Calculating Empirical Formula of an Organic Compound

Did you know that you can use the empirical formula on organic compounds?
Now what is an organic compound?
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compound whose molecules contain carbon.

Ex. When a 3.79 grams of an organic compound is burned, 6.61 grams of CO2 and 3.59 grams of H2O are produced.  What is the empirical formula of this compound?


Molar mass of CO2: 12 + 16 + 16 = 44 g/mol
Mole C = 6.61g x 1 mole CO2 x      1 mole of C   =  0.15023 mole of C
                              44.0 g CO2      1 mole of CO2

Molar mass of H20: 1.0 + 1.0 + 16.0 = 18.0 g/mol
Mole H = 3.59 g x 1 mole of H2O  x     2 moles of H  =  0.39889 mole of H
                              18.0 g H2O          1 mole of H20

Check the mass of C and H
Mass of C = 0.15023 x   12.0 g C = 1.80 g C
                                      1 mole C

Mass of H = 0.39889 x   1.0 g H   =  0.39889 g H
                                     1 mole H
Since the masses of C and H do not add up to 3.79, the rest of the mass must be from O.

Mass of O = 3.79 - 1.80 - 0.39889 = 1.59111

Mole O = 1.59 x 1 mole O  =  0.0994 g O
                           16.0 g O

Now we divide by the smallest molar amount. That is 0.0994

Carbon: 0.15023/0.0994 = 1.5 x 2 = 3
Hydrogen: 0.39889/0.0994 = 4.0 x 2 = 8
Oxygen: 0.0994/0.0994 = 1 x 2 = 2

Empirical formula: C2H8O2

This video will explain determining the empirical formula of an organic compound: