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Question

How is potassium dichromate prepared from a sample of chromite ore? Give balanced equations for the chemical reactions involved.

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Solution

Preparation pf potassium dichromate from chromite ore involves following steps:
(i) Conversion of chromite ore to sodium chromate by roasting finely powdered chromite ore with sodium carbonate and quick lime in excess of air.
4FeCr2O4Chromiteore+8Na2CO3Sodiumcarbonate+7O2CaO+air−−−−2Fe2O3Ferricoxide+8Na2CrO4Sodiumchromate+8CO2
(ii) Conversion of sodium chromate to sodium dichromate by the action of conc.H2SO4.
2Na2CrO4+H2SO4conc.Na2Cr2O7Sodiumdichromate+Na2SO4+H2O
(iii) Conversion of sodium dichromate to potassium dichromate by treating it with KCl solution.
Na2Cr2O7+2KClK2Cr2O7Potassiumdichromate+2NaCl
From resulting solution potassium dichromate is crystallised.

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