas

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Chapter 7 Molar Mass as a Conversion Factor, continued Sample Problem I Solution Section 3 Using Chemical Formulas Given:33 g of C 13 H 18 O 2 molar mass 206.31 g/mol Solution:a. grams moles Unknown:a. moles C 13 H 18 O 2 b. molecules C 13 H 18 O 2 c. total mass of C

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Chapter 7 Molar Mass as a Conversion Factor, continued b. molesmolecules Section 3 Using Chemical Formulas c. moles C 13 H 18 O 2 moles C grams C Sample Problem I Solution, continued

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Chapter 7 Molar Mass as a Conversion Factor, continued Section 3 Using Chemical Formulas a. c. b. Sample Problem I Solution, continued

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 Mass Problem Calculation Do this problem if you still need help.

End Show © Copyright Pearson Prentice Hall 35 Slide of 40 Percent Composition and Chemical Formulas It helps to know the percents of the components in a shirt because they affect how warm it is, whether it will need to be ironed, and how it should be cleaned. You will learn how the percents of the elements in a compound are important in chemistry. 7.3

End Show © Copyright Pearson Prentice Hall 36 Percent Composition and Chemical Formulas > Slide of 40 The Percent Composition of a Compound How do you calculate the percent by mass of an element in a compound? 7.3

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Chapter 7 Percentage Composition It is often useful to know the percentage by mass of a particular element in a chemical compound. To find the mass percentage of an element in a compound, the following equation can be used. Section 3 Using Chemical Formulas The mass percentage of an element in a compound is the same regardless of the sample’s size.

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Chapter 7 Percentage Composition, continued The percentage of an element in a compound can be calculated by determining how many grams of the element are present in one mole of the compound. Section 3 Using Chemical Formulas The percentage by mass of each element in a compound is known as the percentage composition of the compound.

End Show Slide of 40 © Copyright Pearson Prentice Hall 39 Percent Composition and Chemical Formulas > The Percent Composition of a Compound The percent by mass of an element in a compound is the number of grams of the element divided by the mass in grams of the compound, multiplied by 100%. 7.3

End Show Slide of 40 © Copyright Pearson Prentice Hall 42 Percent Composition and Chemical Formulas > The Percent Composition of a Compound Percent Composition from Mass Data The relative amounts of the elements in a compound are expressed as the percent composition or the percent by mass of each element in the compound. 7.3

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Chapter 7 Percentage Composition, continued Sample Problem J Find the percentage composition of copper(I) sulfide, Cu 2 S. Section 3 Using Chemical Formulas

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Chapter 7 Percentage Composition, continued Sample Problem J Solution Given: formula, Cu 2 S Unknown: percentage composition of Cu 2 S Solution: formula molar mass mass percentage of each element Section 3 Using Chemical Formulas

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Chapter 7 Percentage Composition, continued Sample Problem J Solution, continued Section 3 Using Chemical Formulas Molar mass of Cu 2 S = 159.2 g

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Chapter 7 Practice Problems pg. 244 Complete practice problems pg. 244 #1-2 only in your notes. Show all of your work before checking answers on the next slide. You must show work to get any credit also. Rounding might cause answers to be slightly off.