The equivalent weight of oxalic acid in C₂H₂O₄.2H₂O is

[amp_mcq option1=”45″ option2=”63″ option3=”90″ option4=”126″ correct=”option2″]

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UPSC NDA-1 – 2019

The formula given is for hydrated oxalic acid, C₂H₂O₄.2H₂O. Oxalic acid (C₂H₂O₄ or HOOC-COOH) is a diprotic acid, meaning it can donate two protons (H⁺). The equivalent weight of an acid is its molar mass divided by the number of acidic hydrogens that participate in the reaction (also called its basicity). In typical acid-base reactions, oxalic acid dihydrate reacts as a diprotic acid, donating both protons.

– The molar mass of C₂H₂O₄ = (2 × 12.01) + (2 × 1.01) + (4 × 16.00) = 24.02 + 2.02 + 64.00 = 90.04 g/mol.
– The molar mass of 2H₂O = 2 × (2 × 1.01 + 16.00) = 2 × 18.02 = 36.04 g/mol.
– The molar mass of C₂H₂O₄.2H₂O = 90.04 + 36.04 = 126.08 g/mol.
– Oxalic acid is a diprotic acid, meaning it has 2 acidic hydrogens.
– Equivalent weight = Molar mass / number of acidic hydrogens = 126.08 g/mol / 2 = 63.04 g/equivalent.
– Among the options, 63 is the closest value.

If the question were asking for the equivalent weight of *anhydrous* oxalic acid (C₂H₂O₄) reacting as a diprotic acid, the answer would be 90/2 = 45. If it reacted as a monoprotic acid, the equivalent weight would be 90 or 126 depending on whether anhydrous or hydrated form is considered as the basis for molar mass. However, the most common standard use of oxalic acid in volumetric analysis is as the dihydrate, reacting as a diprotic acid.

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