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Empirical & Molecular Formula

Empirical & Molecular Formula. Chapter 11-12. The Empirical Formula. The lowest whole number ratio of elements in a compound. The molecular formula is the actual ratio of elements in a compound. CH 2 empirical formula C 2 H 4 molecular formula C 3 H 6 molecular formula.

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Empirical & Molecular Formula

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  1. Empirical & Molecular Formula Chapter 11-12

  2. The Empirical Formula • The lowest whole number ratio of elements in a compound. • The molecular formula is the actual ratio of elements in a compound. • CH2 empirical formula • C2H4 molecular formula • C3H6 molecular formula

  3. Finding Empirical Formulas • Just find the lowest whole number ratio • C6H12O6 • CH4N2 • It is not just the ratio of atoms, it is also the ratio of moles of atoms.

  4. Calculating Empirical Formulas • Means we can get ratio from percent composition. • Assume you have a 100 g. • The percentages become grams. • Turn grams to moles. • Find lowest whole number ratio by dividing everything by the smallest moles.

  5. Example • Calculate the empirical formula of a compound composed of 38.67 % C, 16.22 % H, and 45.11 %N. • Assume 100 g so • 38.67 g C x 1mol C = 3.22 mole C 12.0 gC • 16.22 g H x 1mol H = 16.1 mole H 1.0 gH • 45.11 g N x 1mol N = 3.22 mole N 14.0 gN

  6. Example • The ratio is 3.22 mol C = 1 mol C 3.22 molN 1 mol N • The ratio is 16.1 mol H = 5 mol H 3.22 molN 1 mol N • C1H5N1

  7. Your Turn • Calculate the empirical formula of a compound composed of 25.14 % C, 7.19 % H, and 67.67 %N.

  8. Empirical to molecular • empirical formula = lowest ratio • the actual molecule would weigh the same or more. • Divide the actual molar mass by the molar mass of the empirical formula. • Get a whole number. • Multiply the empirical formula by this.

  9. Example • A compound has an empirical formula of ClCH2 and a molar mass of 98.96 g/mol. What is its molecular formula? • Cl = 35.5 g/mol X 1 = 35.5 g/mol • C = 12.0 g/mol X 1 = 12.0 g/mol • H = 1.0 g.molX 2 = 2.0 g/mol 49.5 g/mol • 98.96 g/mol ÷ 49.5 g/mol = 2 • 2 (ClCH2) = Cl2C2H4 • Gives us the Molecular Formula

  10. Your Turn • A compound has an empirical formula of CH2O and a molar mass of 180.0 g/mol. What is its molecular formula?

  11. Percent to molecular • Take the percent X the molar mass • This gives you mass in one mole of the compound • Change this to moles • You will get whole numbers • These are the subscripts

  12. Example • Caffeine is 49.48% C, 5.15% H, 28.87% N and 16.49% O. It has a molar mass of 194 g. What is its molecular formula? • 49.48% C x 194g = 96g ÷ 12.0g/mol = 8 • 5.15% H x 194g = 10g ÷ 1.0g/mol = 10 • 28.87% N x 194g = 56g ÷ 14.0g/mol = 4 • 16.49% O x 194g = 32g ÷ 16.0g/mol = 2 • C8H10N4O2 Molecular Formula • Empirical Formula…. • C4H5N2O

  13. Your Turn • Ibuprofen is 75.69 % C, 8.80 % H, 15.51 % O, and has a molar mass of about 207 g/mol. What is its molecular formula?

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