Lecture Notes:

Empirical, Molecular, and Structural Formulas

• Empirical Formulas
  •  An empirical formula shows the smallest whole number ratio of the atoms
  • It does not show how many of each atom are in each molecule
  • All ionic compounds are expressed using empirical formulas
  • Completely different compounds can have the same empirical formula:
    CH₂O can be C₆H₁₂O₆ (glucose) or C₂H₄O₂ (vinegar) or even just CH₂O (formaldehyde)
  • What's different is their total molar mass: glucose is 180.18 g/mol, vinegar is 60.06 g/mol and formaldehyde is 30.03 g/mol
  • See the examples page for problems showing how to find the empirical formula using a mass percentage
• Molecular Formulas
  •  Molecular formulas show exactly how many of each atom is present in each molecule
  • Empirical formulas are related to molecular formulas in that the latter can be found using the former; that is, if you know the molar mass and the empirical formula you can find out the molecular formula
  • If you divide the molar mass of the compound by the formula weight of the empirical formula you find a factor that can be used to calculate the molecular formula’ subscripts
  • Just multiply the subscripts of the empirical formula by the factor you found; again see the examples page.
• Structural Formulas
  •  A structural formula shows the shape of a molecule
  • Molecules with the same molecular formula can have different structural formulas
  • These molecules (called isomers) have different properties from one another by virtue of a rearrangement of their constituent atoms
  • An example is C₄H₁₀O which can be 1-butanol, 2-butanol, diethyl ether, or methyl propyl ether.