 Molecular and Empirical Formulas: (10-3)

Percentage Composition: Mass of each element compared to the mass of the compound. Method
1. Find the mass (molar or given) of the sample
2. Divide the mass of each atom by the mass of the sample weight
3. Multiply by 100%
4. 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
1. Reduce the coefficients method - complex
1. Find the sample weight of the compound
2. 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)
3. Do the mass to mole conversion for each element (see example)
4. Divide all by the smallest and round to whole numbers (see example below)
5. 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%)
1. Find the sample weight of the compound
2. Divide the mass of each atom by the mass of the sample weight
3. Multiply the element percentages (expressed as decimals) by the molar weight of the compound or molecule (see example)
4. Do a mass to mole conversion for each element (see example)
5. Assemble the formula The ratios expressed as whole numbers umber of moles of each element become the subscript numbers of the atoms

example: 