The Empirical Formula of Organic Compounds

Finding the empirical formula of an organic compound (any substance with carbon in it) is pretty similar to calculating the empirical formula to other substance. Except it has one additional step at the end.
It can be found by:
- Burning the compound (reacting it with O2)
- Collecting and weighing the products
- The mass of the products, the moles of each element in the original organic compound can be calculated

There are 4 steps you need to follow if you want to find the empirical formula correctly.
Here is an example question and the steps that follow.


What is the empirical formula of a compound that when a 5g sample is burned produces 15g of CO2 and 8.18g of H2O? 


Step 1
Calculate the moles of CO2 + H2O produced.


mol CO2 = 15g CO2 x (1 mol CO2/44.0g CO2) = 0.341 mol
mol H2O = 8.18g H2O x (1 mol H2O/18.0g H2O) = 0.454 mol


Step 2
Find the moles of C + H in CO2 and in H2O.


mol C = 0.341 mol CO2 x (1 mol C/1 mol CO2) = 0.341 mol C
mol H = 0.454 mol H2O x (2 mol H/ 1 mol H2O) = 0.908 mol H


Step 3
The ratio of moles of C to moles of H in the organic compound is 0.341: 0.908.
Therefore the formula could be: C0.4341H0.908 BUT the ratio must be whole numbers.


Step 4
Change the ratio to a whole number ration by multiplying in turn by 2/2, 3/3, 4/4 until you get a whole number ratio.


Ratio of moles = 0.908 mol H/ 0.341 mol C = (2.66/1) x 3/3 = 8/3
The empirical formula = C3H8