Chapter 3 - Chemical Formula Example benzene Empirical formula = CH Simplest whole-number ratio Molecular formula =(empirical formula)n[n = integer

Chapter 3 - Chemical Formula Example benzene Empirical...

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1 Example: benzene Empirical formula = CH Simplest whole-number ratio Molecular formula = (empirical formula) n [ n = integer] Molecular formula = C 6 H 6 = (CH) 6 Actual formula of the compound Chemical Formula
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Empirical Formula General rules for Empirical formula from starting from mass percent values: Assume 100 g of total compound Calc. # of moles of each element Divide each value of moles by smallest value If all values are whole numbers, the number for each element is the subscript If not, multiply by an integer to get whole numbers 2
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Molecular Formula Obtain empirical formula Calculate mass of empirical formula Divide molar mass by mass of empirical formula Multiply value by empirical formula 3
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4 Empirical and molecular formulae The composition of adipic acid (precursor for industrial production of nylon) is 49.3% C, 6.9% H, and 43.8% O (by mass). The molar mass of the compound is about 146 g/mol. What is the empirical formula ? What is the molecular formula ?
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General guidelines for empirical formula problems Empirical formula Assume 100 g of total compound Calc. # of moles of each element Divide each value of moles by smallest value If all values are whole numbers, the number for each element is the subscript If not, multiply by an integer to get whole numbers Molecular formula Calculate mass of empirical formula Divide molar mass by mass of empirical formula Multiply value by empirical formula 5
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6 Empirical and molecular formulae The composition of adipic acid is 49.3% C, 6.9% H, and 43.8% O (by mass). The molar mass of the compound is about 146 g/mol. What is the empirical formula ? C 3 H 5 O 2 What is the molecular formula ? C 6 H 10 O 4
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Percent Composition of Compounds and Empirical and Molecular Formulas Vinegar is a mixture of acetic acid (HC 2 H 3 O 2 ) and water. Glucose (C 6 H 12 O 6 ) is a product of photosynthesis. Find the percent by mass composition of acetic acid and glucose. How do your results indicate that percent composition data is incomplete for determining the formula of a compound? What else do you need? 7
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The % composition of a chemical compound is not enough information to determine the molecular formula of a compound so you must also know the molecular mass.
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  • Kenney
  • Chemistry, Review, Molecular formula

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