Identify the cases(s) where there in change in oxidation number.

Options

A. Acidified solution of `CrO_(4)^(2-)`
B. `SO_(2)` gas bubbled through an acidic solution of `Cr_(2)O_(7)^(2-)`
C. alkaline solution of `CrO_(4)^(2-)`
D. aqueous solution of `CrO_(2)CI_(2)` in NaOH

Answer

b

Explanation

(a) `2overset(+6)CrO_(4)^(2-)+2H^(+)rarroverset(+6)Cr_(2)O_(7)^(2-)+H_(2)O`
(b)`overset(Cr_(2)O_(7)^(2-)+2H^(+)+3SO_(2)rarroverset(+3)Cr_(2)(SO_(4))^(3)+H_(2)O`
(c ) `overset(+6)Cr_(2)O_(7)^(2-)+2OHA^(-)rarr2overset(+6)CrO_(4)^(2)+H_(2)O`
(d) Ammoniacal solution of `CrO_(4)^(2-)` is
`NH_(3)+H_(2)OrarrNH_(4)^(+)+OH^(-)`
`overset(+6)CrO_(4)^(2-)+2 NH_(4)^(+)rarr(NH_(4))_(2)overset(+6)crO_(4)`
`overset(+6)CrO_(2)CI_(2)+4NaOHrarrNa_(2)overset(+6)CrO_(4)+2NaCI+2H_(2)O`
Thus only in rection (b) O.N of Cr changes form +6 to +3