How does the chemical equilibrium between chromate (CrO42-) and dichromate (Cr2O72-) ions respond to changes in solution pH?

Answer and explanation

Correct answer: C. Yellow chromate converts to orange dichromate in acid; dichromate reverts to chromate in alkaline medium

  1. Step 1: In aqueous solution, chromate and dichromate ions interconvert according to the reversible equilibrium: .
  2. Step 2: Adding acid increases , shifting the equilibrium to the right (Le Chatelier's principle) to form orange dichromate.
  3. Step 3: Adding base () neutralizes , shifting the equilibrium to the left to regenerate yellow chromate.
  4. Step 4: In both ions, chromium remains in the +6 oxidation state.
Answer
Yellow chromate (CrO42-) converts to orange dichromate (Cr2O72-) in acidic medium, and dichromate reverts to chromate in alkaline medium
Why not the others: 'Orange dichromate converts to yellow chromate in acidic medium...' reverses the pH dependence; 'Chromate is stable only at pH < 2...' is wrong because chromate dominates at pH > 7; 'The equilibrium is unaffected by pH...' ignores the direct stoichiometric participation of in the balanced reaction.

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