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Empirical & Molecular Formulas

Empirical & Molecular Formulas. Percent Composition. Determine the elements present in a compound and their percent by mass . A 100g sample of a new compound contains 55g of element X and 45g of element Y Percent by mass can be calculated using

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Empirical & Molecular Formulas

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  1. Empirical & Molecular Formulas

  2. Percent Composition • Determine the elements present in a compound and their percent by mass. • A 100g sample of a new compound contains 55g of element X and 45g of element Y • Percent by mass can be calculated using Mass of element X 100 = percent by mass Mass of compound

  3. Percent Composition • The percents of all elements in a compound must equal 100. • The percent by mass of each element in a compound is called the percent composition of a compound

  4. Percent Composition from the Chemical Formula • If you already know the chemical formula for a compound you can calculate its percent composition • Calculate the molar mass of each element and compound formula unit a. Assume sample size is one mole b. Multiply the molar mass of the element by its subscript in the chemical formula • Divide the mass of the element by the molar mass of the compound unit and multiply by 100

  5. Example • What is the percent composition of water? • Hydrogen – 1.01 x 2 = 2.02 g H in water Oxygen – 15.99 x 1 = 15.99 g O in water Molar mass of water = 2.02 + 15.99 = 18.02 g/mol of H2O • % of Hydrogen 2.02 g H x 100 = 11.2 % H in Water 18.02 g H2O % of Oxygen 15.99 g O x 100 = 88.8 % O in Water 18.02 g H2O

  6. General Equation for Calculating % by mass Mass of element in 1 mol compound X 100 Molar mass of compound

  7. Example Problem • Determine the percent composition of Sodium Hydrogen Carbonate (NaHCO3) Na = 1* 22.99 g = 22.99 g Na H = 1* 1.01 g = 1.01 g H C = 1* 12.01 g = 12.01 g C O = 3* 15.99 g = 47.97 g O 83.98 g/mol NaHCO3

  8. Example Problem Continued Sodium 22.99 g Na x 100 = 27.4 % Na 83.98 g NaHCO3 Hydrogen 1.01 g H x 100 = 1.2 % H 83.98 g NaHCO3 Carbon 12.01 g C x 100 = 14.3 % C 83.98 g NaHCO3 Oxygen 47.97g O x 100 = 57.1 % O 83.98 g NaHCO3

  9. Empirical Formula • The formula with the smallest whole number mole ratio of the elements • May or may not be the same as the molecular formula • If the two formulas are different the molecular formula will always be a simple multiple of the empirical formula • The empirical formula for hydrogen peroxide is HO • The molecular formula for hydrogen peroxide is H2O2

  10. Empirical Formula • If the % composition is given you can assume the total mass of the compound is 100 grams and that the percent by mass of each element is equal to the mass of that element in grams • The mass of each element can be converted to moles by dividing the molar mass of the element

  11. Example • The percent composition of a sulfur oxide is 40.05 % S and 59.95 % O. 40.05 g S = 1.249 mol S 32.07 g/mol S 59.95 g O = 3.749 mol O 15.99 g/mol O

  12. How does 1.249 mol of S and 3.749 mol O transfer into subscripts? They are not in whole numbers. • Divide the mole values by the value of the element with the smallest number of moles. • 1.249 mol S = 1 mol S 1.249 • 3.749 mol O = 3 mol O 1.249

  13. The simplest whole number mole ratio of S atoms to O atoms is 1:3. • The empirical formula for the oxide of sulfur is SO3 • The calculated mole values may not always be whole numbers • In these cases all the mole values must be multiplied by the smallest factor that will make them whole numbers

  14. % to Mass • Mass to Mole • Divide by smallest • Multiply until whole

  15. Practice Problems 48. Propane is hydrocarbon, a compound composed only of carbon and hydrogen. It is 81.82% carbon and 18.18% hydrogen. What is the empirical formula?

  16. The chemical analysis of aspirin indicates that the molecule is 60.00% carbon, 4.44% hydrogen, and 35.56% oxygen. Determine the empirical formula for aspirin.

  17. Molecular Formulas • Two or more substances with distinctly different properties can have the same percent composition and the same empirical formula • The subscripts in an empirical formula indicate the simplest whole-number ratio of moles of the elements in a compound • The simplest ratio does not always indicate the actual number of moles in the compound

  18. Determining Molecular Formulas • A molecular formula specifies the actual number of atoms of each element in one molecule or formula unit of the substance • The molar mass must be determined through experimentation and compared with the mass represented by the empirical formula

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