Experiment – 1
Redox Titration using Mohr’s Salt
Preparation of 0.1 M Mohr’s Salt Solution and Determination of Molarity & Strength of KMnO₄ Solution
1
AIM
Chemical Equations
Molecular Equation
Reduction Half Reaction:
2 KMnO₄ + 3H₂SO₄ → K₂SO₄ + 2 MnSO₄ + 3H₂O + 5 [O]
Oxidation Half Reaction:
2 FeSO₄(NH₄)₂SO₄·6H₂O + H₂SO₄ + [O] → Fe₂(SO₄)₃ + 2(NH₄)₂SO₄ + 13 H₂O] × 5
Overall Balanced Equation:
2KMnO₄ + 8H₂SO₄ + 10FeSO₄(NH₄)₂SO₄·6H₂O → K₂SO₄ + 2MnSO₄ + 5Fe₂(SO₄)₃ + 10(NH₄)₂SO₄ + 68H₂O
Ionic Equation
Reduction Half Reaction:
MnO₄⁻ + 5e⁻ + 8H⁺ → Mn²⁺ + 4H₂O
Oxidation Half Reaction:
Fe²⁺ → Fe³⁺ + e⁻ × 5
Net Ionic Equation
MnO₄⁻ + 5Fe²⁺ + 8H⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O
Indicator
KMnO₄ acts as a self-indicator.
End Point: Colourless to permanent pink colour.
2
PROCEDURE
A. Preparation of Standard Mohr’s Salt Solution
Molar mass of FeSO₄(NH₄)₂SO₄·6H₂O = 392 g/mol
-
1.
Weigh accurately 9.8 g of Mohr’s salt and transfer it into a 250 mL volumetric flask through a funnel. -
2.
Rinse the funnel with distilled water and add dilute H₂SO₄ dropwise to get a clear solution. -
3.
Shake well and make up the volume to the 250 mL mark with distilled water.
B. Titration
-
1.
Rinse and fill the burette with KMnO₄ solution. Remove air bubbles. -
2.
Pipette out 20 mL of Mohr’s salt solution into a conical flask and add ~5 mL of 1M H₂SO₄. -
3.
Titrate with KMnO₄ solution while swirling until a permanent pink colour appears. -
4.
Repeat the titration to get three concordant readings.
Observation Table
| S.No. | Volume of Mohr’s Salt (mL) | Initial Burette Reading (mL) | Final Burette Reading (mL) | Volume of KMnO₄ (mL) |
|---|---|---|---|---|
| 1 | 20 | 0.0 | 19.9 | 19.9 |
| 2 | 20 | 0.0 | 19.8 | 19.8 |
| 3 | 20 | 0.0 | 19.8 | 19.8 |
Concordant Volume of KMnO₄ used (V₂) = 19.8 mL
Calculations
1. Preparation of 0.1 M Mohr’s Salt
W = \(\dfrac{\text{Mol. wt.} \times M \times V(\text{ml})}{1000}\) = \(\dfrac{392 \times 0.1 \times 250}{1000}\) = 9.8 g
2. Molarity of KMnO₄ Solution
Formula: a₁M₁V₁ = a₂M₂V₂
M₂ = \(\dfrac{1 \times 0.1 \times 20}{5 \times 19.8}\) = \(\dfrac{2}{99}\) = 0.0202 M
3. Strength of KMnO₄
Strength = Molarity × Molar mass of KMnO₄
= 0.0202 × 158 = 3.1916 g/L
RESULT
0.0202 M
3.1916 g/L
Experiment completed successfully.