Fe³⁺ cation gives a prussian blue precipitate on addition of potassium ferrocyanide solution due to the formation of :

Solution

Chemistry ma’am · 4 steps

Prussian blue is ferric ferrocyanide: balance Fe³⁺ against [Fe(CN)₆]⁴⁻.

1. Know the reagent. Potassium ferrocyanide is K₄[Fe(CN)₆]. In solution it gives the ferrocyanide ion [Fe(CN)₆]⁴⁻, with iron as Fe²⁺ and six CN⁻ ligands.
2. Balance the charges. Each Fe³⁺ brings +3 and each ferrocyanide brings -4. The smallest neutral combination is 4 of the cation and 3 of the anion, because 4 × 3 = 3 × 4 = 12.

   4Fe³⁺ + 3[Fe(CN)₆]⁴⁻ → Fe₄[Fe(CN)₆]₃ v

3. Name the product. Fe₄[Fe(CN)₆]₃ is ferric ferrocyanide, the deep blue precipitate called Prussian blue. It contains iron in both oxidation states: +3 outside the complex ion and +2 inside it.
4. So. The Prussian blue precipitate is Fe₄[Fe(CN)₆]₃, option (d).

The idea to keep. For Prussian blue, think ferric ion plus ferrocyanide ion. Match +3 against -4: four iron ions outside and three complex ions, Fe₄[Fe(CN)₆]₃.

Answer from the NTA final answer key. Solution by PrepWiser.