Mohr's salt FeSO4·(NH4)2SO4·6H2O: dissolve equimolar FeSO4 + (NH4)2SO4 in dilute H2SO4, evaporate, cool — pale-green double salt crystallises. Potash alum K2SO4·Al2(SO4)3·24H2O: dissolve equimolar K2SO4 + Al2(SO4)3, evaporate, cool — octahedral colourless crystals. Both are double salts (ionise to constituent ions).
-- NCERT Class 12 Chemistry, Ch. 4, p. 118Mohr Salt Potash Alum
8 MCQs2 revision cards9-step worked example
Source: NCERT Unit 20PYQ coverage: NEET 2024, 2025Official key: NTA-verifiedLast reviewed: May 2026
Lesson
The trap that costs marks here: aspirants confuse the indicator choice for titration of Mohr's salt with KMnO₄. Mohr's salt titration is a redox titration — KMnO₄ is self-indicating (decolourises until end-point, then first persistent pink). Picking phenolphthalein or methyl orange is a category error; those are acid-base indicators with pH-range logic that does not apply to permanganimetry.
What is Mohr's salt? Ferrous ammonium sulphate, FeSO₄·(NH₄)₂SO₄·6H₂O — a double salt (NCERT Class 12 Chemistry, Chapter 4, page 118). It is preferred over plain FeSO₄ because the ammonium ion stabilises Fe²⁺ against aerial oxidation. In NEET practical chemistry, it appears as the primary standard for standardising KMnO₄.
What is potash alum? K₂SO₄·Al₂(SO₄)₃·24H₂O — prepared by mixing hot concentrated solutions of K₂SO₄ and Al₂(SO₄)₃ in stoichiometric proportions, then crystallising on slow cooling. Its NEET relevance is limited to identification (colourless octahedral crystals, acidic solution, gives white gelatinous Al(OH)₃ with NaOH).
Bridge to NEET: Questions test (a) why Mohr's salt is preferred over FeSO₄, (b) indicator choice for permanganimetric titration, (c) the normality equation N₁V₁ = N₂V₂ applied to redox end-points.
Watch-out: The end-point of KMnO₄ vs Mohr's salt is the first persistent faint pink — not deep purple. Overshooting (adding excess KMnO₄ past the first colour hold) invalidates the titre value. Slow drop-wise addition near end-point is the procedural safeguard.
Practice MCQs
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
MCQ 1Easy RecallPractice
Mohr's salt has the formula:
MCQ 2Easy RecallPractice
Mohr's salt is preferred over ferrous sulphate as a primary standard because:
MCQ 3Easy RecallPractice
Potash alum has the formula:
MCQ 4Direct ApplicationPractice
In the titration of Mohr's salt against KMnO₄, the indicator used is:
MCQ 5Direct ApplicationPractice
In an acid-base titration of a weak acid with a strong base, which indicator is appropriate?
MCQ 6Direct ApplicationPractice
25.0 mL of 0.1 N Mohr's salt solution requires how many mL of 0.02 N KMnO₄ to reach the end-point?
MCQ 7Concept TrapPractice
A student performing KMnO₄ vs Mohr's salt titration notices the pink colour appears but fades on swirling. The correct action is:
MCQ 8CalculationPractice
A Mohr's salt solution is prepared by dissolving 3.92 g in 100 mL. Given M(Mohr's salt) = 392 g/mol and that Fe²⁺ → Fe³⁺ involves a 1-electron change, what is the normality of the solution with respect to the KMnO₄ titration?
Quick recall before you leave
Worked Example
- 1
Given
- Volume of Mohr's salt solution (analyte): V₁ = 25.00 mL - Normality of Mohr's salt: N₁ = 0.1 N (n-factor = 1 for Fe²⁺ → Fe³⁺) - Normality of KMnO₄ solution: N₂ = unknown - Titre volume (KMnO₄ used): V₂ = 20.00 mL - Medium: dilute H₂SO₄ (acidic), no separate indicator required
- 2
Required
Find the normality (N₂) of the KMnO₄ solution.
- 3
Concept
At the redox equivalence point, equivalents of reducing agent = equivalents of oxidising agent. This is expressed as N₁V₁ = N₂V₂.
- 4
Formula
N₁V₁ = N₂V₂
- 5
Substitution
0.1 × 25.00 = N₂ × 20.00
- 6
Calculation
N₂ = (0.1 × 25.00) / 20.00 = 2.500 / 20.00 = 0.125 N Note on exact values: The volumes 25.00 mL and 20.00 mL are measured values (4 significant figures from a burette). The normality 0.1 N is given as a 1-sig-fig value, so the final answer is limited to 1 significant figure. However, in NEET practical contexts, 0.1 N is conventionally treated as exact (defined concentration), giving the answer as 0.125 N (3 sig figs from the volume ratio).
- 7
Final answer
**N₂ = 0.125 N** The n-factor of KMnO₄ in acidic medium is 5 (Mn⁷⁺ → Mn²⁺), so the corresponding molarity = 0.125 / 5 = 0.025 M. This cross-check confirms the answer is physically reasonable.
- 8
Common trap
Overshooting the end-point: if the student records the titre as, say, 21.5 mL (added KMnO₄ past the first persistent pink), the calculated N₂ would be falsely low (0.116 N). The procedural safeguard is drop-wise addition near end-point and accepting the first 30-second-persistent faint pink.
- 9
Similar NEET-style question
"25.0 mL of a FeSO₄ solution of unknown concentration is titrated against 0.04 N KMnO₄. If 12.5 mL of KMnO₄ is consumed, what is the concentration of FeSO₄ in g/L?" (Apply N₁V₁ = N₂V₂ to find N₁, then convert normality to g/L using equivalent weight of FeSO₄.) ---
Before solving, remember these
Formulas
Molarity-stoichiometry titration
Use when normality is awkward (e.g., diprotic acids). Stoichiometric coefficients from balanced equation.
| Symbol | Quantity | SI Unit |
|---|---|---|
| M | molarity | mol/L |
| V | volume | L |
| n | coefficient | - |
Valid when
- Balanced equation known
- Same end-point
Normality equation in titration
Equivalents of acid = equivalents of base at end-point. Or for redox: equivalents of oxidant = equivalents of reductant.
| Symbol | Quantity | SI Unit |
|---|---|---|
| N | normality | eq/L |
| V | volume | mL or L |
Valid when
- Same titration end-point
- Equivalent factors known
Exam Traps & Common Mistakes
These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.
Category: Inorganic Exception
Cations like Pb²⁺ precipitate in BOTH Group I (with HCl) and Group II (with H2S) — assigning to only one group misses the redundancy.
When it triggers
Cation that appears in two analytical groups, e.g. Pb²⁺ (Group I + Group II) or Hg²⁺ vs Hg2²⁺.
How to avoid
Apply confirmatory tests for each candidate group; do not assume mutual exclusivity.
Category: Overthinking
Continuing to add titrant past the first persistent colour change because the colour seemed to fade after a swirl.
When it triggers
Question describes 'colour faded after swirling' or 'persistent colour' — distinguishes transient vs end-point.
How to avoid
End-point = first PERSISTENT colour change (lasts ≥30 s). Transient fades back to original on swirling.
Category: Similar Terms
Phenolphthalein (pH 8.2–10) and methyl orange (pH 3.1–4.4) only mark equivalence when the eq-pt pH falls within their range; using the wrong indicator gives an end-point that disagrees with the actual equivalence point.
When it triggers
Titration prompt mentions a specific weak/strong combination but asks which indicator is suitable.
How to avoid
Match the indicator's pH-change range to the equivalence-point pH: phenolphthalein for eq-pt > 7, methyl orange for eq-pt < 7.
Root cause: concept gap
Correction
Pb²⁺ appears in both Group I (PbCl2 white ppt, soluble in hot water) and Group II (PbS black ppt). Confirm with hot-water solubility test.
Root cause: concept gap
Correction
Strong-acid + strong-base: any indicator (eq pt = 7). Weak-acid + strong-base: phenolphthalein (eq pt > 7). Strong-acid + weak-base: methyl orange (eq pt < 7).
Root cause: rushed under time pressure
Correction
Slow drop-wise addition near the end-point; first persistent colour change is the end-point. Re-do if overshot.
Past Year Questions
3 questions from NEET 2024, 2025. Answers verified against NTA official keys.
NEET 2025
1A-III, B-II, C-I, D-IV
2A-III, B-IV, C-II, D-I
3A-III, B-IV, C-I, D-II
4A-III, B-II, C-IV, D-I
NTA Answer: Option 3(final)
NEET 2024Revised key
1dilute hydrochloric acid
2concentrated sulphuric acid
3dilute nitric acid
4dilute sulphuric acid
NTA Answer: Option 4(revised_final)
NEET 2024Revised key
1B, A, D, C, E
2B, C, A, D, E
3E, C, D, B, A
4E, A, B, C, D
NTA Answer: Option 1(revised_final)
How NEET usually asks this
Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.
Practical chemistry — titrimetric end-point selection, indicator–pH matching, qualitative-analysis cation/anion identification, organic functional-group tests.
Direct ApplicationMedium
Common distractors
wrong zero error direction
Sign convention is easily flipped.
Sources
NCERT refs: Class 12 Chemistry Chapter 4, p.118
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