When 50 ml  ${\text{CO}}_{\text{2}}$ gas is passed through red hot charcoal, the volume becomes 70 ml, the volume of ${\text{CO}}_{\text{2}}$  converted into CO is

               ${\text{CO}}_{\text{2}}\text{ + C }\to \text{ 2CO}$
Ini.          50ml.                      0ml
Fin.         50 - x.                      2x
        Total = 50 + x  = 70 
      x = 20ml