4.3 - Determining Empirical and Molecular Formulas

What is an empirical formula?

The empirical formula expresses the simplest whole-number ratio of atoms in a compound.

What is a molecular formula?

The molecular formula shows the actual number of each type of atom in a molecule.

Can the empirical and molecular formulas be the same? Give an example.

Yes. For water (H₂O), both the empirical and molecular formulas are the same because the 2:1 ratio cannot be simplified.

What is the molecular formula of hydrogen peroxide?

H₂O₂

What is the empirical formula of hydrogen peroxide (H₂O₂)?

HO (simplified 2:2 ratio to 1:1)

Do ionic compounds have molecular formulas?

No. Ionic compounds like CaCl₂ or CuO only have empirical formulas because they form extended structures, not molecules.

What is the empirical formula of silicon dioxide (quartz)?

SiO₂ (1:2 ratio of silicon to oxygen)

Why can’t molecular formulas be written for quartz or copper(II) oxide?

Because these substances form extended networks or crystal lattices, not individual molecules.

What is the molecular and empirical formula of benzene?

Molecular: C₆H₆; Empirical: CH

What is the molecular and empirical formula of acetylene?

Molecular: C₂H₂; Empirical: CH

What is the empirical formula of a compound with the molecular formula C₄H₁₀?

C₂H₅ (divided by 2)

Why is the molecular formula more informative than the empirical formula?

It shows the actual number of atoms in a molecule, not just the ratio.

Substance: Cyclopentane. What are the formulas?

Molecular formula: C₅H₁₀
Empirical formula: CH₂

How do you determine the empirical formula from a molecular formula?

Divide all subscripts by the greatest common factor.