Molecular Formula of Gaseous Hydrocarbons

The molecular formula of gaseous hydrocarbons can be determined even without knowing their percentage composition. A known volume of hydrocarbon is taken in eudiometer tube and exploded with an excess of known volume of oxygen. The carbon and hydrogen of the hydrocarbon are oxidized to carbon dioxide and water respectively according to the following general reaction.

After explosion, the tube is allowed to cool to room temperature. The volume of the gaseous mixture is recorded. The volume corresponds to unreacted oxygen and carbon dioxide. The volume of carbon dioxide is determined by the decrease in volume of the gaseous mixture when caustic potash solution is introduced into the eudiometer tube. The caustic potash solution absorbs whole of carbon dioxide. The gas left behind is the unreacted oxygen. Volume of oxygen used is then calculated by subtracting the volume of unreacted oxygen from the total volume of the oxygen taken initially.

The molecular formula of the hydrocarbon can be evaluated by knowing

i) the volume of gaseous hydrocarbon taken

ii) the volume of oxygen used, and

iii) the volume of carbon dioxide produced.