Common Ion

[bajoneswadup]

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I'm a little confused as to how I'm supposed to know when to use "like dissolves like" vs common ion effect. So far, if it's in the BS section I go with "like dissolves like" and in PS I do common ion, but is there a better way to do this?

For example, if it says a cpd is insoluble in acid, in the bio section I think "oh it's nonpolar", but in the PS section I think "oh it's an different acid"

Is this thinking correct?

 

[syoung]

Well if it's insoluble in acid, yes it may be nonpolar, but it can also just have no H+ to donate or free lone pairs to accept protons

 

[Buttafuoco]

The OP is a little hard to follow, but couldn't cpd (not sure what that is) be nonpolar in acidic solution because it's accepted protons in all the polar areas (like you see in extractions)?

 

[syoung]

Nonpolar would be like acetone in HCl. Acetone doesn't accept protons, but its still not miscible in HCl right?
Or another example like one of those precipitates that are only slightly soluble in Water (Pb(OH)2) or something similar. I mean that's polar isn't it?

But OP, generally for the PS they will be asking you about acids/bases/common ion effect. What's the pH of this solution, etc. etc. what's the conc of that compound...

 

[gettheleadout]

Well if it's insoluble in acid, yes it may be nonpolar, but it can also just have no H+ to donate or free lone pairs to accept protons

Well a compound doesn't have to react with a solvent for it to be soluble. Two polar compounds will be miscible without reaction if one is an acid and the other is something like 2,2,2-trifluoroethane.

The OP is a little hard to follow, but couldn't cpd (not sure what that is) be nonpolar in acidic solution because it's accepted protons in all the polar areas (like you see in extractions)?

OP is abbreviating "compound" as "cpd."

 

[Temperature101]

That is a good question that I never thought of... The logic I follow is that: If both compounds can dissociate into ions. For instance, NaCl and Na2SO4 I use the common ion effect. But if they are organic compounds like CH3OH and CH3CH2OH, I use like disolve in like. That usually works for me.

 

[Temperature101]

Bump...

 

[MedPR]

They are complementary concepts. The common ion effect does not necessarily make something insoluble, just less-soluble than another similar solvent without a common ion. In other words, HNO3 is less soluble in an aqueous solution of NO3- compared to pure water, but it is still more soluble in that same aqueous solution of NO3- than in pure hexane. "Like dissolve like" always prevails.

 

[Temperature101]

They are complementary concepts. The common ion effect does not necessarily make something insoluble, just less-soluble than another similar solvent without a common ion. In other words, HNO3 is less soluble in an aqueous solution of NO3- compared to pure water, but it is still more soluble in that same aqueous solution of NO3- than in pure hexane. "Like dissolve like" always prevails.

MedPR...Didn't you take the mcat last week? If so, how was it?

 

[MedPR]

MedPR...Didn't you take the mcat last week? If so, how was it?

Yea I took it on 4/5. I thought it was ok; the PS was outrageously calculation heavy, but the concepts weren't too bad.