$\begin{array}{llll}\text { SOLUTION }: & 2 CO ( g )+ O _{2}( g )= & 2 CO _{2}( g ) \\& 2 vol . & 1 vol . & 2 vol . \\& 80 mL & 40 mL & 80 mL\end{array}$
Volume of $C O$ reacted $=80 mL$, Hence volume of $CO _{2}$ formed $=80 mL$
Thus the mixture after reaction : $80 mL$ of $CO _{2}$ and $20 mL$ of $CO$.
This mixture is shaken with $KOH$ solution which absorbs $CO _{2}$. Hence after this treatment $C O$ will remain. Vol. of $C O$ remained after treatment with $K O H=20$ mL.
