A mixture of 2.3 g formic acid and 4.5 g oxalic acid is treated with conc. $H_2SO_4$. The evolved gaseous mixture is passed through KOH pellets. Weight (in g) of the remaining product at STP will be

HCOOH $\xrightarrow{Conc. H_2SO_4}$ CO(g) + $H_2O$(l). Moles of HCOOH = 2.3/46 = 1/20 mol. COOH-COOH $\xrightarrow{Conc. H_2SO_4}$ CO(g) + $CO_2$(g) + $H_2O$(l). Moles of oxalic acid = 4.5/90 = 1/20 mol. Gaseous mixture is CO and $CO_2$. KOH absorbs $CO_2$. Remaining gas is CO. Total moles of CO = 1/20 + 1/20 = 2/20 mol. Weight = (2/20) * 28 = 2.8 g.