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Empirical & Molecular Formulas

Empirical & Molecular Formulas. Percent Composition. Def – the percent by mass of each element in a compound Percent by mass = mass of element x 100 mass of compound Example : Find the percent composition of the following compound: NH 4 Cl N = 26.1%, H = 7.5%, Cl = 66.4%

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Empirical & Molecular Formulas

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  1. Empirical & Molecular Formulas

  2. Percent Composition • Def – the percent by mass of each element in a compound Percent by mass = mass of element x 100 mass of compound • Example: Find the percent composition of the following compound: NH4Cl • N = 26.1%, H = 7.5%, Cl = 66.4% • Make sure all percentages equal 100%!!

  3. Empirical Formula • Def – the smallest whole number mole ratio of each element in a compound • Assume that all percentages are out of 100 g, meaning the percentages can now be the amount of grams (assuming there is 100 grams present) • Example: The percent composition of a compound is 40.05% S and 59.95% O. What is the empirical formula of the compound? • SO3

  4. Molecular Formula Part I • Def – the actual number of atoms of each element in one molecule or formula unit of the substance • Molar mass of the unknown formula MUST be given!! n = experimentally determined molar mass mass of empirical formula • Multiply the empirical formula by n to get the molecular formula [molecular formula = (empirical formula) x n] • Example: A colorless liquid composed of 46.6% N and 53.32% O has a molar mass of 60.01 g/mol. What is the molecular formula?

  5. Molecular Formula Part II • If you are not given percent composition, you must find them first and then proceed • Example: A compound was found to contain 49.98 g of carbon and 10.47 g hydrogen. The molar mass of the compound is 58.12 g/mol. Determine the molecular formula.

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