Acidified `K_2Cr_2O_7` solution turns green when sodium sulphite is added to it. Explain.

Options

A. `CrSO_(4)`
B. `Cr_(2)(SO_(4))_(3)`
C. `CrO_(4)^(2-)`
D. `Cr_(2)(SO_(3))_(3)`

Answer

B

Explanation

Sulphites are oxidized to sulphates:
`{:(K_(2)Cr_(2)O_(7) + 4H_(2)SO_(4) rarr K_(2)SO_(4) + Cr_(2)(SO_(4))_(3) + 4H_(2)O + 3[O]),(" "[Na_(2)SO_(3) + [O] rarr Na_(2)SO_(4)]xx3),(bar(K_(2)Cr_(2)O_(7) + 3Na_(2)SO_(3) + 4H_(2)SO_(4) rarr 3Na_(2)SO_(4) + K_(2)SO_(4) + Cr_(2)(SO_(4))_(3) + 4H_(2)O)):}`
or `Cr_(2)O_(7)^(2-) + 3SO_(3)^(2-) + 8H^(+) rarr 3SO_(4)^(2-) + 2Cr^(3+) + 4H_(2)O`