Department of Pre-University Education, KarnatakaPUC Karnataka Science Class 12
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When chromite ore FeCr2O4 is fused with NaOH in presence of air, a yellow-coloured compound (A) is obtained, which on acidification with dilute sulphuric acid gives a compound (B). Compound (B) on reaction with KCl forms an orange coloured crystalline compound (C). - Chemistry

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When chromite ore FeCr2O4 is fused with NaOH in presence of air, a yellow-coloured compound (A) is obtained, which on acidification with dilute sulphuric acid gives a compound (B). Compound (B) on reaction with KCl forms an orange coloured crystalline compound (C).

(i) Write the formulae of the compounds (A), (B) and C.

(ii) Write one use of compound (C).

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Solution

Chromite ore, on fusion with NaOH, forms sodium chromate (yellow-coloured compound):

4 FeCr2O4 + 16 NaOH + 7 O2 8 Na2CrO4 + 2 Fe2O3 + 8 H2O

(A) = Na2CrO4

Sodium chromate, on acidification, gives sodium dichromate (B): 2 Na2CrO4 + H2SO4 Na2Cr2O7 + Na2SO4 + H2O

(B) = Na2Cr2O7

Sodium dichromate, on reaction with KCl, forms potassium dichromate (orange-coloured crytalline compound):

Na2Cr2O7 + 2 KCl K2Cr2O7 + 2 NaCl

(C) = K2Cr2O7

One use of potassium dichromate is that it is a powerful oxidising agent. It is used in preparation of azo compounds. It is also used in leather industry.

Disclaimer: Students can write any one use of potassium dichromate.

Concept: Some Important Compounds of Transition Elements - Oxides and Oxoanions of Metals
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