EMPIRICAL AND MOLECULAR FORMULASPart 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• molecular compounds

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.

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.

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 C6H6.

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.

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.

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 C6H6 and its empirical formula is CH

(we divide all subscripts by 6).

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).

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 no common divisor for all subscripts).

Examples

Find the empirical formulas of each of the following molecular compounds.

1. acetic acid, CH3CO2H (= C2H4O2):

2. glucose, C6H12O6:

3. methane, CH4:

4. ethane, C2H6:

5. Ethanol, CH3CH2OH (= C2H6O):

CH2O

CH2O

CH4

CH3

C2H6O

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.