1 Chapter 6 Chemical Quantities 6.5 Percent Composition and Empirical Formulas Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings

2 Percent Composition Percent composition Is the percent by mass of each element in a formula. Example: Calculate the percent composition of CO 2. CO 2 = 1 C(12.01g) + 2 O(16.00 g) = g/mol) g C x 100 = % C g CO g O x 100 = % O g CO %

3 What is the percent composition of lactic acid, C 3 H 6 O 3, a compound that appears in the blood after vigorous activity? Learning Check

4 STEP 1 3C(12.01) + 6H(1.008) + 3O(16.00) = g/mol g C g H g O STEP 2 %C = g C x 100= 40.00% C g %H = g H x 100 = 6.714% H g %O = g O x 100 = 53.29% O g Solution

5 Learning Check The chemical isoamyl acetate C 7 H 14 O 2 gives the odor of pears. What is the percent carbon in isoamyl acetate? 1) %C 2) %C 3) %C Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings

6 3) %C Molar mass C 7 H 14 O 2 = 7C(12.01) + 14H(1.008) + 2O(16.00) = g/mol Total C = 7C(12.01) = g % C = total g C x 100 total g % C= g C x 100 = % C g Solution

7 The empirical formula Is the simplest whole number ratio of the atoms. Is calculated by dividing the subscripts in the actual (molecular) formula by a whole number to give the lowest ratio. C 5 H 10 O 5  5 = C 1 H 2 O 1 = CH 2 O actual (molecular) empirical formula formula Empirical Formulas

8 Some Molecular and Empirical Formulas The molecular formula is the same or a multiple of the empirical. Table 6.3 Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings

9 A. What is the empirical formula for C 4 H 8 ? 1) C 2 H 4 2) CH 2 3) CH B. What is the empirical formula for C 8 H 14 ? 1) C 4 H 7 2) C 6 H 12 3) C 8 H 14 C. Which is a possible molecular formula for CH 2 O? 1) C 4 H 4 O 4 2) C 2 H 4 O 2 3) C 3 H 6 O 3 Learning Check

10 A. What is the empirical formula for C 4 H 8 ? 2) CH 2 C 4 H 8  4 B. What is the empirical formula for C 8 H 14 ? 1) C 4 H 7 C 8 H 14  2 C. Which is a possible molecular formula for CH 2 O? 2) C 2 H 4 O 2 3) C 3 H 6 O 3 Solution

11 A compound has an empirical formula SN. If there are 4 atoms of N in one molecule, what is the molecular formula? Explain. 1) SN 2) SN 4 3) S 4 N 4 Learning Check

12 A compound has an empirical formula SN. If there are 4 atoms of N in one molecule, what is the molecular formula? Explain. 3) S 4 N 4 In this molecular formula 4 atoms of N and 4 atoms of S and N are related 1:1. Thus, it has an empirical formula of SN. Solution

13 A compound contains 7.31 g Ni and 20.0 g Br. Calculate its empirical (simplest) formula. Learning Check

14 Convert 7.31 g Ni and 20.0 g Br to moles g Ni x 1 mol Ni = mol Ni g Ni 20.0 g Br x 1 mol Br = mol Br g Br Divide by smallest: mol Ni = 1 Ni0.250 mol Br = 2 Br Write ratio as subscripts: NiBr 2 Solution

15 Converting Decimals to Whole Numbers When the number of moles for an element is a decimal, all the moles are multiplied by a small integer to obtain whole number. Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings Table 6.4

16 Aspirin is 60.0% C, 4.5 % H and 35.5 % O. Calculate its empirical (simplest) formula. Learning Check

17 STEP 1. Calculate the moles of each element in 100 g. 100 g aspirin contains 60.0% C or 60.0 g C, 4.5% H or 4.5 g H, and 35.5% O or 35.5 g O g C x 1 mol C = 5.00 mol C g C 4.5 g H x 1 mol H = 4.5 mol H g H 35.5 g O x 1mol O = 2.22 mol O g O Solution

18 Solution (continued) STEP 2. Divide by the smallest number of mol mol C= 2.25 mol C (decimal) mol H = 2.0 mol H mol O = 1.00 mole O 2.22

19 Solution (continued) 3. Use the lowest whole number ratio as subscripts When the moles are not whole numbers, multiply by a factor to give whole numbers, in this case x 4. C: 2.25 mol C x 4 = 9 mol C H: 2.0 mol Hx 4 = 8 mol H O: 1.00 mol O x 4 = 4 mol O Using these whole numbers as subscripts the simplest formula is C 9 H 8 O 4