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Question

1.6 g of pyrolusite ore was treated with 50 mL of 0.1 N oxalic acid and some sulphuric acid. The oxalic acid left in excess was raised to 250 mL in a flask. 25 mL of this solution when titrated with 0.01 N KMnO4 required 30 mL of the solution. The percentage of pure MnO2 in the sample is :

A
10.86%
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B
5.43%
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C
1.086%
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D
none of the above
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Solution

The correct option is B 5.43%
MnO2(1.6 g)+H2C2O450 mL/0.1 Nexcess25 mL0.01 N/30 mLofKMnO4Mn2+
mEq of KMnO4 mEq of H2C2O4]excessin25 mL
Excess mEq of H2C2O4 in 25 mL
=0.01×30=0.30
Excess mEq of H2C2O4 in 250 mL=3.0
mEq of H2C2O4 used =53=2
mEq of MnO2=2
Weight87/2×1000=2Weight=0.087 g
% MnO2=0.0871.6×100=5.43%

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