Which of the following would be more soluble in an acidic solution?

Question

Which of the following would be more soluble in an acidic solution?

CsF(s) or RbI(s)

I assume it would be CsF and I also make the assumption that it means more soluble in an acidic solution as opposed to a basic or pure water solution.

My question is why would CsF(s) be more soluble in an acidic solution? Would it have something to do with the conjugate bases? and how would you identify that?

Answer 1

CsF is the salt of a weak acid, HF. RbI is the salt of a weak acid, HI. Therefore, you would expect CsF to be more soluble, and you are correct in saying that it has to do with the conjugate base. Let's take a look at each salt dissolved in water and then the addition of acid, H⁺.

CsF ==> Cs⁺ + F^-

F- + H₂O ==> HF + OH^-

Remember HF is weak so will not ionize much. Addition of H⁺ will react with F- making more HF and this will increase solubility. Ksp = [Cs⁺][F^-] and if you decrease F^-, you decrease Ksp. Another way to look at it, is that addition of H⁺ will remove OH- from the right side, and according to Le Chatelier, this will push the reaction toward the product side, increasing the solubility.

RbI ==> Rb+ + I-

I^- + H⁺ ==> HI which ionizes completely back to H⁺ + I^- so acid will not affect the solubility.

So, to answer the last part of your question, you identify it by knowing if the salt is from a weak acid or a strong acid, If it's from a weak acid, the conjugate base will be strong and this will increase solubility in acid solution.