Molecular and Empirical Formulas: (103)
Percentage Composition:
 Mass of each element compared to the mass of
the compound. 
 Method 
 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%
example:
Empirical Formulas:
 Formula that gives the simplest whole
number ratio of the atoms in a compound 
 method  simple 
 Reduce the coefficients
 method  complex 
 Find the sample weight of the compound
 If not given find the mass of each element in
the compound (multiply the sample weight by the percent compositions of the
elements in the compound)
 Do the mass to mole conversion for each
element (see example)
 Divide all by the smallest and round
to whole numbers (see example below)
 Assemble the formula
 The ratios expressed as whole numbers
become the subscript numbers of the atoms 
example: (Step 3)
(Step 4)
(www.iun.edu/.../C101webnotes/ quantchem/images/empcal1.jpg)
Molecular Formulas:
 Formula that gives the exact number of
elements in a molecule or compound. 
 method (start with the % composition steps
without multiplying by 100%) 
 Find the sample weight of the compound
 Divide the mass of each atom by the mass of
the sample weight
 Multiply the element percentages (expressed as
decimals) by the molar weight of the compound or molecule (see example)
 Do a mass to mole conversion for each element
(see example)
 Assemble the formula
 The ratios expressed as whole numbers
umber of moles of each element become the subscript numbers of the atoms 
example:
