Question

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).

Answer 1

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.