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

Percent Composition Percent composition Is the percent by mass of each element in a formula. Example: Calculate the percent composition of CO2. CO2 = 1 C(12.01g) + 2 O(16.00 g) = 44.01 g/mol) 12.01 g C x 100 = 27.29 % C 44.01 g CO2 32.00 g O x 100 = 72.71 % O 44.01 g CO2 100.00 %

CFU What is the percent composition of lactic acid, C3H6O3, a compound that appears in the blood after vigorous activity?

Solution #1 STEP 1 3C(12.01) + 6H(1.008) + 3O(16.00) = 90.08 g/mol 36.03 g C + 6.048 g H + 48.00 g O STEP 2 %C = 36.03 g C x 100 = 40.00% C 90.08 g %H = 6.048 g H x 100 = 6.714% H %O = 48.00 g O x 100 = 53.29% O

CFU #2 The chemical isoamyl acetate C7H14O2 gives the odor of pears. What is the percent carbon in isoamyl acetate? 1) 7.102 %C 2) 35.51 %C 3) 64.58 %C Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings

Solution #2 3) 64.58 %C Molar mass C7H14O2 = 7C(12.01) + 14H(1.008) + 2O(16.00) = 130.18 g/mol Total C = 7C(12.01) = g % C = total g C x 100 total g % C = 84.07 g C x 100 = 64.58 % C 130.18 g

Empirical Formulas 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. C5H10O5  5 = C1H2O1 = CH2O actual (molecular) empirical formula formula

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

CFU #3 A. What is the empirical formula for C4H8? 1) C2H4 2) CH2 3) CH B. What is the empirical formula for C8H14? 1) C4H7 2) C6H12 3) C8H14 C. Which is a possible molecular formula for CH2O? 1) C4H4O4 2) C2H4O2 3) C3H6O3

Solution #3 A. What is the empirical formula for C4H8? 2) CH2 C4H8  4 B. What is the empirical formula for C8H14? 1) C4H7 C8H14  2 C. Which is a possible molecular formula for CH2O? 2) C2H4O2 3) C3H6O3

CFU #4 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) SN4 3) S4N4

Solution #4 A compound has an empirical formula SN. If there are 4 atoms of N in one molecule, what is the molecular formula? Explain. 3) S4N4 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.

CFU #5 optional with %Comp A compound contains 7.31 g Ni and 20.0 g Br. Calculate its empirical (simplest) formula.

Solution #5 Convert 7.31 g Ni and 20.0 g Br to moles. 7.31 g Ni x 1 mol Ni = 0.125 mol Ni 58.69 g Ni 20.0 g Br x 1 mol Br = 0.250 mol Br 79.90 g Br Divide by smallest: 0.125 mol Ni = 1 Ni 0.250 mol Br = 2 Br 0.125 0.125 Write ratio as subscripts: NiBr2

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. Table 6.4 Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings

CFU #6 Aspirin is 60.0% C, 4.5 % H and 35.5 % O. Calculate its empirical (simplest) formula.

Solution #6 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. 60.0 g C x 1 mol C = 5.00 mol C 12.01 g C 4.5 g H x 1 mol H = 4.5 mol H 1.008 g H 35.5 g O x 1mol O = 2.22 mol O 16.00 g O

Solution #6 (continued) STEP 2. Divide by the smallest number of mol. 5.00 mol C = 2.25 mol C (decimal) 2.22 4.5 mol H = 2.0 mol H 2.22 mol O = 1.00 mole O

Solution #6 (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 H x 4 = 8 mol H O: 1.00 mol O x 4 = 4 mol O Using these whole numbers as subscripts the simplest formula is C9H8O4