Q.49 Chemistry ⭐ 360° Option Elimination Active
NEET 2026 Chemistry Question 49 Solution
Topic: Coordination Compounds & Equilibrium · Official NTA Booklet Code 50
Question
In potash alum, the ratio of K⁺ and SO₄²⁻ ions is
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Verified Against NTA Final Answer Key
Option (1) : 1:2
📚 Coordination Compounds & Equilibrium Easy
📋 Given Parameters & Problem Setup
Substance Potash Alum [K₂SO₄·Al₂(SO₄)₃·24H₂O]
Target Ions Ratio of K⁺ to SO₄²⁻ ions
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⏱️ Save ~45 seconds The 30-Second Exam Hall Shortcut: ⚡ Formula Inspection Shortcut
Potash alum has formula KAl(SO₄)₂·12H₂O -> Exactly 1 K⁺ for every 2 SO₄²⁻!
1 Write the empirical double-salt formula: KAl(SO₄)₂·12H₂O.
2 Count ions: 1 K⁺ ion and 2 SO₄²⁻ ions.
3 Ratio of K⁺ : SO₄²⁻ = 1 : 2 immediately!
4 Even in the dual-formula K₂SO₄·Al₂(SO₄)₃·24H₂O: 2 K⁺ and (1 + 3) = 4 SO₄²⁻ -> 2 : 4 = 1 : 2.
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Why You Can 100% Trust This Shortcut: Standard definition of double salts: In aqueous solution, potash alum completely dissociates into simple hydrated ions retaining stoichiometric molar proportions.
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Step-by-Step Rigorous Workflow & Mathematical Proof
Step 1
Chemical Formula of Potash Alum
Potash alum is a classic double salt containing potassium sulfate and aluminium sulfate crystallized with 24 molecules of water of hydration.
- • Full crystalline unit: K₂SO₄·Al₂(SO₄)₃·24H₂O.
- • Empirical unit: KAl(SO₄)₂·12H₂O.
Step 2
Complete Aqueous Dissociation
Unlike coordination complexes where inner-sphere ligands do not dissociate, double salts dissociate completely into their constituent simple ions in aqueous solution.
- • Total K⁺ ions per formula unit = 2.
- • Total Al³⁺ ions per formula unit = 2.
- • Total SO₄²⁻ ions per formula unit = 1 (from K₂SO₄) + 3 (from Al₂(SO₄)₃) = 4.
Step 3
Compute the Molar Ratio of K⁺ to SO₄²⁻
Divide the stoichiometric coefficient of potassium ions by that of sulfate ions.
- • Ratio = 1:2.
- • Corresponds to Option (1).
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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
✓ 100% Correct Verdict & Scientific Reason:
Completely correct. There are two K⁺ ions for every four SO₄²⁻ ions, yielding a 1:2 ratio.
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Why this is airtight: None; mathematically exact.
2 B
✗ Incorrect Verdict & Scientific Reason:
Inverted ratio. 2:1 is the ratio of SO₄²⁻ to K⁺, not K⁺ to SO₄²⁻.
⚠️
Student Fallacy (Why candidates pick this trap): Reading the question order in reverse.
3 C
✗ Incorrect Verdict & Scientific Reason:
Incorrect. This represents the ratio of K⁺ to sulfate ions from Al₂(SO₄)₃ alone, neglecting the sulfate from K₂SO₄.
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Student Fallacy (Why candidates pick this trap): Omitting the sulfate ion originating from the potassium sulfate component.
4 D
✗ Incorrect Verdict & Scientific Reason:
Incorrect. Does not represent any valid stoichiometric ion ratio in potash alum.
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Student Fallacy (Why candidates pick this trap): Confusing valency of aluminium (+3) with potassium ion counts.
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NTA Trap Alert: Counting Only Sulfates in Al₂(SO₄)₃
A common slip is counting only the 3 sulfates in aluminium sulfate and concluding the ratio is 2:3, forgetting that K₂SO₄ provides the 4th sulfate ion.
🛡️ How to avoid the -1 mark: Always tally all ions: 1 sulfate from K₂SO₄ plus 3 sulfates from Al₂(SO₄)₃ equals 4 sulfates total.
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Core NEET Syllabus Standard
NEET Chemistry Core Syllabus · Coordination Compounds Standard Benchmark
Core Scientific Principle Tested:
"Double salts lose their identity in aqueous solution and dissociate into all constituent ions."
Syllabus Focus: Double salts vs Coordination complexes
✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗
Potash Alum: 1 K⁺ and 1 Al³⁺ balanced by 2 Sulfates (1:2 ratio)!
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