like dissolves like vs. common ion

[Mae16]

hi -- I need some help answering questions about which solvents will best dissolve a given substance. I keep getting mixed up between taking a "like dissolves like" approach to the question, and considering it from a common ion effect standpoint.

For instance, a salt with OH in it dissolves better in acid and in base because of the common ion effect. (right?)

But then I see other problems where hydrocarbons are best dissolved in benzenes and other substances that are similar to hydrocarbons.

Can anyone give me an easy way not to get mixed up here? Is it that "like dissolves like" just pertains to polarity -- and common ion effect, only pertains to literally having the same ions in solution as in the thing that's dissolving? (thus common ion effect doesn't apply to hydrocarbons?)

thanks.

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[matth87]

hi -- I need some help answering questions about which solvents will best dissolve a given substance. I keep getting mixed up between taking a "like dissolves like" approach to the question, and considering it from a common ion effect standpoint.

For instance, a salt with OH in it dissolves better in acid and in base because of the common ion effect. (right?)

But then I see other problems where hydrocarbons are best dissolved in benzenes and other substances that are similar to hydrocarbons.

Can anyone give me an easy way not to get mixed up here? Is it that "like dissolves like" just pertains to polarity -- and common ion effect, only pertains to literally having the same ions in solution as in the thing that's dissolving? (thus common ion effect doesn't apply to hydrocarbons?)

thanks.

You are dealing with two different scenarios here.

Like does dissolve like. But then...

You have to look at the K(sp) - Solubility Product at Equilibrium. Even though something is dissolved the common ion effect can cause some of it to precipitate out although some will still be dissolved.

If your just looking at will it dissolve then use like dissolve like.
If your looking at common ion effect or Le Chartlears(or w/e his name is) then you need to how the reaction will shift into account.