Empirical Formula of Organic Compounds

Organic Compounds are classified as any substance that contains carbon.
They can be found by:

ʘ Burning (as it reacts with oxygen gas [O2])

ʘ Collecting and weighing the products

                From the mass of the product, the mass of each element in the original Organic Compound can be found.

Keep in mind that the Empirical Formula of a Organic compound is CxHy.

The balanced chemical equation for the burning of a organic compound can be written as:

CxHy + 2O2 => XCO2 + Y/2H2O

The aim is to find the x and the y (which are the simplest ratio of atoms of mole in the elements)

From the equation, we can see that all of the C and H atoms in CxHy we used while making CO2 and H2O. It can therefore be said that the moles of carbon and hydrogen in the product are equal to the moles of Carbon in CO2 and the moles of Hydrogen in H2O.

Example:
A 3.27g sample is burned and produces 10.27 g of CO2 and 4.20g of H20. What is it's empirical formula?
Step 1: Find the moles of each
CO2  =>  C = 12                   = 44     10.27g CO2 x 1mole CO2/44g CO2 = 0.233 moles of CO2
                O = 16 x 2 = 32

H2O  =>  H = 1 x 2 = 2         = 18     4.20g H2O x 1mole H2O/18g H2O = 0.233 moles of H2O
                O = 16
Step 2: Find the moles of Carbon and Hydrogen
0.233 mol CO2 x 1 mol C/1 mol CO2 = 0.233 mol H
0.233 mol H2O x 2 mol H/1 mol H2O = 0.466 mol C
Step 3: Divide by smaller number
C 0.233 / 0.233 = 1
H 0.466 / 0.233 = 2
Therefore Empirical Formula = CH2
Step 4: Check Masses
0.466 mol H  x 1g H/1 mol H    = 0.466 g H
0.233 mol C x 12g C/1 mol C  = 2.80 g H
0.466g H + 2.80g C = 3.27g
If there is a difference between the mass of carbon and hydrogen versus the mass of the compound, there is oxygen in the compound. To find the amount of oxygen,
Mass of O = Mass of compound - Mass of C + H

For further explanations, please watch this video:

http://www.youtube.com/watch?v=9qGo9YTdnMg