what sort of information does molar solubility tell you that the ksp doesn't?

[mrh125]

on the AAMCs exams I've mainly been using the ksp to compare the solubility of compounds and questions about what precipitates first. In TPBR however, they use the molar solubility to determine this. What does the molar solubility tell us that the ksp doesn't? I know the molar solubility is the x-term in the ksp the formula.

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[sazerac]

Molar solubility is handier if you have pure water and want to know how much solute you can pour in before it precipitates.

Ksp is more detailed and would be more useful if you had an ionic solute that breaks up into parts, especially if the solution already has some of the ions floating around or maybe you are thinking about adding more ions later.

 

[Cmdr_Shepard]

What he said.

 

[dudewheresmymd]

Molar solubility can be used to compare any two compounds REGARDLESS of how many ions a compound dissociates into. The Ksps of NaCl and KCl can be compared because their Ksp = x^2, however Ksp for CaCl2 will equal 4x^3. You cannot compare the relative Ksps of NaCl (2 ion system) and CaCl2 (3 ion system), you must use the molar solubility "x" as you mentioned to compare two compounds that dissociate into different numbers of ions.