A mixture of 2.3 g formic acid and 4.5 g oxalic acid is treated with conc. ${\mathrm{H}}_2{\mathrm{SO}}_4$. The evolved gaseous  mixture is passed through KOH pellets. Weight (in g) of the remaining product at STP will be

 

$H_{2}O$ absorbed by $H_{2}S{O}_4$. Gaseous mixture (containing $CO$ and $C{O}_4$) when passed through KOH pellets, $C{O}_2$ gets absorbed.

$\text{ Moles of CO left (unabsorbed) }=\frac{1}{20}+\frac{1}{20}=\frac{1}{10}$

$\text{ Mass of }\mathrm{CO}=\text{ moles }\times \text{ molar mass }$

                             $=\frac{1}{10}\times 28=2.8 \mathrm{g}$