10 ml of gaseous hydro carbon is exploded with 100 ml of oxygen. The residual gas on cooling is found to measure 95 ml. of which 20 ml is absorbed by caustic soda and the remaining by alkaline pyrogallol. The formula of the hydrocarbon is _____

 $C_a{H}_b-(a+\frac{b}{4})O_2\to aC{O}_2+\frac{b}{2}{H}_{2}O$
1 ml       $(a+\frac{b}{4})ml\to aml\frac{b}{2}ml$
10 ml      $10(a+\frac{b}{4})ml\to 10aml\text{'}0ml\text{'}$
Volume absorbed by  $KOH=V_{C{O}_2}=20ml$
  $\Rightarrow 10a=20$
           $a=2$
Volume absorbed by alkaline pyrogallol = 75 ml
$V_{O_2}$  reacted = 100 – 75 = 25
 $\Rightarrow 10(a+\frac{b}{4})=25\Rightarrow \frac{10b}{4}-5\Rightarrow b=2$