SECTION 4.4 CONTINUED
Molecular formula (True formula)
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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
CH
CH2O
Molecular Formula
Molar Mass
none
C2H2 (ethyne or
acetylene)
C6H6 (benzene)
C8H8 (styrene)
CH2O (methanol)
C2H4O2 (ethanoic
acid)
C6H12O6
(glucose)
13
2 X 13 = 26
6 X 16 = 78
8 X 13 = 104
30
2 X 30 = 60
6 X 30 = 180
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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 mass
3. 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
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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
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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 +
3.94 g
CO2
9.62 g
Element
Number of
moles
Ratio
Formula
Assignment
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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