0.5 g mixture of oxalic acid (H₂C₂O₄) and some sodium oxalate (Na₂C₂O₄) with some impurities requires 40 ml of 0.1M NaOH for complete neutralization and 6ml of 0.2 M KMnO₄ for complete oxidation. Calculate the % of Na₂C₂O₄ in the mixture

No.of milli equivalents of H₂C₂O₄
= No.of milli equivalents of NaOH = 4
No. of milli equivalents of H₂C₂O₄ + Na₂C₂O₄
= No. of milli equivalents of KMnO₄
= 6 × 0.2 × 5 = 6
Milli equivalent of Na₂C₂O4 = 2
Weight of${\mathrm{Na}}_2{\mathrm{C}}_2{\mathrm{O}}_4=2\times {10}^{-3}\times 67=0.134\mathrm{g}$
$\mathrm{\%}{\mathrm{Na}}_2{\mathrm{C}}_2{\mathrm{O}}_4=\frac{0.134}{0.5}\times 100=26.8$