SECTION 4.4 CONTINUED

Molecular formula (True formula)

For covalent compounds the empirical formula may or may not be the true molecular formula.

The molecular formula gives the actual number of atoms of each type of element found in one molecule of the compound

Empirical Formula Molecular Formula Molar Mass

CH

none 13

C2H2 (ethyne or acetylene)

2 X 13 = 26

C6H6 (benzene)

6 X 16 = 78

C8H8 (styrene) 8 X 13 = 104

CH2OCH2O (methanol)

30

C2H4O2 (ethanoic acid)

2 X 30 = 60

C6H12O6 (glucose)

6 X 30 = 180

If the molar mass found using the empirical formula is the same as the actual molar mass of the compound then the empirical formula is the molecular formula.

If they are not the same then the actual molar mass will be some whole number multiple of the empirical mass and the molecular formula will be the same whole number multiple of the empirical formula.

We will follow the following steps to find the molecular formula.

1. Write down the empirical formula and calculate the empirical mass.

2. Set up the following ratio. actual molar mass empirical mass3. Multiply the subscripts in the empirical

formula by the whole number from this ratio.

The empirical formula for a compound of sulfur and chlorine is SCl. The known molar mass of this compound 135 g/mol. Determine the molecular formula of the compound.

A compound contains 40% carbon, 6.70% hydrogen, and 53.3% oxygen by mass. The mass of 1 mole of the compound is 90.1 g. Determine the molecular formula of the compound

Element Mass Molar Mass

Number of moles

Ratio Empirical formula

A hydrated salt is a salt with water molecules attached to it. i.e. CuSO4

.5H2O An anhydrous salt is a salt with no water

molecules attached to it. i.e. CuSO4

Hydrated salt anhydrous salt + water

When a 5.742 g of hydrated magnesium sulfate is heated until all the water has been released, 2.801 g of anhydrous magnesium sulfate remains. Determine the formula of hydrated magnesium.

Mass Molar mass

Number of moles

Ratio Formula

Carbon-Hydrogen analysis

A hydrocarbon is any compound that contains only hydrogen and carbon.

When a hydrocarbon undergoes complete combustion the only products will be carbon dioxide and water.

Hydrocarbon + oxygen carbon dioxide + water

A sample of an unknown compound containing only C and H is burned in a C-H analyzer: 3.94 g of water and 9.62 g of CO2 are produced. The measured molar mass is 84.0 g/mol. Determine the molecular formula of the compound

C?H? + O2 H2O + CO2

3.94 g 9.62 g

Element Number of moles

Ratio Formula

Assignment

Lab prep is to be completed for Tuesday The following questions must be

completed for the day after the lab. Page 159 practice problem Page 162 practice problem Page 16#5 1,3,4,6,7,8,10,11 Quiz on empirical and molecular formula

on Wed. April 27