Chapter 11: The Mole IV. Empirical Formulas

Learning Check 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 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) = 84.07g % C = 84.07 g C x 100 = 64.58 % C 130.18 g

A. 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 CH2O actual (molecular) formula empirical 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

Learning Check 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 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

B. Determining an Empirical Formula Percent to mass (assume 100 g sample) Mass to mole Divide by small Multiply until whole

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

Solution Convert 7.31 g Ni and 20.0 g Br to moles. ______ = = Divide by smallest: 0.125 mol Ni = 1 Ni 0.250 mol Br = 2 Br 0.125 0.125 Write ratio as subscripts: NiBr2 7.31 g Ni 1 mol Ni 0.125 mol Ni 58.69 g Ni 20. 0 g Br 1 mol Br 0.250 mol Br 79.90 g Br

C. 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

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

Solution 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 1 mol C = 5.00 mol C 12.01 g C 4.5 g H 1 mol H = 4.5 mol H 1.008 g H 35.5 g O 1mol O = 2.22 mol O 16.00 g O

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

Solution (continued) Step 3: Multiply until whole numbers (this case, x4) 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

Practice A compound is found to contain 30.5% nitrogen and 69.5% oxygen. Determine its empirical formula.