CBSE 2025, Set 2

Question

When FeCr$_2$O$_4$ is fused with Na$_2$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 coloured (C). An acidified solution of compound (C) oxidises Na$_2$SO$_3$ to (D). Identify (A), (B), (C) and (D).

Edvaya · Accepted Answer

The correct answer is Subjective. This is a series of chromium chemistry reactions: **Step 1:** FeCr₂O₄ + Na₂CO₃ + air (O₂): $$4	ext{FeCr}_2	ext{O}_4 + 8	ext{Na}_2	ext{CO}_3 + 7	ext{O}_2 ightarrow 8\underbrace{	ext{Na}_2	ext{CrO}_4}_{(A)\,	ext{yellow}} + 2	ext{Fe}_2	ext{O}_3 + 8	ext{CO}_2$$ **Step 2:** Na₂CrO₄ + acid (acidification): $$2	ext{Na}_2	ext{CrO}_4 + 	ext{H}_2	ext{SO}_4 ightarrow \underbrace{	ext{Na}_2	ext{Cr}_2	ext{O}_7}_{(B)\,	ext{orange}} + 	ext{Na}_2	ext{SO}_4 + 	ext{H}_2	ext{O}$$ **Step 3:** Na₂Cr₂O₇ + KCl: $$	ext{Na}_2	ext{Cr}_2	ext{O}_7 + 2	ext{KCl} ightarrow \underbrace{	ext{K}_2	ext{Cr}_2	ext{O}_7}_{(C)\,	ext{orange}} + 2	ext{NaCl}$$ **Step 4:** K₂Cr₂O₇ (acidified) oxidises Na₂SO₃: $$	ext{K}_2	ext{Cr}_2	ext{O}_7 + 	ext{Na}_2	ext{SO}_3 + 	ext{H}_2	ext{SO}_4 ightarrow \underbrace{	ext{Na}_2	ext{SO}_4}_{(D)} + 	ext{Cr}_2(	ext{SO}_4)_3 + 	ext{K}_2	ext{SO}_4 + 	ext{H}_2	ext{O}$$ **Answers:** - **(A)** = Na₂CrO₄ (Sodium chromate) — yellow - **(B)** = Na₂Cr₂O₇ (Sodium dichromate) — orange - **(C)** = K₂Cr₂O₇ (Potassium dichromate) — orange - **(D)** = Na₂SO₄ (Sodium sulphate)

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