0.435 g of an impure pyrolusite (MnO2, molar mass 87) is heated with 50.0 mL of 0.100 M oxalic acid in dilute H2SO4 (MnO2 + H2C2O4 + 2H+ -> Mn^2+ + 2CO2 + 2H2O). The unreacted oxalic acid is titrated with 0.0200 M KMnO4, needing 20.0 mL. The percentage of MnO2 in the sample is:

JEE Advanced levelSingle correct optionDifficulty 3 of 3rx199

0.435 g of an impure pyrolusite ( $MnO X_{2}$ , molar mass 87) is heated with 50.0 mL of 0.100 M oxalic acid in dilute $H X_{2} SO X_{4}$ ( $MnO X_{2} + H X_{2} C X_{2} O X_{4} + 2 H X^{+} Mn X^{2 +} + 2 CO X_{2} + 2 H X_{2} O$ ). The unreacted oxalic acid is titrated with 0.0200 M $KMnO X_{4}$ , needing 20.0 mL. The percentage of $MnO X_{2}$ in the sample is:

Total oxalic acid = 5.00 mmol. Excess: mmol

MnO X_{4} X^{-}

= 0.400, so oxalic acid =

0.400 \times 2.5 = 1.00

mmol. Oxalic acid used by

MnO X_{2}

= 4.00 mmol, equal to mmol

MnO X_{2}

(1:1). Mass =

4.00 \times 87 = 348

mg, so purity = 348/435 x 100 = 80%.