[neobenzene]

Hi. I recently came across the fact that dichromate ion (Cr2O7 2−) is stable in an acidic medium, whereas chromate ion
(CrO4 2-), is stable only in basic medium.

What is stable in neutral medium, i.e, pH = 7?

[Borek]

I am not convinced calling them "stable" is a correct term, but perhaps that's just my English failing me. There is an equilibrium in the solution:

2CrO₄^2- + 2H⁺ Cr₂O₇^2- + H₂O

If memory serves me well, Kc is around 10^-15 - you should be able to calculate relative concentrations of both forms at pH 7.

[neobenzene]

The concept sounds familiar...but I don't know how exactly I should proceed. What do I do?

[Borek]

Write formula for the equilibrium constant (reaction quotient), plug everything you know in, solve for chromate/dichromate ratio.

[neobenzene]

So, that means Kc = [Cr2O7 2-] = 10^-15 ?
                            -----------
                             [CrO4 2-]^2

Is that right?

[Borek]

No, reaction quotient should also contain concentration of H⁺ (and perhaps of water).