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CHEMISTRY

Learn how to determine empirical and molecular formulas of compounds based on percent composition and molar mass calculations. Practice problems included for better comprehension.

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CHEMISTRY

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  1. CHEMISTRY November 1st, 2011

  2. Brainteaser 11/1/11 • A compound is 30.4% Nitrogen and 69.6% Oxygen. Determine the empirical formula

  3. Molecular Formula • Molecular Formula-actual number of elements in the formula • Can be determined following the same steps as used for empirical formula • The empirical formula of the compound must be known first!! • Once the empirical formula of the compound has been determined, calculate the molar mass of the empirical formula and compare it to the actual molar mass (given).

  4. Empirical vs. Molecular • Empirical: • Like knowing you have 3 bags of apples, but you don’t know how many apples you have in each bag. • Molecular: • Knowing that each apple is 1 gm, and the weight of all the bags combines is 9 grams. • How many apples are in each bag??

  5. Practice Problem • What is the molecular formula of a compound with a percent composition by mass of 65.2% Arsenic and 34.8% Oxygen and has a molar mass of 460 grams?

  6. Practice Problem • A blue solid is found to contain 36.84% Nitrogen and 63.16% Oxygen, what is the empirical formula of this compound?

  7. Molecular formula • Molecular Formulas • To determine the molecular formula of a compound Experimentally determined molar mass = multiple of smallest whole Mass of empirical formula number ratio • For example: • Molar mass of acetylene is 26.04 g/mol and mass of the empirical formula, CH, is 13.02 g/mol • This tells you that the molecular formula is 2X that of the empirical formula. Molecular formula: (CH)2 = C2H2

  8. Molecular Formula • Molecular Formula-actual number of elements in the formula • Can be determined following the same steps as used for empirical formula • The empirical formula of the compound must be known • Once the empirical formula of the compound has been determined, calculate the molar mass of the empirical formula and compare it to the actual molar mass (given).

  9. Practice Problem • What is the molecular formula of a compound with a percent composition by mass of 65.2% Arsenic and 34.8% Oxygen and has a molar mass of 460 grams? • A blue solid is found to contain 36.84% Nitrogen and 63.16% Oxygen, what is the empirical formula of this compound?

  10. Molecular formula • Molecular Formulas • To determine the molecular formula of a compound Experimentally determined molar mass = multiple of smallest whole Mass of empirical formula number ratio • For example: • Molar mass of acetylene is 26.04 g/mol and mass of the empirical formula, CH, is 13.02 g/mol • This tells you that the molecular formula is 2X that of the empirical formula. Molecular formula: (CH)2 = C2H2

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