Question

When \( \text{FeCr}_2\text{O}_4 \) is fused with \( \text{Na}_2\text{CO}_3 \) in the presence of air, it gives a yellow solution of compound (A). Compound (A) on acidification gives compound (B). Compound (B) on reaction with KCl forms an orange-colored compound (C). An acidified solution of compound (C) oxidizes \( \text{Na}_2\text{SO}_3 \) to (D). Identify (A), (B), (C), and (D).

Hint:

Chromates and dichromates are strong oxidizing agents. Chromates are yellow in alkaline conditions, while dichromates are orange in acidic conditions.

Answer 1

Step 1: Identification of Compound (A). Fusion of \( \text{FeCr}_2\text{O}_4 \) with \( \text{Na}_2\text{CO}_3 \) in the presence of air yields a yellow solution of sodium chromate (\( \text{Na}_2\text{CrO}_4 \)). Therefore, \( A = \text{Na}_2\text{CrO}_4 \).

Step 2: Identification of Compound (B). Acidification of \( \text{Na}_2\text{CrO}_4 \) produces orange-colored potassium dichromate (\( \text{K}_2\text{Cr}_2\text{O}_7 \)). Thus, \( B = \text{K}_2\text{Cr}_2\text{O}_7 \).

Step 3: Identification of Compound (C). Reaction of \( \text{K}_2\text{Cr}_2\text{O}_7 \) with KCl results in the formation of orange-colored \( \text{Cr}_2\text{O}_7^{2-} \) ions. Consequently, \( C \) is an orange-colored chromium compound. Therefore, \( C = \text{K}_2\text{Cr}_2\text{O}_7 \).

Step 4: Identification of Compound (D). An acidified solution of \( C \) oxidizes sodium sulfite (\( \text{Na}_2\text{SO}_3 \)) to sodium sulfate (\( \text{Na}_2\text{SO}_4 \)). This demonstrates the oxidizing capability of chromium compounds.
Thus, \( D = \text{Na}_2\text{SO}_4 \).
The identified compounds are:
\( A = \text{Na}_2\text{CrO}_4 \)
\( B = \text{K}_2\text{Cr}_2\text{O}_7 \)
\( C = \text{K}_2\text{Cr}_2\text{O}_7 \)
\( D = \text{Na}_2\text{SO}_4 \)