Class 12 Practical Titration | KMnO4 Mohr's Salt | Indian students Guide | Calculations + Viva 2026

The Class 12 Chemistry Practical on titration of KMnO₄ (potassium permanganate) vs Mohr's salt (ferrous ammonium sulfate, FeSO₄·(NH₄)₂SO₄·6H₂O) is a standard redox titration experiment in CBSE and most Indian boards. The goal is usually:

Prepare a standard solution of Mohr's salt (e.g., M/20 or 0.05 M).
Titrate it against the given KMnO₄ solution to find the molarity and strength (g/L) of KMnO₄.

This is a self-indicated titration (KMnO₄ is purple; endpoint is permanent faint pink due to excess MnO₄⁻).

Theory & Redox Reaction
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Mohr's salt provides Fe²⁺ ions (reducing agent), and KMnO₄ in acidic medium is a strong oxidizing agent (oxidizes Fe²⁺ to Fe³⁺ and reduces MnO₄⁻ to Mn²⁺).
Ionic half-reactions (most important for viva):

Reduction: MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O
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(n-factor of KMnO₄ = 5 in acidic medium)
Oxidation: Fe²⁺ → Fe³⁺ + e⁻
(n-factor of Fe²⁺ / Mohr's salt = 1)

Balanced ionic equation:
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MnO₄⁻ + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O
Molecular equation (for reference):
2KMnO₄ + 10FeSO₄(NH₄)₂SO₄·6H₂O + 8H₂SO₄ → K₂SO₄ + 2MnSO₄ + 5Fe₂(SO₄)₃ + 10(NH₄)₂SO₄ + 68H₂O
Dilute H₂SO₄ is added to provide acidic medium and prevent precipitation of MnO₂.
Endpoint: Appearance of permanent faint pink color (self-indicator).
Apparatus Required

Burette (50 mL), pipette (20 mL or 10 mL), conical flask (250 mL)
Measuring/volumetric flask (250 mL), weighing bottle, funnel
White glazed tile, burette stand, glass rod
Chemicals: Mohr's salt, dilute H₂SO₄ (~1–2 M), given KMnO₄ solution, distilled water

Procedure (Step-by-Step)
A. Preparation of standard Mohr's salt solution (usually M/20 or 0.05 M in 250 mL)

Molar mass of Mohr's salt = 392 g/mol
For M/20 solution (0.05 M): Weight required = (392 × 0.05 × 250)/1000 = 4.90 g (approximately 4.9 g for 250 mL)
Weigh accurately ~4.90 g Mohr's salt in a weighing bottle.
Transfer to 250 mL volumetric flask using funnel.
Add small amount of dilute H₂SO₄ (to prevent hydrolysis/oxidation), then add distilled water and shake to dissolve.
Make up the volume to the mark with distilled water. Shake well. This is your standard solution (Mohr's salt = reducing agent).

B. Titration

Rinse and fill the burette with the given KMnO₄ solution (remove air bubbles).
Pipette out 10 mL or 20 mL of prepared Mohr's salt solution into a conical flask.
Add ~10–20 mL dilute H₂SO₄ (about half test tube) to make the medium acidic.
Place the conical flask on a white tile under the burette.
Note initial burette reading.
Run KMnO₄ from burette slowly into the conical flask with constant gentle swirling.
The purple color disappears initially due to reduction; continue until a permanent faint pink color appears throughout the solution (endpoint).
Note final burette reading. Volume of KMnO₄ used = final – initial.
Repeat for 2–3 concordant readings (difference less than 0.1 mL).

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