In the above figure variation of potential of the system MnO 4 – /Mn 2+ with pH is shown when concentrations of other products and reactants are equal to 1 M.
(i)What is the standard reduction potential of MnO 4 – /Mn 2+ ?
Text Solution
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(i)
Sol. MnO 4 – + 8H + + 5e ⎯→ Mn 2+ + 4H 2 O
= 
– 
log 
= 
– 
log [H + ] –8
or 
= 
+ 
× 0.059 log [H + ]
or 
= 
– 
× 0.059 × pH
From the above figure it is seen at pH = 3,
= 1.2
∴ 1.2 = 
– 
× 0.059 × 3
∴ 
= 1.2 + 
× 0.059 × 3 = 1.48 V
(ii)
Sol. At pH greater than one but lesser than five
(iii)
Sol. 
= 
– 
log 
= 1.7 – 0.059 × 
× pH
= 1.48 – 
× 0.059 × pH
∴ 1.7 – 0.059 × 
× pH
= 1.48 – 
× 0.059 × pH
or 0.059 × pH × (1.6 – 1.33) = 1.48 – 1.7
= – 0.22
or pH = 
= – 13.8
At pH = –15
= 1.7 – 0.059 × 
× (– 15) = 2.88
= 1.48 – 
× 0.059 × (– 15) = 2.896
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