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Question

1.6 g of pyrolusite ore was treated with 50 cm3 of 1.0 N oxalic acid and some sulphuric acid. The oxalic acid left undecomposed was raised to 250 cm3 in a flask. 25 cm3 of this solution when titrated with 0.1 N KMnO4 required 32 cm3 of the solution. Find the percentage of pure MnO2 in the sample and also the percentage of available oxygen.

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Solution

25 cm3 of undecomposed oxalic acid required
=32 cm30.1 N KMnO4 solution
Thus, 250 cm3 of undecomposed oxalic acid required
=320 cm30.1 N KMnO4 solution
32 cm31 N KMnO4 solution
=32 cm31 N oxalic acid solution
Oxalic acid used by pyrolusite
=(5032)cm31 N solution
=18 cm31 N solution
18 cm31 N MnO2 solution
Mass of MnO2=N×E×V1000=1×18×871000×2=0.783 g
Percentage of MnO2=0.7831.6×100=48.9
MnO287 gMnO+O16 g
Oxygen given by 0.783 g MnO2=1687×0.783=0.144 g
% of available oxygen =0.1441.6×100=9.0.

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