common ion effect

[puffylover]

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if you want to separate AgCl from a mixture of AgCl and Ag2SO4, then you can do so by adding NaCl. This will selectively crash out AgCl by the common ion effect (Cl- being the common ion).


can someone explain this?

AgCl is insoluble, right?

Ag2SO4 is soluble?

My thought process:

AgCl --> Ag+ + Cl-

Ksp= [Ag+][Cl-]
if you add Cl-, you exceed the concentration "allowed" by Ksp and precipitate AgCl. SO42- would be left behind in solution?

 

[loveoforganic]

This isn't so much common ion effect as it is single displacement reaction.

You're essentially reacting silver sulfate (a little soluble) with sodium chloride (very soluble) to form silver chloride (essentially insoluble) and sodium sulfate (very soluble).

Any excess sodium chloride will have somewhat of a common ion effect and cause silver chloride to be even less soluble, but it's already so insoluble that hardly matters.

 

[Phantastic]

Any excess sodium chloride will have somewhat of a common ion effect and cause silver chloride to be even less soluble, but it's already so insoluble that hardly matters.

I think the question is more about purification. To precipitate any impurities in a solution, the common ion effect is a good strategy, and a common motif for MCAT. AgCl was present as an impurity beforehand (albeit in low concentration), NaCl was used to precipitate it out (due to the high dissociation of NaCl as LoveOfOrgo pointed out), and now the only silver in our solution should be that from the sulfate form (our pure product).

 

[loveoforganic]

If you add sodium chloride, you aren't going to have any silver in solution left 😛

 

[TFS]

adding NaCl introduces more Cl- into solution, which reduces the solubility of AgCl and helps remove it from solution. Cl- is the common ion.

 

[loveoforganic]

If you have silver sulfate though, that chloride is going to react to precipitate those soluble silver ions in solution out as silver chloride before any common ion effect comes into play.