Hi, @MC789. I took the MCAT last June and have been working as a tutor this year. This question has to do with the concept of Ksp aka the solubility product constant along with LeChatelier's principle. Just like Ka and Kb are the equilibrium constants which tell us how strong or weak an acid/base is based on how much an acid dissociates or a base associates with a proton, Ksp is the equilibrium constant which tells us how much some ionic solid will dissociate into its ions in an aqueous solution. Barium Chromate and Sodium Chromate have Ksp values that are independent of each other. In a saturated solution of Barium Chromate, the ion product aka [Ba2+][CrO42-] would be equal to Ksp for Barium Chromate. The sodium chromate would dissociate based on its Ksp but this is where both would start interacting. Since the sodium chromate would be providing a common ion, the chromate ion, and since the barium chromate solution is saturated, to maintain equilibrium according to LeChatelier's principle, the equilibrium for the barium chromate solution would be pushed to the left, thus precipitating the solid form of Barium Chromate. Let me know if you have any further questions or feel free to PM me.
