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Percent Composition, Empirical Formula and Molecular Formula

Percent Composition, Empirical Formula and Molecular Formula. 10.4 summary. Percent Composition. 1 pizza 2 slices pineapple 6 slices pepperoni 4 slices cheese What % of the pizza slices are pineapple? pepperoni? just cheese?.

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Percent Composition, Empirical Formula and Molecular Formula

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  1. Percent Composition, Empirical Formula and Molecular Formula 10.4 summary

  2. Percent Composition 1 pizza • 2 slices pineapple • 6 slices pepperoni • 4 slices cheese What % of the pizza slices are • pineapple? • pepperoni? • just cheese? (2/12) x 100 = 17% pineapple, (6/12) x 100 = 50% pepperoni, (4/12) x 100 = 33% cheese

  3. Percent Composition of a Compound from experimental data • ex: a lab sample contains 24.02 g carbon and 64.00 g oxygen. • compound mass = 24.02 g + 64.00 g = 88.02 g • % C: (24.02 g / 88.02 g) x 100 = 27.29% C • % O: (64.00 g / 88.02 g) x 100 = 72.71 % O • good because 27.29% + 72.71% = 100%

  4. Percent Composition of a Compound from a chemical formula • ex: find the percent composition of CO2. • MM of C = 12.01 g/mole • MM of O = 16.01 g/mole • molar mass of compound = 12.01 + 2(16.00) = 44.01 g/mole • % C: (12.01 g / 44.01 g) x 100 = 27.29% • % O: (2X16.00 g / 44.01 g) x 100 = 72.71% • good because 27.29% + 72.71% = 100%

  5. Empirical Formula • Empirical means determined experimentally. • If an unknown sample is analyzed and the amount of each element is determined, an “elementary” formula can be created. • smallest, whole-number ratio of moles of each element in the compound. • hydrogen peroxide = HO

  6. Calculating Empirical Formula • If given %, assume 100g sample. • Convert mass of each element to moles. • Divide each of these numbers by the smallest number to force one of them to = 1. • If necessary, multiply by the smallest number possible to make each a whole number. • These whole numbers are the subscripts in the empirical formula called mole ratio. empirical formula = XAYBZC

  7. Molecular Formula • Actual number of atoms of each element in a molecule or formula unit. • hydrogen peroxide = H2O2 • experimental molar mass needs to be provided.

  8. Calculating Molecular Formula • Find molar mass of empirical formula. • Divide experimentally determined molar mass (given), by molar mass of empirical formula. • Has to equal a whole number “n” • Multiply each subscript by n. molecular formula = XnxAYnxBZnxC

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