# Which one of the compounds listed would we use in order to get the highest concentration of sulphate ion in a saturated solution?

Solutions are prepared by dissolving a sulphate salt in water. Which one of the compounds listed would we use in order to get the highest concentration of sulphate ion, SO42-, in a saturated solution? What would be the concentration of sulphate ion in this case?

A. $$\ce{CaSO4}$$ $$K_{sp}$$ = 7.1E-5

B. $$\ce{SrSO4}$$ $$K_{sp}$$ = 3.4E-7

C. $$\ce{BaSO4}$$ $$K_{sp}$$ = 1.1E-10

D. $$\ce{RaSO4}$$ $$K_{sp}$$ = 4.0E-11

My first instinct was to simply look at the $$K_{sp}$$ values.. Since $$\ce{CaSO4}$$ has the largest $$K_{sp}$$ value, it should be the most soluble, right?

But then I looked at a data page referring to the solubility of common compounds in water (Please see image. According to the table, $$\ce{SO4^2-}$$ has low solubility with $$Ca$$, $$Sr$$, and $$Ba$$.

My question is- does the solubility depend on the $$K_{sp}$$ value (was my first instinct right), or is the table correct?

Any ideas would be appreciated. The $K_{sp}$ values are the way to go .. the other information you mentioned are just some convenient rules of thumb for predicting relative solubilities of compounds when you don't know the $K_{sp}$ values. Also, the 4th choice in your list .. radium .. probably also belongs on that list, since it's the next alkali earth element after barium, but it's included because it's a radioactive element, and people don't commonly use or encounter it.