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Empirical and Molecular Formulas

Learn how to calculate the percent composition of a compound, determine empirical and molecular formulas from mass percent and actual mass data.

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Empirical and Molecular Formulas

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  1. Empirical and Molecular Formulas Section 11.4 Chemistry

  2. Objectives • Explain what is meant by the percent composition of a compound. • Determine the empirical and molecular formulas for a compound from mass percent and actual mass data.

  3. Key Terms • Percent composition • Empirical formula • Molecular formula

  4. Percent Composition • Percent by mass of each element in a compound. • Mass of Element x 100 Mass of Compound

  5. Example • Calculate percent composition of H in H2O • Molar mass of water: 18.02 g/mol • Determine mass of H in 1 mol of H2O

  6. Mass of H • 1.01 x 2 = 2.02 g H in 1 mol water Atomic mass of H from periodic table Number of H in H2O

  7. Percent Composition of H • 2.02 g of H x 100 = 11.2% H 18.02 g of H2O

  8. Empirical Formula • Formula for a compound with the smallest whole-number ratio of elements. • Percent composition can be used to find the chemical formula.

  9. Empirical Formula • When given percent composition, assume: • The total mass of the compound is 100 g • The percent composition of the element is equal to the mass in grams of the element.

  10. Example • A compound has a percent composition of 40.05% S and 59.95% O. • So in 100 g of the compound, 40.05 g are S and 59.95 g are O. • Find the amount of mol for each element.

  11. Empirical Formula • 40.05g S x 1 mol S = 1.249 mol S 32.07g S • 59.95g O x 1 mol O = 3.747 mol O 16.00g O

  12. Empirical Formula • The element with the smallest number of mol gets the subscript 1.

  13. Empirical Formula • S has a subscript 1 • Then divide the mol O by the mol S • 3.747 mol O/ 1.249 mol S = 3 mol O • Then write your empirical formula using your subscripts: SO3

  14. Practice Problems • Pg. 333

  15. Molecular Formula • Formula that specifies the actual number of atoms of each element in one molecule or formula unit of a substance. • n = molecular formula mass empirical formula mass

  16. Example • Compound is composed of 40.68% carbon, 5.08% hydrogen, and 54.24% oxygen and has a mass of 118.1 g/mol. Determine the empirical and molecular formulas for succinic acid.

  17. Example • 40.68 g C x 1 mol C = 3.387 mol C 12.01 g C • 5.08 g H x 1 mol H = 5.04 mol H 1.008 g H • 54.24 g O x 1 mol O = 3.390 mol O 16.00 g O

  18. Example • 3.387 mol C/ 3.387 = 1 mol C • 5.040 mol H/ 3.387 = 1.49 = 1.5 mol H • 3.390 mol O/ 3.387 = 1.001 = 1 mol O • Ratio of C : H : O = 1 : 1.5 : 1

  19. Example • Empirical Formula: C2H3O2

  20. Example • To find molecular formula, calculate n. • n = molecular formula mass empirical formula mass • Molecular mass is in the problem! • Calculate molar mass of empirical

  21. Example • n = 118.1 = 2 59.04 • Multiply the subscripts of the empirical by n to find the molecular formula. • Molecular Formula: C4H6O4

  22. Practice Problems • Pg 335

  23. Homework • Section 11.4 Problems 27-29 on page 877

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