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Percent Composition, Empirical Formulas, Molecular Formulas

Learn about determining the percentage by mass of elements in compounds, empirical formulas, molecular formulas, and calculations for molecular formulas with examples and step-by-step explanations.

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Percent Composition, Empirical Formulas, Molecular Formulas

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  1. Percent Composition, Empirical Formulas, Molecular Formulas

  2. Percent Composition • Percent Composition – the percentage by mass of each element in a compound (*note: not the ratio of elements in the compound) • Law of Definite Proportions – compounds always contain the same proportions of elements by mass. Part _______ Percent = x 100% Whole So… Percent composition of a compound or = molecule Mass of element in 1 mol ____________________ x 100% Mass of 1 mol of compound

  3. Formulas Percent composition allow you to calculate the simplest ratio among the atoms found in compound. Empirical Formula – formula of a compound that expresses lowest whole number ratio of atoms. Molecular Formula – actual formula of a compound showing the number of atoms present Examples: C6H12O6 - molecular C4H10 - molecular - empirical - empirical C2H5 CH2O

  4. Formulas Is H2O2 an empirical or molecular formula? Molecular, it can be reduced to HO HO = empirical formula

  5. Calculating Molecular Formula A white powder is analyzed and found to have an empirical formula of P2O5. The compound has a molar mass of 283.88g/mol. What is the compound’s molecular formula? Step 3: Multiply Step 1: Molar Mass P = 2 x 30.97 g = 61.94g O = 5 x 16.00g = 80.00 g 141.94 g (P2O5)2 = P4O10 Step 2: Divide MM by Empirical Formula Mass 238.88 g = 2 141.94g

  6. Calculating Molecular Formula A compound has an experimental molar mass of 78 g/mol. Its % composition is found to be 92.24% carbon and 7.76% hydrogen. What is its molecular formula of this compound? First, determine the empirical formula using the % composition. Empirical formula of this compound is CH. Determine the multiple (CH)6 = C = 12.01 g/mol H = 1.01 g/mol 13.02 g/mol C6H6 78 g/mol = 6 13.02 g/mol

  7. Homework: Page 298 #1 Page 300 #2

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