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Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Using Analytical Data The percentage composition is the percentage by mass of each element in a compound. Percentage composition helps verify a substance’s identity. Percentage composition also can be used to compare the ratio of masses contributed by the elements in two different substances. Section 3 Formulas and Percentage Composition Chapter 7

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Using Analytical Data, continued Determining Empirical Formulas Section 3 Formulas and Percentage Composition An empirical formula is a chemical formula that shows the simplest ratio for the relative numbers and kinds of atoms in a compound. An actual formula shows the actual ratio of elements or ions in a single unit of a compound. For example, the empirical formula for ammonium nitrate is NH 2 O, while the actual formula is NH 4 NO 2. Chapter 7

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Using Analytical Data, continued Determining Empirical Formulas, continued Section 3 Formulas and Percentage Composition You can use the percentage composition for a compound to determine its empirical formula. 1.Convert the percentage of each element to g. 2.Convert from g to mol using the molar mass of each element as a conversion factor. 3.Compare these amounts in mol to find the simplest whole-number ratio among the elements. Chapter 7

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Determining an Empirical Formula from Percentage Composition Sample Problem G Chemical analysis of a liquid shows that it is 60.0% C, 13.4% H, and 26.6% O by mass. Calculate the empirical formula of this substance. Section 3 Formulas and Percentage Composition Chapter 7

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Determining an Empirical Formula from Percentage Composition Sample Problem G Solution Assume that you have a 100.0 g sample, and convert the percentages to grams. for C:60.0%  100.0 g = 60.0 g C for H:13.4%  100.0 g = 13.4 g H for O:26.6%  100.0 g = 26.6 g O Section 3 Formulas and Percentage Composition Chapter 7

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Determining an Empirical Formula from Percentage Composition Sample Problem G Solution, continued Convert the mass of each element into the amount in moles, using the reciprocal of the molar mass. Section 3 Formulas and Percentage Composition Chapter 7

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Determining an Empirical Formula from Percentage Composition Sample Problem G Solution, continued The formula can be written as C 5 H 13.3 O 1.66, but you divide by the smallest subscript to get whole numbers. Section 3 Formulas and Percentage Composition The empirical formula is C 3 H 8 O. Chapter 7

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Using Analytical Data, continued Molecular Formulas Are Multiples of Empirical Formulas Section 3 Formulas and Percentage Composition The formula for an ionic compound shows the simplest whole-number ratio of the large numbers of ions in a crystal of the compound. A molecular formula is a whole-number multiple of the empirical formula. The molar mass of any compound is equal to the molar mass of the empirical formula times a whole number, n. Chapter 7

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Determining a Molecular Formula from an Empirical Formula Sample Problem H The empirical formula for a compound is P 2 O 5. Its experimental molar mass is 284 g/mol. Determine the molecular formula of the compound. Section 3 Formulas and Percentage Composition Chapter 7

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Determining a Molecular Formula from an Empirical Formula Sample Problem H Solution Find the molar mass of the empirical formula P 2 O 5. 2  molar mass of P = 61.94 g/mol + 5  molar mass of O = 80.00 g/mol molar mass of P 2 O 5 = 141.94 g/mol Section 3 Formulas and Percentage Composition Chapter 7

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Determining a Molecular Formula from an Empirical Formula Sample Problem H Solution, continued Section 3 Formulas and Percentage Composition Chapter 7 n (empirical formula) = 2 (P 2 O 5 ) = P 4 O 10

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Using Analytical Data, continued Chemical Formulas Can Give Percentage Composition Section 3 Formulas and Percentage Composition If you know the chemical formula of any compound, then you can calculate the percentage composition. From the subscripts, determine the mass contributed by each element and add these to get molar mass. Divide the mass of each element by the molar mass. Multiply by 100 to find the percentage composition of that element. Chapter 7

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu CO and CO 2 are both made up of C and O, but they have different percentage compositions. Using Analytical Data, continued Chemical Formulas Can Give Percentage Composition Section 3 Formulas and Percentage Composition Chapter 7

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Using a Chemical Formula to Determine Percentage Composition Sample Problem I Calculate the percentage composition of copper(I) sulfide, Cu 2 S, a copper ore called chalcocite. Section 3 Formulas and Percentage Composition Chapter 7

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Using a Chemical Formula to Determine Percentage Composition Section 3 Formulas and Percentage Composition Chapter 7 Sample Problem I Solution Find the molar mass of Cu 2 S. 2 mol  63.55 g Cu/mol = 127.10 g Cu + 1 mol  32.07 g S/mol = 32.07 g S molar mass of Cu 2 S = 159.17 g/mol

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Using a Chemical Formula to Determine Percentage Composition Sample Problem I Solution, continued Calculate the fraction that each element contributes to the total mass by substituting the masses into the equations below and rounding correctly. Section 3 Formulas and Percentage Composition 79.852% Cu Chapter 7

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Using a Chemical Formula to Determine Percentage Composition Sample Problem I Solution, continued Section 3 Formulas and Percentage Composition 20.15% S Chapter 7