Question

Calculate the molality of Ferrous ammonium sulphate dissolved 140g in 5000ml

Answer 1

Answer:

Prepare M/20 solution of ferrous ammonium sulphate (Mohr's Salt). Using this solution, find out the molarity and strength of the given KMnO4solution.

Theory:

Molecular mass of Mohr's Salt FeSO4.(NH4)2SO4.6H2O = 392

Therefore to prepare 1000 ml of 1M Mohr's Salt solution, we need 392g of it.

Therefore to prepare 250 ml of M/20 Mohr's Salt solution, we need

392/1000 * 250 * 1/20 = 4.9g

Apparatus required:

Watch glass, beaker, measuring cylinder, pipette, burette, wash bottle, conical flask.

Chemicals required:

Mohr's salt, concentrated H2SO4, M/100 KMnO4solution.

Chemical equations:

Molecular equations-

2KMnO4 + 3H2SO4 -> K2SO4 + 2MnSO4 + 3H2SO4+ 5[O].

[2FeSO4(NH4)2SO4.6H2O + H2SO4 + [O] -> Fe(SO4)3 + 2(NH4)2SO4 + 13H2O]*5

2KMnO4 + 8H2SO4 + 10 FeSO4.6H2O -> K2SO4 + 2MnSO4 + 5Fe2(SO4)3 + 10(NH4)2 + 68H2O

Ionic equations-

MnO4- + 8H+ + 5e- -> Mn2+ + 4H2O

[Fe2+ -> Fe3+ ]*5

MnO4- + 8H+ + 5Fe2+ -> 5Fe3+ + Mn2+ + 4H2O

Indicator:

KMnO4 is a self indicator.

End point:

Colorless to permanent pink color (KMnO4 in the burette).

Observations:

Weight of watch glass = 18g Weight of watch glass + Mohr's Salt = 22.9g Weight of Mohr's Salt = 4.9g

Volume of Mohr's salt solution prepared = 250 ml Molarity of Mohr's salt solution = M/20 Volume of Mohr's salt solution atken for each titration = 10 ml

Calculations:

a) Molarity of KMnO4 solution- From the overall balanced equation, it is clear that 2 moles of KMnO4 react with 10 moles of Mohr's Salt

(MKMnO4 * VKMnO4) / (MMohr's Salt * VMohr's Salt) = 2/10 where M stands for molarity, and V for volume.

Therefore MKMnO4 = 0.01M (Approximately).

b) Strength of the KMnO4 solution. Strength (in g/L) = Molarity * Molar Mass = 0.01 * 158 = 1.58 g/L

Result:

The molarity of the given KMnO4 solution is 0.01M. The strength of the given KMnO4 solution is 1.58g/L.

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