Molecular and Empirical Formulas
Percentage Composition:
• Mass of each element compared to the mass of the
compound (m/m) or volume of each compared to the
total volume (v/v)
Method (m/m)
• Find the mass (molar or given) of the sample
• Divide the mass of each atom by the mass of the
sample weight
• Multiply by 100%
• The total of all percentages must equal 100%
Molecular Formula
• Formula that gives the exact number of elements in a molecule or
compound.
• Find the % Composition for each element in the compound.
– If given % Composition: start with the % composition listed as decimals
• Find the contributing mass of each element
– Multiply each % Composition by the sample
weight of the compound to find the contributing
mass
• Do a mass to mole conversion for each element
• Assemble the formula
– The moles of each become the new subscripts
of your compound
Empirical Formula
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Same steps as finding the molecular formula
Find the sample weight of the compound
Use % composition to find the contributing mass
Do the mass to mole conversion for each element
Divide all by the smallest and round to whole numbers
Assemble the formula
– The ratios expressed as whole numbers become the subscript numbers of the
atoms