Air contains 20% $O_2$ by volume.  How much volume of air will be required for the oxidation of 100 cc of acetylene        

$2C_2{H}_2+5O_2\to 4C{O}_2+2H_{2}O$

$2volof{C}_2{H}_2\to 5Volof{O}_2$

100 cc $\to$ ?

$\therefore$ 250cc of oxygen.

$\therefore$ Air contains 20%  $O_2$ by volume

100 cc air $\to$ 20 cc $O_2$

?  $\leftarrow$     250C of $C_2$

$\left(\frac{250\times 100}{20}\right)=1250ccofair.$