How can you precipitate BaCrO4(s) from a solution of barium nitrate and potassium dichromate? I don't get it.. it's a question from a lab we did on chromate-dichromate equilibrium.

Answer 1

There is an equilibrium represented as follows:
2CrO4^2- + 2H^+ ==> Cr2O7^2- + H2O
So in acid solution the Cr2O7^2- predominates and in neutral or basic solution the CrO4^2- is the main ion.
Barium ion forms a ppt with CrO4^2- but not with Cr2O7^2-. So to get BaCrO4 you are depending upon the equilibrium conditions to help you get enough CrO4^2- to exceed the Ksp of BaCrO4.

Answer 2

Thank you! :D