Prepare M/20 Ferrous Ammonium Sulphate (Mohr’s salt) Solution

Prepare M/20 Ferrous Ammonium Sulphate (Mohr’s salt) Solution. Find out the Percentage Purity of Impure KMnO₄ Sample 2.0 g of  Which have been Dissolved per Litre 

Chemical Equations

Indicator
KMnO₄ is a self-indicator.

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

Procedure
1. Prepare 250 ml of M/20 Mohr’s salt solution by dissolving 4.9 g of Mohr’s salt in water as described in experiment 11.4. Rinse the pipette with the M/20 Mohr’s salt solution and pipette out 20.0 ml of it in a washed titration flask.
2. Rinse and fill the burette with the given 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 last single drop of KMnO₄ solution.
6. Note the final reading of the burette.
7. Repeat the above steps 4-5 times to get a set of three concordant readings.

Observations
Weight of watch glass =……. g
Weight of watch glass + Mohr’s salt =…………..g
Weight of Mohr’s salt = 4.9 g
Volume of Mohr’s salt solution prepared = 250 ml
Solution taken in burette = KMnO₄ solution
Volume of Mohr’s salt solution taken for each titration = 20.0 ml

Calculations
Calculation of molarity of KMnO₄ solution
From the overall balanced chemical equation, it is clear that 2 moles of KMnO₄ react with 10 moles of Mohr’s salt.

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