25 ml of a gaseous hydrocarbon requires 125 ml of oxygen for complete combustion.  The hydrocarbon is

$$\begin{gathered} C_3{H}_8+5O_2\stackrel{ }{\to }3C{O}_2+4H_{2}O \\ \mathit{1}\mathit{ }vol\mathit{ }\mathit{ }\mathit{ }\mathit{ }\mathit{ }\mathit{ }\mathit{ }\mathit{5}\mathit{ }vol \end{gathered}$$

Volume of Hydrocarbon and Oxygen requirement for its combustion are in 1:5 ratio. This ratio is maintained only for $C_3{H}_8$.

1 mole of $C_3{H}_6$ needs 4.5 mole of Oxygen for combustion; 1 mole of $C_2{H}_4$ needs 3.0 mole of Oxygen for combustion; 

1 mole of $C_4{H}_{10}$ needs 6.5 mole of Oxygen for combustion.