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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. enter image description here

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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.

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