When an orange solution containing Cr2O72– ion is treated with an alkali,

Question:

When an orange solution containing Cr2O72– ion is treated with an alkali, a yellow solution is formed and when H+ ions are added to a yellow solution, an orange solution is obtained. Explain why does this happen?

Solution:

When Cr2O72–is treated with an alkali:

(orange) Cr2O72–+ OH-→ 2CrO42-(yellow)

When the yellow solution is treated with an acid, we get back the orange solution:

(yellow)2CrO42- +2H+→Cr2O72–(orange)+ H2O

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