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Question

Permanganate (VII) ion,MnO4 oxidies I ion to I2 and gives manganese (IV) oxide MnO2 in basic medium. The skeletal ionic equation is given as
pMnO4(aq)+xH2O(l)rMnO2(s)+sI2(s)+yOH(aq)The values of p,q,r and s are:

A
p (1), q (2), r (8), s (4)
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B
p (2), q (6), r (2), s (3)
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C
p (2), q (4), r (2), s (8)
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D
p (1), q (4), r (8), s (2)
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Solution

The correct option is B p (2), q (6), r (2), s (3)
pMnO4(aq)+qI+xH2O(l)rMnO2(s)+sI2(s)+yOH(aq)
in a balanced chemical reaction number of all atoms at right side should be equal to the left side of the reaction.
Balancing a chemical reaction as:
Step 1: Assign oxidation numbers to each of the atoms in the equation and write the numbers above the atom as:
+7MnO4(aq)+1I+H2O(l)+4MnO2(s)+0I2(s)+OH(aq)
Step 2: Identify the atoms that are oxidized and those that are reduced as:
Reduction: +7MnO4(aq)+4MnO2(s)
Oxidation: 1I0I2(s)
Step 3: oxidation-number change is:
Reduction: +7MnO4(aq)+4MnO2(s) Gain of 3 electron per Mn atom
Oxidation: 1I0I2(s): Loss of total 2 electrons
Step 4: Balance the total change in oxidation number as:
Reduction: +7MnO4(aq)+4MnO2(s)×2: Gain of 6 electron total
Oxidation: 1I0I2(s)×3: Loss of 6 electron
2+7MnO4(aq)2+4MnO2(s)
61I30I2(s):
Step 5: Balance O atoms in reduction reaction by adding H2O and then balance H by H+ as:
2+7MnO4(aq)+8H+2+4MnO2(s)+4H2O
Step 6: For base catalysed reaction add OH to both side to neutralize H+ as:
2+7MnO4(aq)+8H++8OH2+4MnO2(s)+4H2O+8OH
or, 2+7MnO4(aq)+4H2O2+4MnO2(s)+8OH
Thus overall reaction is:
2+7MnO4(aq)+61I+4H2O2+4MnO2(s)+30I2(s)+8OH
p+7MnO4(aq)+q1I+xH2Or+4MnO2(s)+s0I2(s)+yOH
thus p=2,q=6,r=2,s=3,x=4,y=8

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