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Empirical and Molecular Formulas

Empirical and Molecular Formulas. Part 1: Definitions. Objectives. Upon completion of this presentation, you will be able to define empirical formula define molecular formula use the terms empirical formula and molecular formula to describe the composition of ionic compounds

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Empirical and Molecular Formulas

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  1. Empirical and Molecular Formulas Part 1: Definitions

  2. Objectives • Upon completion of this presentation, you will be able to • define empirical formula • define molecular formula • use the terms empirical formula and molecular formula to describe the composition of • ionic compounds • molecular compounds

  3. Introduction • Chemical formulas tell us about the number of atomsin a compound. • In general, there are two kinds of chemical formulas. • In molecularformulas, the total number of atoms in the compound is used. • In empiricalformulas, the lowest whole number ratio of atoms in the compound are used.

  4. Introduction • Molecular formulas are used with molecular(covalent) compounds. • The formula reflects the totalnumberof atoms in the compound. • We do not use molecular formulas with ioniccompounds. • Empirical formulas are most often used with ioniccompounds. • The formula reflects the lowest whole number ratioof the atoms or ions in the compound. • We use empirical formulas because the ioniccrystalstructureis composed of a certain whole number ratio of cations and anions. • Occasionally, we will use empirical formulas with molecularcompounds.

  5. Molecular Formulas • In molecular formulas, the totalnumber of atomsin the compound is used. • For example, benzene has 6 carbon atoms and 6 hydrogen atoms in each molecule. • Therefore, its molecular formula is C6H6.

  6. Molecular Formulas • In molecular formulas, the total number of atoms in the compound is used. • For example, acetic acid has 2 carbon atoms, 4 hydrogen atoms, and 2 oxygen atoms in each molecule. • Therefore, its molecular formula is C2H4O2.

  7. Molecular Formulas • In molecular formulas, the total number of atoms in the compound is used. • For example, propane gas has 3 carbon atoms and 8 hydrogen atoms in each molecule. • Therefore, its molecular formula is C3H8.

  8. Empirical Formulas • In empirical formulas, the lowestwholenumberratioof atoms in the compound are used. • For example, benzene has 6 carbon atoms and 6 hydrogen atoms in each molecule. • Therefore, its molecular formula is C6H6 and its empirical formula is CH(we divide all subscripts by 6).

  9. Empirical Formulas • In empirical formulas, the lowest whole number ratio of atoms in the compound are used. • For example, acetic acid has 2 carbon atoms, 6 hydrogen atoms, and 2 oxygen atoms in each molecule. • Therefore, its molecular formula is C2H4O2. and its empirical formula is CH2O(we divide all subscripts by 2).

  10. Empirical Formulas • In empirical formulas, the lowest whole number ratio of atoms in the compound are used. • For example, propane gas has 3 carbon atoms and 8 hydrogen atoms in each molecule. • Therefore, its molecular formula is C3H8 and its empirical formula is also C3H8(there is nocommondivisorfor all subscripts).

  11. Examples Find the empirical formulas of each of the following molecular compounds. • acetic acid, CH3CO2H (= C2H4O2): • glucose, C6H12O6: • methane, CH4: • ethane, C2H6: • Ethanol, CH3CH2OH (= C2H6O): CH2O CH2O CH4 CH3 C2H6O

  12. Summary • Chemical formulas tell us about the number of atoms in a compound. • In general, there are two kinds of chemical formulas. • In molecular formulas, the total number of atoms in the compound is used. • In empirical formulas, the lowest whole number ratio of atoms in the compound are used.

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