Combustion of a Hydrocarbon


1. Determine the % C and % H in an unknown compound.

2.  Determine the empirical formula of the unknown compound

3.  Determine the molecular formula of the unknown compound

Resources Needed


Hydrocarbons are compounds containing only carbon and hydrogen.  The alkane pictured above, propane, is a hydrocarbon.  Notice propane is SATURATED.  Saturated means that all the bonds around the carbon atoms are single bonds.  There are no double or triple bonds in propane.

Organic compounds containing carbon and hydrogen will react with O2 to produce carbon dioxide and water.  This reaction is called COMBUSTION.

Organic Compound + O2   -->    CO2  +  H2O

Notice that all of the CARBON in the organic compound becomes incorporated into the CO2.  And all of the hydrogen in the organic compound becomes incorporated into the H2O.

In this experiment we will use a combustion apparatus that will allow us to collect and weigh the combustion products, CO2 and H2O.  We will place a weighed amount of our unknown compound in the FURNACE.  Then we will TARE (set to zero) both balances.  Notice we have an H2O  TRAP and and a CO2 TRAP.  These traps contain substances that absorb the H2O and CO2 as the gases pass through them.  These traps allow us to find the weight of H2O and CO2 produced in the combustion reaction.

You will be able to determine the % H and % C from the mass of water and CO2 produced in the reaction.  What if there is another element in the compound?  Since you know the mass of the COMPOUND, H, and can find the mass of the other element by subtracting from the mass of the Compound.

mass of other element = mass of compound - mass of H - mass of C

The Simulation Website will give you additional information about combustion analysis of organic compounds.  You'll also find examples of how to determine the percentages, empirical formula, and molecular formula from the experimental data.

Your textbook is also a good source of information about how to determine empirical formulas and molecular formulas.

Go to this web address and review the sample calculations.

1. Select Compound A..

2. Zero the readings on the balances.

3. Initiate the combustion reaction by selecting the "Burn Sample" button.

4. Wait a few seconds for the combustion reaction to occur, the water and carbon dioxide to be absorbed, and the mass readings to stabilize.

5. Record the masses of water and carbon dioxide produced by the combustion of the sample.

6. Calculate the empirical formula for the unknown compound. The molecular mass of the compound is provided; use this value and the empirical formula to determine the molecular formula for the unknown compound.

7. Repeat the procedure for Compounds B, C, and D.