Adding OH- ions to the solution and concentrating the solution by allowing water to evaporate will decrease the concentration of CrO2- ions.
In the equilibrium system shown in the image, CrO2- is a base.
Adding OH- ions to the solution will shift the equilibrium to the left, decreasing the concentration of CrO2- ions.
Concentrating the solution by allowing water to evaporate will also shift the equilibrium to the left, decreasing the concentration of CrO2- ions.
The equilibrium constant for the system is:
K = [CrO2-]^2 / [Cr2O3] [H3O+]^3
When OH- ions are added to the solution, they react with H3O+ ions to form water molecules.
This decreases the concentration of H3O+ ions, which shifts the equilibrium to the left.
OH- + H3O+ -> 2H2O
When water is evaporated from the solution, the concentration of all of the ions in the solution increases.
This shifts the equilibrium to the left.
In conclusion, both adding OH- ions to the solution and concentrating the solution by allowing water to evaporate will decrease the concentration of CrO2- ions.
Question
Consider the following equilibrium system: Evaluate each stress to determine if it will will result in an increase or decrease in the concentration of chromate ion, CrO₄²⁻, as equilibrium is re-established Cr₂O₇²⁻(aq)+3H₂O(l)⇌2CrO₄²⁻(aq)+3H₃O+(aq).
a) Addition of OH−ion
b) Concentrating the solution by allowing water to evaporate.