Self-indicator titration: KMnO4 (purple) → Mn²⁺ (colourless) in acidic medium (dilute H2SO4). Half-reaction: MnO4⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H2O. Oxalate: C2O4²⁻ → 2CO2 + 2e⁻. Heat to 60–70 °C to initiate the auto-catalysed reaction. End-point: first persistent pink colour.
-- NCERT, p. 252Titrimetric Redox
8 MCQs1 revision card9-step worked example
Source: NCERT Unit 20PYQ coverage: NEET 2024, 2025Official key: NTA-verifiedLast reviewed: May 2026
Lesson
The trap that costs marks: In KMnO₄ vs oxalic acid titrations, aspirants frequently overshoot the end-point — they see a pink flash, swirl, watch it fade, and add more titrant. That extra drop converts a correct concordant reading into a discordant one, and the calculated molarity drifts high.
The reaction. In acidic medium (dilute H₂SO₄), permanganate oxidises oxalic acid:
2 KMnO₄ + 5 H₂C₂O₄ + 3 H₂SO₄ → 2 MnSO₄ + K₂SO₄ + 10 CO₂ + 8 H₂O
KMnO₄ is self-indicating: purple MnO₄⁻ reduces to colourless Mn²⁺. The end-point is the first persistent pale pink colour that survives at least 30 seconds of swirling (NCERT Class 11, Chemistry Part II, Chapter 8, page 252).
Normality approach. The equivalent factor (n-factor) of KMnO₄ in acidic medium is 5 (Mn goes from +7 to +2). For oxalic acid, n-factor is 2 (C goes from +3 to +4, two carbon atoms). The normality equation applies:
N₁V₁ = N₂V₂
where subscripts 1 and 2 refer to KMnO₄ and oxalic acid respectively (or vice versa — consistency matters).
Why heating matters. The reaction is slow at room temperature. The flask is heated to 60–70 °C before titration begins. Overheating (>70 °C) decomposes oxalic acid, giving falsely low titre values.
Watch-out: A transient pink that vanishes on swirling is NOT the end-point. Only a persistent colour change (≥30 s) signals equivalence. Overshooting even by one drop inflates KMnO₄ volume and the calculated normality of oxalic acid drops below its true value.
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
In the titration of oxalic acid with KMnO₄ in acidic medium, what is the n-factor (equivalent factor) of KMnO₄?
MCQ 2Easy RecallPractice
Why is no separate indicator needed in the titration of oxalic acid against KMnO₄?
MCQ 3Easy RecallPractice
The balanced equation for the reaction between KMnO₄ and oxalic acid in acidic medium is: 2 KMnO₄ + 5 H₂C₂O₄ + 3 H₂SO₄ → products. What is the molar ratio of KMnO₄ to H₂C₂O₄?
MCQ 4Direct ApplicationPractice
During KMnO₄ vs oxalic acid titration, the solution is heated to 60–70 °C before adding KMnO₄. What happens if the temperature exceeds 70 °C?
MCQ 5Direct ApplicationPractice
A student standardises KMnO₄ solution against standard oxalic acid (0.1 N). KMnO₄ is in the burette. The student overshoots the end-point by one drop. How does this affect the calculated normality of KMnO₄?
MCQ 6Direct ApplicationPractice
In KMnO₄ titration with oxalic acid, dilute H₂SO₄ is used to provide acidic medium. Why is HCl NOT used instead?
MCQ 7Direct ApplicationPractice
25.0 mL of 0.1 N oxalic acid is titrated against KMnO₄ solution. The end-point is reached at 20.0 mL of KMnO₄. What is the normality of the KMnO₄ solution?
MCQ 8Concept TrapPractice
In an acidic-medium KMnO₄ titration, a student observes that the initial drops of KMnO₄ decolourise very slowly, but after some oxalic acid has reacted, subsequent drops decolourise almost instantly. What explains this acceleration?
Quick recall before you leave
Worked Example
Pattern: NEET pattern: practical bundle (redox titration calculation)
- 1
Given
- Volume of oxalic acid solution (V₂) = 25.0 mL - Normality of oxalic acid (N₂) = 0.05 N (prepared from 0.63 g H₂C₂O₄·2H₂O in 200 mL) - Volume of KMnO₄ at end-point (V₁) = 18.5 mL - Medium: dilute H₂SO₄ (acidic)
- 2
Required
Find the normality and molarity of the KMnO₄ solution.
- 3
Concept
At the equivalence point, equivalents of oxidant = equivalents of reductant. In acidic medium, KMnO₄ has n-factor = 5 (Mn: +7 → +2). Oxalic acid has n-factor = 2 (each C: +3 → +4, two C atoms).
- 4
Formula
N₁V₁ = N₂V₂ Molarity = Normality / n-factor
- 5
Substitution
N₁ × 18.5 = 0.05 × 25.0
- 6
Calculation
N₁ = (0.05 × 25.0) / 18.5 N₁ = 1.25 / 18.5 N₁ = 0.0676 N Molarity of KMnO₄ = N₁ / n-factor = 0.0676 / 5 = 0.01351 M **Note on exact values:** The stoichiometric coefficients (2, 5) and n-factors (5, 2) are exact integers and do not limit significant figures. The result is limited by the 3 significant figures in the measured volumes and normality.
- 7
Final answer
Normality of KMnO₄ = 6.76 × 10⁻² N Molarity of KMnO₄ = 1.35 × 10⁻² M (Scientific notation used to avoid ambiguity in trailing zeros — per Rule 2.)
- 8
Common trap
**End-point overshoot (trap: end point overshoot):** If the student recorded 19.0 mL instead of 18.5 mL by overshooting, the calculated N₁ would be (1.25/19.0) = 0.0658 N — about 2.7% lower than the true value. In NEET MCQs, the "overshoot" distractor typically gives the next-lower option.
- 9
Similar NEET-style question
"20.0 mL of an acidified oxalic acid solution requires 16.0 mL of 0.02 M KMnO₄ for complete oxidation. Find the molarity of the oxalic acid solution." (Approach: Convert M(KMnO₄) to normality using n=5, apply N₁V₁ = N₂V₂, then convert back to molarity of oxalic acid using n=2.) ---
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.
Test yourself on this topic with real past-paper questions:
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