Empirical and Molecular FormulasTextbook Section 7.4

Today’s Learning Target

1. Determine the empirical formula given percent data.

Empirical Formula

Definition: Formula with the lowest whole-number ratio between the elements

Examples:

Empirical Formula Calculations

Reverse of Percent Composition:

1. Change percent signs to grams.2. Using PT, convert grams to moles.3. Divide by the smallest number of moles.4. Will give you the lowest ratio between the elements.

Empirical Formula Examples

1. Determine the empirical formula of the compound that contains 17.15% carbon, 1.44% hydrogen and 81.41% fluorine.

Empirical Formula Examples

2. Determine the empirical formula for the compound that is 40.00% C, 6.72% H, and 53.29% O.

HOMEWORK

Worksheet Questions 1-4, 10

Today’s Learning Target

1. Determine the molecular formula given the molar mass and empirical formula.

Molecular Formula

Definition: Formula showing the actual number of each element present in the compound.

To Calculate, need two things:

1. empirical formula2. molar mass of compound [given in problem]

Molecular Formula Examples

1. Determine the molecular formula of a compound with an empirical formula of NH2 and a molar mass of 32.06 g/mol.

2. The empirical formula of hexane is C3H7. It’s molar mass is 86.2 g/mol. What is the molecular formula?

Combining the Two

Caffeine is 49.5% C, 5.15% H, 16.5% O and 28.9% N by mass. From this data calculate the empirical formula. The molar mass of the compound is known to be 194.2 g/mol; determine the molecular formula for caffeine.

HOMEWORK

FINISH WORKSHEET