Determine the Equivalent Mass and Number of Molecules of Water of Crystallisation in a Sample of Mohr’s salt, FeSO4(NH4)2 SO4 . nH20. Provided KMnO4
Prepare a solution of Mohr’s salt with known strength (g/litre). Molarity of ferrous ammonium sulphate can be determined by directly titrating it against standard (M/100) KMnO4 solution.
Molecular mass = strength/molarity.
Substituting the value of strength and value of molarity as calculated above, the molecular mass of Mohr’s salt can be calculated. Suppose it comes out to be M.
Theoretical molecular mass of Mohr’s salt, FeSO4.(NH4)2SO4. nH20
= 284 + 18n
∴ 284 + 18n = M
whence, n can be calculated.
In case of Mohr’s salt equivalent mass is equal to its molecular mass. Therefore, Equivalent mass of Mohr’s salt = M.
KMnO4 is a self-indicator.
Colourless to permanent pink colour (KMnO4 in burette).
- Weigh exactly 4.90 g of Mohr’s salt and dissolve in water to prepare exactly 250 ml of solution, using a 250 ml measuring flask. Rinse the pipette with the prepared Mohr’s
salt solution and pipette out 20.0 ml of it in a washed titration flask.
- Rinse and fill the burette with M/100 KMnO4 solution.
- Add one test-tube (~ 20 ml) full of dilute sulphuric acid (~ 4 M) to the solution in titration flask.
- Note the initial reading of the burette.
- 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.
- Note the final reading of the burette.
- Repeat the above steps 4-5 times to get a set of three concordant readings.
Weight of watch glass =……. g
Weight of watch glass + Mohr’s salt =…………..g
Weight of Mohr’s salt = 4.90 g
Volume of Mohr’s salt solution prepared = 250 ml
Volume of Mohr’s salt solution taken for each titration = 20.0 ml
Molarity of KMnO4 solution =M/100