Empirical and Molecular Formulas

• Molecular Formula: The true number of atoms of each element in the formula of a compound

• Empirical Formula: The lowest whole number ratio of atoms in a compound.

  • Molecular formula = (empirical formula)_n

  • Molecular formula = C₆H₆ = (CH)₆

  • Empirical formula = CH

Empirical and Molecular Formulas

To go from a molecular formula to an empirical formula we divide the subscripts by the greatest common factor.

Ex. If the molecular formula is C₂H₂O₄, What is the empirical formula?

1.    Determine the greatest common factor _______

2.    Divide subscripts by that factor to get the empirical formula __________________

Formulas (continued)

• Formulas for ionic compounds are ALWAYS empirical (lowest whole number ratio).

• Examples: NaCl, MgCl₂, Al₂(SO₄)₃, K₂CO₃

Formulas for molecular compounds MIGHT be empirical (lowest whole number ratio)

• Molecular: H₂O Empirical: H₂O

• Molecular: C₆H₁₂O₆ Empirical: CH₂O

Try These

Formula GCF Empirical

H₂O₂

Fe₂(SO₄)₃

Na₂B₄O₇

6 NaCl

2 C₄H₅N₂O

What is the GCF which allows you to switch from HO to H₂O₂? ______________

What is the difference in mass between HO & H₂O₂? ___________________

The GCF which we multiply each atom to find the molecular formula will be the same as the multiple which allows us to switch from the empirical formula mass to the molecular formula mass.

Ex. Empirical formula of a substance is CH₂O. Molar mass is 180 g. What is the molecular formula?

1. Empirical mass ________________

2. Molecular mass/ empirical mass = ______(this is our multiple)

3. Multiply empirical formula by the multiple

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