Empirical and Molecular

Formulas

Definitions

• Empirical Formula: the lowest whole number ratio of elements in a compound. It can be determined from experimental data (either mass composition or % composition)

• Molecular Formula: a formula which describes the actual molecule. May be the same as the empirical formula or a whole # multiple of it

Empirical Molecular

NaClMgCl2

H20CH4

CH

HO

CH2O

None

None

H20CH4

C6H6

H2O2

C6H12O6

To find an empirical formula

A. When given a mass of each element:– Convert grams to moles (use at least 3

significant figures)– Find small whole numbers ratio of

moles. (if ratio is not obvious, divide by the smallest)

B. When given percentage of each element:– Assume sample size is 100 grams– Proceed as above

Example

• A sample was analyzed and found to contain 24.0 grams of nitrogen 4.0 grams of hydrogen. What is the empirical formula?

• An experimental analysis gives the following data for a sample of Freon: .423 g C: 1.34 g F: 2.5 g Cl. What is the empirical formula?

To find a molecular formula

Find the empirical formula

Find the molar mass of the empirical formula (called empirical mass)

Divide the given molecular mass (given)by the empirical mass. This answer will give the appropriate multiple.

Examples

• The empirical formula of a certain nitrogen compound was found to be NO2. The molecular mass is 92.0 g/mol. What is the molecular formula?

• A certain compound was found to contain 84.98% mercury and 15.02% chlorine. Its molecular mass is 472. Find the empirical formula and the molecular formula.