Question

# Find the number of moles of KMnO4 needed to oxidise one mole Cu2S in acidic medium. The reaction is KMnO4+Cu2S→Mn2++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 1.6K+7MnO4++1Cu2−2S→Mn2++Cu2+++4SO2 Formula used for the n-factor calculation, nf=(|O.S.Product−O.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

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