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
prepare-m20-ferrous-ammonium-sulphate-mohrs-salt-solution-1

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
prepare-m20-ferrous-ammonium-sulphate-mohrs-salt-solution-2

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.
prepare-m20-ferrous-ammonium-sulphate-mohrs-salt-solution-3
prepare-m20-ferrous-ammonium-sulphate-mohrs-salt-solution-4

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