Q.

3.92 g of ferrous ammonium sulphate are dissolved in 100 ml water. 20ml of this solution requires 18 ml of potassium permaganate during titration for complete oxidation. The weight of KMnO4 present in one litre of the solution is

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a

34.76 g

b

12.38 g

c

1.238 g

d

3.511 g

answer is D.

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Detailed Solution

Mohr's salt is FeSO4.(NH4)2SO4.6H2O
GMW of Mohr's salt = 392g
n-factor Mohr's salt = 1(Fe+2 →Fe+3)
GMW of KMnO4 = 158g
n-factor KMnO4/H+ = 5(MnO4- →Mn+2 )
I WMohr's salt = 3.92g
Vsol = 100ml
 

\large M\;=\;\frac{3.92}{392}\times \frac{1000}{100}\;=\;0.1M


20ml of this solution required 18ml of KMnO4/H+ for complete oxidation
Gram equivalents of Mohr's Salt = Gram equivalents of KMnO4 H+
VaNa = VbNb
20(0.1)1 = 18(Mb)5
 

\large M_b\;=\;\frac{2}{90}M


 

Molarity of KMnO4 solution = \frac{2}{90}M


 

\large Weight of KMnO4 in 1 liter solution =\frac{2}{90}\times 158


                                                                                     = 3.511g

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