---
title: "Complex vs double salts; EAN of Fe(CN)₆"
subject: "Engineering Chemistry"
chapter: 6
key: "semester_2/engineering_chemistry:complex-double-ean"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/complex-double-ean
site: BCT Engineering
---

# Complex vs double salts; EAN of Fe(CN)₆

Engineering Chemistry · Chapter 6: Coordination Complexes

- Asked in: 2069 Bhadra, 2.5 marks
- Video: [Effective Atomic Number - Co-Ordination Compounds - Chemistry Class 12](https://www.youtube.com/watch?v=v5zO3gZExfU) — Ekeeda
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/complex-double-ean
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-6

## Question

Differentiate between complex salts and double salts. Calculate EAN of the central metal atom in Fe(CN)₆.

## Short answer

Complex salts retain complex ions in solution, while double salts dissociate completely into simple ions. For Fe in [Fe(CN)₆]⁴⁻: Fe²⁺ (Z=26), EAN = 26 - 2 + 2×6 = 36.

## Answer

| Basis | Complex salt | Double salt |
|---|---|---|
| Formation | Metal ion bonded to ligands by coordinate bonds | Two simple salts crystallised together |
| In water | Complex ion stays intact | Splits into all simple ions |
| Example | K₄[Fe(CN)₆] | Mohr's salt, potash alum |

#### EAN of Fe in Fe(CN)₆
The complex ion is [Fe(CN)₆]⁴⁻, with Fe²⁺ (Z = 26).
EAN = (Z − oxidation state) + 2 × (number of ligands) = 26 − 2 + 2 × 6 = 36

**Answer: EAN = 36**, the atomic number of krypton. (For [Fe(CN)₆]³⁻ with Fe³⁺ the EAN would be 35.)

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How does a complex salt differ from a double salt in water?
   A. A complex salt splits into all simple ions, a double salt does not
   B. The complex ion stays intact, while a double salt splits into all its simple ions
   C. Both stay intact
   D. Neither dissolves
   Answer: B. K4[Fe(CN)6] gives K+ and [Fe(CN)6]4-, whereas Mohr's salt gives all its simple ions. (Key point: Behaviour in water)

2. How is a double salt formed?
   A. A metal ion bonds to ligands by coordinate bonds
   B. By oxidising a complex
   C. By neutralising an acid with a base only
   D. Two simple salts crystallise together
   Answer: D. A double salt such as potash alum is two simple salts crystallised together; complex salts have coordinate bonds. (Key point: Formation of double salt)

3. Which expression gives EAN for monodentate ligands?
   A. Z - oxidation state + 2 x number of ligands
   B. Z + oxidation state - 2 x number of ligands
   C. Z - 2 x ligands
   D. Z + oxidation state + number of ligands
   Answer: A. EAN = electrons left on the metal ion plus 2 per donor ligand. (Key point: EAN formula)

4. What is the EAN of Fe in [Fe(CN)6]4-?
   A. 24
   B. 26
   C. 36
   D. 35
   Answer: C. Fe2+ has 24 e-, plus 6 x 2 = 12 gives 36, the krypton configuration. (Key point: EAN of Fe in [Fe(CN)6]4-)

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