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Question

Find the number of moles of KMnO4 needed to oxidise one mole Cu2S in acidic medium. The reaction is KMnO4+Cu2SMn2++Cu2++SO2


A
0.4
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B
1.6
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C
0.5
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D
1.2
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Solution

The correct option is B 1.6
K+7MnO4++1Cu22SMn2++Cu2+++4SO2
Formula used for the n-factor calculation,
nf=(|O.S.ProductO.S.Reactant|)×number of atoms
nf of KMnO4=|(+7)(+2)|×1=5

If in a reaction, two elements of same reactant molecule are undergoing either oxidation or reduction then the n-factor of the molecule will be the sum of the individual n-factors of both the elements.
nf of Cu2S=(|(+1)(+2)|×2)+(|(2)(+4)|×1)=8

From law of equivalence 

Equivalents of Cu2S=Equivalents of KMnO4
Moles of Cu2S×nf=Moles of KMnO4×nf
Moles of KMnO4 needed per mole of Cu2S=1×85=1.6

Chemistry

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