Empirical and Molecular Formulas

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.