Q.68 Chemistry ⭐ 360° Option Elimination Active
NEET 2026 Chemistry Question 68 Solution
Topic: Coordination Compounds · Official NTA Booklet Code 50
Question
The formula of tetraammineaquachloridocobalt(III) chloride is
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Verified Against NTA Final Answer Key
Option (4) : [Co(NH₃)₄(H₂O)Cl]Cl₂
📚 Coordination Compounds Easy
📋 Given Parameters & Problem Setup
IUPAC Name tetraammineaquachloridocobalt(III) chloride
Central Metal Cobalt in +3 oxidation state [Co³⁺]
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⏱️ Save ~45 seconds The 30-Second Exam Hall Shortcut: ⚡ Charge Balancing in Coordination Formulas
Sum the charges inside the bracket to find the required counter-ions outside!
1 Central atom: Co³⁺ (+3).
2 Ligands inside bracket: 4 NH₃ (neutral = 0) + 1 H₂O (neutral = 0) + 1 Cl⁻ (-1).
3 Net charge of complex cation = +3 + 0 + 0 - 1 = +2.
4 To balance the +2 cation, we need TWO Cl⁻ ions outside: Cl₂!
5 Chemical formula MUST be [Co(NH₃)₄(H₂O)Cl]Cl₂.
6 Option (4) is identified in 10 seconds!
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Why You Can 100% Trust This Shortcut: Electroneutrality of ionic coordination compounds: Total cationic charge + Total anionic charge = 0.
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Step-by-Step Rigorous Workflow & Mathematical Proof
Step 1
Identify Central Metal and Ligands from IUPAC Name
Break down the name "tetraammineaquachloridocobalt(III) chloride": 1. Central metal: Cobalt, oxidation state = +3. 2. tetraammine = 4 NH₃ ligands (neutral). 3. aqua = 1 H₂O ligand (neutral). 4. chlorido = 1 Cl⁻ ligand (charge -1). 5. Counter-ion: chloride (Cl⁻).
- • Coordination number = 4 + 1 + 1 = 6 (octahedral complex).
Step 2
Calculate the Net Charge of the Coordination Entity
Sum the oxidation state of the metal and the formal charges of all coordinated ligands.
- • The complex ion is [Co(NH₃)₄(H₂O)Cl]²⁺.
Step 3
Balance with Outer-Sphere Counter-Ions
To neutralize the +2 charge of the complex cation, two chloride ions (Cl⁻, each -1) are required outside the coordination sphere.
- • Final formula: [Co(NH₃)₄(H₂O)Cl]Cl₂.
- • Matches Option (4).
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4-Option Deep Scan
360° Option Elimination Engine Key Feature
Diagnose every choice: Know exactly why the right option works and why distractors fail.
1 A
✗ Incorrect Verdict & Scientific Reason:
Incorrect. Contains two chlorido ligands inside and water of crystallization outside (hydrate isomer).
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Student Fallacy (Why candidates pick this trap): Placing aqua outside as water of crystallization.
2 B
✗ Incorrect Verdict & Scientific Reason:
Incorrect. Missing chlorido ligand inside the coordination sphere.
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Student Fallacy (Why candidates pick this trap): Placing all chloride ions outside the brackets.
3 C
✗ Incorrect Verdict & Scientific Reason:
Incorrect. Only one chloride outside, which implies Co is in the +2 oxidation state instead of +3.
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Student Fallacy (Why candidates pick this trap): Failing to balance charges for Cobalt(III).
4 D
✓ 100% Correct Verdict & Scientific Reason:
Exact formula matching the IUPAC name with Cobalt(III) oxidation state.
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Why this is airtight: None; perfect nomenclature translation.
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NTA Trap Alert: Confusing Ligand Aqua with Water of Hydration
Students often see "aqua" and mistakenly write "· H₂O" outside the bracket (Option 1). In IUPAC nomenclature, "aqua" inside the prefix name refers to a coordinated ligand inside the bracket [ ... (H₂O) ... ].
🛡️ How to avoid the -1 mark: Prefixes in the complex name (aqua, ammine) belong INSIDE the square bracket.
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Core NEET Syllabus Standard
NEET Chemistry Core Syllabus · Coordination Compounds Standard Benchmark
Core Scientific Principle Tested:
"The coordination entity is enclosed in square brackets; charge is balanced by counter-ions outside."
Syllabus Focus: Nomenclature and formulation of coordination compounds
✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗
Inside the bracket: 4 ammines, 1 aqua, 1 chloro leaves +2; needs 2 chlorides outside to make it true!
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