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 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.

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

5. Molecular Formulas In molecular formulas, the total number of atoms in the compound is used. For example, benzene has 6 carbon atoms and 6 hydrogen atoms in each molecule. Therefore, its molecular formula is C 6 H 6.

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 C 2 H 4 O 2.

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 C 3 H 8.

8. Empirical Formulas In empirical formulas, the lowest whole number ratio of 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 C 6 H 6 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 C 2 H 4 O 2. and its empirical formula is CH 2 O (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 C 3 H 8 and its empirical formula is also C 3 H 8 (there is no common divisor for all subscripts).

11. Examples Find the empirical formulas of each of the following molecular compounds. 1. acetic acid, CH 3 CO 2 H (= C 2 H 4 O 2 ): 2. glucose, C 6 H 12 O 6 : 3. methane, CH 4 : 4. ethane, C 2 H 6 : 5. Ethanol, CH 3 CH 2 OH (= C 2 H 6 O): CH 2 O CH 4 CH 3 C2H6OC2H6O

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.