Carbon and hydrogen are the only two elements in a hydrocarbon. When a hydrocarbon combusts completely in excess oxygen, the products would be and . The and would come from the hydrocarbon, while the atoms would come from oxygen.
Look up the relative atomic mass of these three elements on a modern periodic table:
: .
: .
: .
Calculate the molar mass of and :
.
Calculate the number of moles of molecules in of :
.
Similarly, calculate the number of moles of molecules in of :
.
Note that there is one carbon atom in every molecule. Approximately of molecules would correspond to the same number of atoms. That is: .
On the other hand, there are two hydrogen atoms in every molecule. approximately of molecules would correspond to twice as many atoms. That is: .
The ratio between the two is: .
The empirical formula of a compound gives the smallest whole-number ratio between the elements. For this hydrocarbon, the empirical formula would be .