CBSE Class 12 · 2016 · Delhi · d and f Block ElementsShort Answer

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When chromite ore, FeCr₂O₄, is fused with NaOH in the presence of air, a yellow compound (A) is obtained, which on acidification with dilute sulphuric acid gives compound (B). Compound (B) on reaction with KCl forms an orange crystalline compound (C).
i) Write the formulae of compounds (A), (B) and (C).
ii) Write one use of compound (C).

Solution

Fusion:

4FeC r_{2} O_{4} + 16NaOH + 7 O_{2} \to 8N a_{2} Cr O_{4} + 2F e_{2} O_{3} + 8 H_{2} O

(A) = sodium chromate, Na

_{2}

CrO

_{4}

(yellow).
Acidification:

2N a_{2} Cr O_{4} + H_{2} S O_{4} \to N a_{2} C r_{2} O_{7} + N a_{2} S O_{4} + H_{2} O

(B) = sodium dichromate, Na

_{2}

_{2}

_{7}

.
With KCl:

N a_{2} C r_{2} O_{7} + 2KCl \to K_{2} C r_{2} O_{7} + 2NaCl

_{2}

_{2}

_{7}

(orange crystals).
(ii) K

_{2}

_{2}

_{7}

is used as a powerful oxidising agent — a primary standard in volumetric analysis (also in leather tanning).

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CBSE Class 12 · 2016 · Delhi · d and f Block ElementsShort Answer

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