Empirical and Molecular
Formulas
PERCENT COMPOSITION TOO!
Percent Composition
 Remember…..The percent (by mass) of each element
found in a compound
 Fe2O3
Empirical vs. Molecular Formulas
 Empirical Formula - The chemical formula that
shows the composition of a compound in the
simplest ratio
 Molecular Formula – The chemical formula that
shows the exact number and kinds of atoms in a
compound
 Many compounds will have the same empirical
formulas, but different molecular formulas
Finding Empirical Formulas
 Using percent composition:
 Convert % to grams (since you know 100% of the atoms,
convert that % to grams – 47% = 47 g)
 Now you are working with a 100 g sample
 Use the molar mass of each element to determine the number
of moles of each
 Empirical formulas MUST be written in whole numbers. If the
number of moles of each is not an even number, then you must
adjust……
 Divide all moles by the smallest number of moles
 This will usually give you numbers that are close to whole
numbers – adjust accordingly.
Examples
 69.6% Mn and 30.4% O
 38.77% Cl and 61.23% O
 18.0% C, 2.26% H, and 79.7% Cl
Molecular Formulas
 Are whole number MULTIPLES of empirical
formulas
 Can be found from the Empirical Formula:
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Need to know molar mass of the compound
Find the molar mass of the Empirical Formula
Divide: Molar Mass of Compound by molar mass of
empirical formula
Multiply the empirical formula by the whole number that you
found
Examples
 Empirical Formula – CH
 Molar mass – 78 g/mol
 Empirical Formula – CH2O
 Molar mass – 90.0 g/mol
 Empirical Formula – NO2
 Molar mass – 46 g/mol