Q). 100 ml of an unknown hydrocarbon when burned completely under 300 ml of oxygen produces 200 ml volume of carbon dioxide. What is the probable molecular formula of the gas?

Dear Student 

The approximate reaction of the unknown hydrocarbon is : 

$C_x{H}_y + O_2 \to C{O}_2 +H_{2}O$

The volume ratio also meets the stoichiometry So, accordingly 

1 part of the unknown hydrocarbon reacts with 3 part of oxygen to give 2 part of carbon dioxide. 
So, the probable reaction may be: 


$$\begin{gathered} C_x{H}_y + 3O_2 \to 2C{O}_2 +H_{2}O \\ So, to balance the oxygen both side H_{2}O must be multiplied with the factor 2. \\ The new reaction will be: \\ {C}_x{H}_y + 3O_2 \to 2C{O}_2 +2H_{2}O \end{gathered}$$

So, now from the reaction it is found that Carbon atoms are 2 and hydrogen are 4. 
And hence the probable molecular formula of the unknown hydrocarbon is : 
                        $C_2{H}_4$ or ethene. 

Regards