The number of moles of `KMnO_(4)` that will be needed to react with one mole of ferrous sulphite in acidic solution is

Options

A. `1`
B. `3//5`
C. `4//5`
D. `2//5`

Answer

B

Explanation

`overset(+7)(M)nO_(4)^(-) rarr overset(+2)(M)n^(2+)`
Since the oxidation number of Mn changes by 5 units `(+7 rarr +2)` , 1 mol of `KMnO_4` contains five equivalents of `KMnO_4`.
`overset(+4)(S)O_(3)^(2-) rarr overset(+6)(S)O_(4)^(2-)`
Since the oxidation number of S changes by 2 units `(+4 rarr +6)`, 1 mol of sulphite contains two equivalents .
This means we need 2 equivalents of ` KMnO_4` to react with 1 mol of sulphite . As mentioned above, 5 equivalents of ` KMnO_4` are present in 1 mol . thus , 2 equivalents will be present in ` 2//5` mol.
Alternatively, we can get the answer from the balanced chemical equation
`2 MnO_4^(-) + 6 H^(+) + 5SO_3^(2-) rarr 2Mn^(2+) + 3H_2O +5SO_4^(2-)`
` 2MnO_4^(-) + 6H^(+) + 5SO_3^(-2) rarr 2Mn^(2+) + 3H_2O + 5SO_4^(2-)`
5 mol of ` SO_3^(2-)` react eith 2 mol of ` MnO_4^(-) `. Thus , 1 mol of ` SO_3^(2-)` will react with ` 2//5` mol of `MnO_4^(-).`