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Empirical Formulas

Learn about empirical formulas, molecular formulas, and how to calculate them using examples such as formaldehyde, acetic acid, and glucose. Discover the process to determine molecular formulas from given percentages of elements by mass.

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Empirical Formulas

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  1. Empirical Formulas formaldehyde CH2O (toxic preservative) acetic acid C2H4O2 (vinegar) glucose C6H12O6 (simple sugar) all have the same basicor “empirical” formula CH2O

  2. empirical formula: • thesmallest whole-number ratio of the atoms in a compound. • Examples: • CH3 HO PbO2 C2H6N • Non-Examples: • C2H6 H2O2 Pb2O4 C6H18N3

  3. Acetylene (C2H2) is a gas used in welder’s torches. • These two compounds of carbon both have the same empirical formula(CH) but different molecular formulas. • Styrene (C8H8) is used in making polystyrene.

  4. What is the empirical formula of a compound that is analyzed and found to contain 25.9% N and 74.1% O? Percent to Mass Mass to Mole Divide by Small Times ‘till Whole

  5. What is the empirical formula of a compound that is analyzed and found to contain 25.9% N and 74.1% O? Percent to Mass Mass to Mole Divide by Small Times ‘till Whole 25.9 g N 74.1 g O x 1 mol N 14.01 g N x 1 mol O 16.00 g O = 1.85 mol N = 4.63 mol O = 1 N = 2.5 O x 2 = 2 N x 2 = 5 O ________ 1.85 ________ 1.85 N2O5

  6. 1,6-diaminohexane is used to make nylon. What is the empirical formula if it is 62.1% C, 13.8% H, and 24.1% N? 62.1 g C 13.8 g H 24.1 g N x 1 mol C 12.01 g C x 1 mol H 1.01 g H x 1 mol N 14.01 g N = 5.17 mol C = 13.66 mol H = 1.72 mol N = 3C = 8H = 1N ________ 1.72 _________ 1.72 ________ 1.72 C3H8N

  7. molecular formula: • awhole-number multiple of the empirical formulaof a compound formaldehyde CH2O acetic acid C2H4O2 glucose C6H12O6 all have the same empirical formula CH2O

  8. Calculate the molecular formula of a compound with the empirical formula CH4N and a molar mass of 60.0 g/mol. molecular mass = multiple of emp. form. empirical mass empirical mass 1(C) + 4(H) + 1(N) = 12.01 + 4(1.01) + 14.01 = 30.06 g/mol 60.0 = 30.06 1.996 ≈ 2 2 (CH4N) = C2H8N2

  9. Calculate the molecular formula of benzene with the empirical formula CH and a molar mass of 78 g/mol. molecular mass = multiple of emp. form. empirical mass empirical mass 1(C) + 1(H) = 12.01 + 1.01 = 13.02 g/mol 78 = 13.02 5.991 ≈ 6 6 (CH) = C6H6

  10. Quick Quiz! • 1. An empirical formula shows the __________ of atoms in a compound. • lowest common number • highest whole ratio • lowest whole number ratio • average ratio

  11. Quick Quiz. • 2. Which of the following is NOT an empirical formula? • CH3 • H2N • CH • C3H6

  12. Quick Quiz. • 3. True or False: • A molecular formula can be the same as an empirical formula. True False Example: Formaldehyde has the empirical formula CH2O, which is its molecular formula as well.

  13. Quick Quiz. • 4. Determine the molecular formula of a compound that contains 40.0 % C, 6.71 % H, and 53.29 % O by mass and has a molar mass of 60.05 g/mol. • C2H4O2 • CH2O • C2H3O • C2H4O

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