Empirical and Molecular Formulas

Topic 8: Empirical and Molecular Formulas

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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 __________________

Multiple Choice

A chemical formula that shows the actual # and kinds of atoms present in one molecule of a compound is called_________

• ionic formula

• covalent formula

• empirical formula

• molecular formula

Multiple Choice

What is the empirical formula for C₄H₆?

• CH

• CH₃

• C₂H₃

• C₄H₆

Multiple Choice

Which pair has the same empirical formula?

• NaCrO₄ and Na₂Cr₂O₇

• C₂H₄O₂ and C₆H₁₂O₆

• C₃H₆O₃ and C₂H₆O₂

• CH₄ and C₂H₆

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

Multiple Choice

What is the empirical formula for N₂S₃?

• NS

• N₃S₂

• N₂S₃

Multiple Choice

C₂H₆O

Which is the correct empirical formula

• CHO

• C₂H₆O

• C₄H₁₄O₂

• None

Multiple Select

Which of the following compounds are both an empirical AND molecular formula (check all that apply)

• C₆H₁₂O₆

• C₁₂H₂₂O₁₁

• H₂O₂

• S₂O₄

• H₂O

Try These

Formula GCF Empirical

H₂O₂

Fe₂(SO₄)₃

Na₂B₄O₇

6 NaCl

2 C₄H₅N₂O

Fill in the Blanks

We can also calculate the molecular formula given the molecular mass and empirical formula.

            What is the molecular mass of HO (in grams)?

Fill in the Blanks

What is the molecular mass of H₂O₂ (in grams)?

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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