You are Provided with a Partially Oxidised Sample of Ferrous Sulphate Crystals

You are Provided with a Partially Oxidised Sample of Ferrous Sulphate (FeSO₄.7H₂0) Crystals. Prepare a Solution by Dissolving 14.0 g of these Crystals per litre and Determine the Percentage Oxidation of the given Sample. Given M/100 KMnO₄ Solution

Chemical Equations

Theory
Since the given sample contains partially oxidized ferrous sulphate, it contains both ferrous ions, Fe²⁺ (unoxidised) and ferric ions Fe³⁺ (oxidised). The strength of partially oxidised sample is known. The solution of partially oxidised FeSO₄ of known strength is titrated against standard KMnO₄ solution to determine the molarity and strength of the unoxidised ferrous sulphate. From this the percentage oxidation of the sample can be calculated.

Indicator
KMnO₄ is a self-indicator.

End Point
Colourless to permanent pink (KMnO₄ in burette).

Procedure

1. Weigh exactly 3.50 g of the given sample of ferrous sulphate on a watch glass and dissolve in water to prepare exactly 250 ml of solution using a 250 ml measuring flask. Rinse and fill the pipette with prepared ferrous sulphate solution and pipette out 20.0 ml of it in a washed titration flask.
2. Rinse and fill the burette with the M/100 KMnO₄ solution.
3. Add one test-tube (~ 20 ml) full of dilute sulphuric acid (- 2 M) to the solution in titration flask.
4. Note the initial reading of the burette.
5. Now add KMnO₄ solution from the burette till a permanent light pink colour is imparted to the solution in the titration flask on addition of a last single drop of KMnO₄ solution.
6. Note the final reading of the burette.
7. Repeat the above steps 4—5 times to get three concordant reading.

Observations
Weight of watch glass =……. g
Weight of watch glass + Mohr’s salt =…………..g
Weight of mixture = 3.50 g
Volume of solution prepared = 250 ml
Molarity of KMnO₄ solution =M/100
Volume of oxalate solution taken for each titration = 20.0 ml.

Calculations
Molarity of the standard KMnO₄ solution = M/100