Molecular & Empirical Formulas

• Molecular formula of a substance:

  Shows the actual number of atoms of each element present in a compound or molecule.
• The molecular formula for glucose is C6H12O6:


  Meaning a glucose molecule contains:

  • 6 carbon atoms
  • 12 hydrogen atoms
  • 6 oxygen atoms
• Empirical formula of a substance:

  The simplest, whole number ratio of atoms of each element in a compound.

  • Meaning it doesn't tell you exactly how many atoms there are, just the ratio of the atoms of each element. 
• The empirical formula for glucose is CH2O:

  Meaning the ratio of carbon : hydrogen : oxygen is 1 : 2 : 1.

  • for every 1 carbon atom, there's 2 hydrogen atoms & 1 oxygen atom. 
• Molecular formulas:

  • CO2
  • H2O
  • N2O4
  • P4O10
  • C10H22
  • C6H18
  • O3

  Empirical formulas:

  • CO2
  • H2O
  • NO2
  • P2O5
  • C5H11
  • C2H6O
• To find the empirical formula from the molecular formula, find the simplest whole number ratio of the atoms in the compound.
• Find the empirical formula of ethane (C2H6):

  With the molecular formula, the ratio of C : H atoms is 2 : 6

  To get the empirical formula, simplify the ratio:

  • 2 : 6 turns into 1 : 3
  • Meaning 1 carbon atom for every 3 hydrogen atoms.

  The empirical formula for ethane is CH3