Which of the following compounds is used as the starting material for the preparation of potassium dichromate?

K_{2} S O_{4}

C r_{2} (S O_{4})_{3} .24 H_{2} O

(Chromealum)

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P b C r O_{4}

(Chromite yellow)

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F e C r_{2} O_{4}

(Chromite)

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P b C r O_{4} . P b O

(Chrome red)

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Solution

The correct option is C $F e C r_{2} O_{4}$ (Chromite)
Following step reactions are involved in preparation of $K_{2} C r_{2} O_{7}$ from Chromite ore (Fe $C r_{2} O_{4}$ )
(i)4 $F e C r_{2} O_{4}$ + 4 $N a_{2} C O_{3}$ +9 $O_{2}$ $\to$ $8 N a C r O_{4}$ + $2 F e_{2} O_{3}$ + $4 C O_{2}$ (chromite ore).
(ii) $2 N a_{2} C r O_{4}$ + $H_{2} S O_{4}$ $\to$ $N a_{2} C r_{2} O_{7}$ + $N a_{2} S O_{4}$ + $H_{2} O$
(iii) $2 N a_{2} C r_{2} O_{7}$ + $2 K C l$ $\to$ $2 K_{2} C r_{2} O_{7}$ + $2 N a C l$

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