Molecular Formulas

An empirical formula shows the lowest whole-number ratio of the elements in a compound, but may not be the actual formula for the molecular compound, called the molecular formula. The molecular formula is always some whole number multiple of the empirical formula.

For Example CompoundE.F.M.F.Multiple WaterH2OH2OH2OH2O1 Hydrogen PeroxideHOH2O2H2O2 2 GlucoseCH 2 OC 6 H 12 O 6 6

Review of Empirical and Molecular Formulas

To find the molecular formula it is necessary to know the empirical formula and the multiple. Often the empirical formula is calculated from the percent composition, and then the multiple is calculated by knowing the actual molar mass of the molecular compound.

For example: A compound has a formula mass of g/mol and is 92.24% carbon and 7.76% hydrogen. What is the molecular formula?

Step 1: Determine the empirical formula:

92.24 g C = 7.68 mols g/mol 7.76 g H = 7.68 mols 1.01 g/mol

Step 1: Determine the empirical formula: Dividing both by 7.68 moles gives an empirical formula of CH g C = 7.68 mols g/mol 7.76 g H = 7.68 mols 1.01 g/mol

Step 2: Determine the molar mass of the empirical formula:

CH = g/mol

Step 3: Determine the factor:

molecular formula mass = empirical formula mass =13.02 = or 6

Step 3: Determine the factor: Thus the actual molecular formula is 6 times more than the empirical formula, or C 6 H 6. molecular formula mass = empirical formula mass =13.02 = or 6