Unit 6.6-6.7

Empirical and Molecular Formulas

Empirical FormulasConsists of the symbols for the elements

combined in a compound, with subscripts showing the smallest whole-number ratio of the different atoms in the compound.

Huh?!?In other words, its a chemical formula in

the simplest ratio.C2H6 → CH3

Empirical Formula from Percent Composition1) Convert the percent to grams.2) Convert grams to moles by dividing by

the molar mass of each element.3) Divide each amount by the smallest

molar value to determine ratios.4) Multiply each molar ratio until all

elements are whole numbers.Example:What is the empirical formula for a compound

that is 38.77% Cl and 61.23% O by mass?

Empirical Formula Practice ProblemA hydrate is analyzed and has a percent

composition of 13.5% Ca, 10.8 % O, and 0.675% H. The rest of the compound is made of water and is not considered relevant. What is the empirical formula of this compound?

Molecular FormulaWhole number multiple of the empirical

formula.Gives the exact number of atoms in the molecule not just the simplest ratio.

Example:A compound has an empirical formula of

CH2O, but has a mass of 90.0 g/mol. What is the correct molecular formula?

Molecular Formula Practice ProblemHydrogen peroxide is a common household

disinfectant. When analyzed, it has a composition of 5.88% H and 94.12% O. It has a molar mass of 34 g/mol. What is the molecular formula for hydrogen peroxide?