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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 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.

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