3.4 Empirical and Molecular
Formulas
Empirical and Molecular Formulas
• How do we find a chemical formula for and unknown substance?
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Identify elements & # of atoms of each element
Determine % composition by mass
Convert mass values  moles
Formula = empirical formula
• Empirical Formula
• Simplest whole-number ratio of atoms or ions in a compound.
Empirical and Molecular Formulas
• An Empirical Formula does not necessarily provide correct
information about the number of atoms in a molecule – only the
ratio of atoms.
• Different molecules may have the same % composition but contain
different numbers of atoms
• A Molecular Formula is needed to tell us the number and kind of
atoms in a molecule.
Empirical and Molecular Formulas
• A Molecular Formula represents the number of atoms of each
element that make up a single molecule of that compound.
• Example
Formaldehyde
(poison)
Acetic Acid
(vinegar)
Empirical
CH2O
CH2O
Molecular
CH2O
CH3COOH
Calculating Empirical Formulae
• Convert % comp. data to mass data, assuming total mass is 100g.
• Determine number of moles of each element, by dividing mass by
molar mass
• Convert # of moles into whole numbers by dividing each amount in
moles by the smallest number
• Use these as subscripts in formula
Calculating Empirical Formulae - Example
• A compound is 50.91% Zn, 16.04% P, 33.15% O. Find its Empirical
Formula.
Calculating Molecular Formulae
• Determine Molar Mass of Empirical Formula
• Determine Molar Mass of actual compound
• Done experimentally – mass spec.
• Divide experimental molar mass by empirical molar mass & multiply
answer by each subscript.
Determining Molecular Formula - Example
• A compound with the empirical formula CH was analyzed using a
mass spectrometer. Its molar mass was found to be 78 g/mol.
Determine the molecular formula.