8.36 Indicate the steps in the preparation of: (i) K2Cr2O7 from chromite ore. (ii) KMnO4 from pyrolusite ore.
8.36 Indicate the steps in the preparation of: (i) K2Cr2O7 from chromite ore. (ii) KMnO4 from pyrolusite ore.
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1 Answer
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8.36 (i) The chromite ore (FeCr2O4) is fused with sodium hydroxide (NaOH)
4FeCr2O4 +16 NaOH+ 7O2 → 8Na2CrO4 +2Fe2O3 +8H2O
The yellow solution of sodium chromate is then filtered and acidified with sulphuric acid giving its dichromate.
2Na2CrO4 +H2SO4→ Na2Cr2O7 + Na2SO4 +H2O
On cooling, sodium sulphate crystallizes out as Na2SO4.10H2O and is removed.
Na2Cr2O7 +2KCl → K2Cr2O7 + 2NaCl
(ii) The pyrolusite ore(MnO2) is oxidised in the presence of potassium hydroxide by heating.
2MnO2 + 4KOH +O2→ 2K2MnO4 + 2H2O
The green potassium manganate (K2MnO4) is then treated with a current of chlorine or ozone to oxidise potassium manganat
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