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Empirical and Molecular Formulas: Understanding Compound Ratios

This chapter discusses empirical and molecular formulas for compounds, their differences and definitions, and how to determine the molecular formula from the empirical formula. It also covers finding the simplest form using mass percentages and moles.

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Empirical and Molecular Formulas: Understanding Compound Ratios

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  1. Chapter 9 Empirical Formulas Timberlake LecturePLUS

  2. Types of Formulas The formulas for compounds can be expressed as an empirical formula and as a molecular(true) formula. Empirical Molecular (true) Name CH C2H2 acetylene CH C6H6 benzene CO2 CO2 carbon dioxide CH2O C5H10O5 ribose Timberlake LecturePLUS

  3. Empirical Formulas Write your own one-sentence definition for each of the following: Empirical formula Molecular formula Timberlake LecturePLUS

  4. An empirical formula represents the simplest whole number ratio of the atoms in a compound. • The molecular formula is the true or actual ratio of the atoms in a compound. Timberlake LecturePLUS

  5. Learning Check EF-1 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. What is a molecular formula for CH2O? 1) CH2O 2) C2H4O2 3) C3H6O3 Timberlake LecturePLUS

  6. Solution EF-1 A. What is the empirical formula for C4H8? 2) CH2 B. What is the empirical formula for C8H14? 1) C4H7 C. What is a molecular formula for CH2O? 1) CH2O 2) C2H4O2 3) C3H6O3 Timberlake LecturePLUS

  7. Learning Check EF-2 If the molecular formula has 4 atoms of N, what is the molecular formula if SN is the empirical formula? Explain. 1) SN 2) SN4 3) S4N4 Timberlake LecturePLUS

  8. Solution EF-2 If the molecular formula has 4 atoms of N, what is the molecular formula if SN is the empirical formula? Explain. 3) S4N4 If the actual formula has 4 atoms of N, and S is related 1:1, then there must also be 4 atoms of S. Timberlake LecturePLUS

  9. Empirical and Molecular Formulas molar mass = a whole number = n simplest mass n = 1 molar mass = empirical mass molecular formula = empirical formula n = 2 molar mass = 2 x empirical mass molecular formula = 2 x empirical formula molecular formula = or > empirical formula Timberlake LecturePLUS

  10. Molecular Formula Empirical Formula Molecular Mass Empirical Mass Timberlake LecturePLUS

  11. Learning Check EF-3 A compound has a formula mass of 176.0 and an empirical formula of C3H4O3. What is the molecular formula? 1) C3H4O3 2) C6H8O6 3) C9H12O9 Timberlake LecturePLUS

  12. Solution EF-3 A compound has a formula mass of 176.0 and an empirical formula of C3H4O3. What is the molecular formula? 2)C6H8O6 C3H4O3 = 88.0 g/EF 176.0 g = 2.00 88.0 Timberlake LecturePLUS

  13. Learning Check EF-4 If there are 192.0 g of O in the molecular formula, what is the true formula if the EF is C7H6O4? 1) C7H6O4 2) C14H12O8 3) C21H18O12 Timberlake LecturePLUS

  14. Solution EF-4 If there are 192.0 g of O in the molecular formula, what is the true formula if the EF is C7H6O4? 3) C21H18O12 192 g O = 3 x O4 or 3 x C7H6O4 64.0 g O in EF Timberlake LecturePLUS

  15. Finding the Molecular Formula A compound is Cl 71.65%, C 24.27%, and H 4.07%. What are the empirical and molecular formulas? The molar mass is known to be 99.0 g/mol. 1. State mass percents as grams in a 100.00-g sample of the compound. Cl 71.65 g C 24.27 g H 4.07 g Timberlake LecturePLUS

  16. 2. Calculate the number of moles of each element. 71.65 g Cl x 1 mol Cl = 2.02 mol Cl 35.5 g Cl 24.27 g C x 1 mol C = 2.02 mol C 12.0 g C 4.07 g H x 1 mol H = 4.04 mol H 1.01 g H Timberlake LecturePLUS

  17. Why moles? Why do you need the number of moles of each element in the compound? Timberlake LecturePLUS

  18. 3. Find the smallest whole number ratio by dividing each mole value by the smallest mole values: Cl: 2.02 = 1 Cl 2.02 C: 2.02 = 1 C 2.02 H: 4.04 = 2 H 2.02 4. Write the simplest or empirical formula CH2Cl Timberlake LecturePLUS

  19. 5. EM (empirical mass) = 1(C) + 2(H) + 1(Cl) = 49.5 6. n = molar mass/empirical mass Molar mass = 99.0 g/mol = n = 2 E M 49.5 g/EM 7.Molecular formula (CH2Cl)2 = C2H4Cl2 Timberlake LecturePLUS

  20. Learning Check EF-5 Aspirin is 60.0% C, 4.5 % H and 35.5 O. Calculate its simplest formula. In 100 g of aspirin, there are 60.0 g C, 4.5 g H, and 35.5 g O. Timberlake LecturePLUS

  21. Solution EF-5 60.0 g C x ___________= ______ mol C 4.5 g H x ___________ = _______mol H 35.5 g O x ___________ = _______mol O Timberlake LecturePLUS

  22. Solution EF-5 60.0 g C x 1 mol C= 5.00 mol C 12.0 g C 4.5 g H x 1 mol H = 4.5 mol H 1.01 g H 35.5 g O x 1mol O = 2.22 mol O 16.0 g O Timberlake LecturePLUS

  23. Divide by the smallest # of moles. 5.00 mol C = ________________ ______ mol O 4.5 mol H = ________________ ______ mol O 2.22 mol O = ________________ ______ mol O Are are the results whole numbers?_____ Timberlake LecturePLUS

  24. Divide by the smallest # of moles. 5.00 mol C = ___2.25__ 2.22 mol O 4.5 mol H = ___2.00__ 2.22 mol O 2.22 mol O = ___1.00__ 2.22 mol O Are are the results whole numbers?_____ Timberlake LecturePLUS

  25. Finding Subscripts A fraction between 0.1 and 0.9 must not be rounded. Multiply all results by an integer to give whole numbers for subscripts. (1/2) 0.5 x2 = 1 (1/3) 0.333 x 3 = 1 (1/4) 0.25 x4 = 1 (3/4) 0.75 x 4 = 3 Timberlake LecturePLUS

  26. Multiply everything 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 Use the whole numbers of mols as the subscripts in the simplest formula C9H8O4 Timberlake LecturePLUS

  27. Learning Check EF-6 A compound is 27.4% S, 12.0% N and 60.6 % Cl. If the compound has a molar mass of 351 g/mol, what is the molecular formula? Timberlake LecturePLUS

  28. Solution EF 6 0.853 mol S /0.853 = 1 S 0.857 mol N /0.853 = 1 N 1.71 mol Cl /0.853 = 2 Cl Empirical formula = SNCl2 = 117.1 g/EF Mol. Mass/ Empirical mass 351/117.1 = 3 Molecular formula = S3N3Cl6 Timberlake LecturePLUS

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