UPSC NDA I 2019 General Ability Solved Paper

Concept:Equivalent weight is the molecular weight divided by the basicity (number of replaceable $\mathrm{H}^+$ ions).Explanation:Oxalic acid exists as a dihydrate: $\mathrm{C}_2\mathrm{H}_2\mathrm{O}_4 \cdot 2\mathrm{H}_2\mathrm{O}$.The molecular weight of anhydrous $\mathrm{C}_2\mathrm{H}_2\mathrm{O}_4$ is $90$ g/mol.Two water molecules add $2 \times 18 = 36$ g/mol, giving a total molecular weight of $90 + 36 = 126$ g/mol.Oxalic acid is a dibasic acid, releasing two $\mathrm{H}^+$ ions per molecule, so basicity $= 2$.Equivalent weight $= \frac{126}{2} = 63$ g/equiv.Answer:$63$ (Option B).