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Topic: Separating solutions exploiting solubility  (Read 9140 times)

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Offline unistudent

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Separating solutions exploiting solubility
« on: September 20, 2010, 09:58:30 AM »
A solution of benzoic acid in 5% NaOH is placed in a flask. An equal volume of diethyl ether is then added and the mixture is stoppered, shaken and then allowed to stand and reach equilibrium. Ether is immiscible with aqueous solvents, and the mixture of liquids forms two layers: the ether layer and an aqueous layer.

(a) which liquid forms the top layer? briefly explain?

(b) which solvent contains the benzoic acid? briefly explain?

If hydrochloric acid is now carefully added to the mixture in order to neutralise the mixture, then the flask is stoppered, shaken and then allowed to stand to reach equilibrium again,

(c) now which solvent contains the benzoic acid? briefly explain?

Outline a procedure for separating a mixture of para-nitrphenol and naphthylene by exploiting their different solubilities.

So what I have is, for part a) the ether forms the top layer because the acid and base mix to create aqueous solution so it is more dense and so for part b) benzoic acid is in the bottom layer because acid and bases react and want to neutralize each other so for part c) when HCl is added, benzoic acid is reprotonated and so it is back to normal and NaCl salt is formed and then the benzoic acid makes up the top layer with diethyl ether.  But what I don't know is does benzoic acid form a homogeneous mixture with diethyl ether?

And so the last part with making a procedure I have this so far, the para-nitrophenol and napthylene make a homogeneous mixutre and then to separate, you deprotonate para-nitrophenol with NaOH and it makes an aqueous solution and thus forms the bottom layer which you would drain off and then separate further.  To separate further you add HCl to reprotonate the para-nitrophenol.  Here is where I get really stuck.  I think it forms a salt (NaCl) and I am assuming it is in solution, not a precipitate but I was told that we had to re-add ether but I don't know why or what its purpose is.


Offline Doc Oc

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Re: Separating solutions exploiting solubility
« Reply #1 on: September 20, 2010, 11:36:56 AM »
Your answers look correct.  Yes, benzoic acid would be homogeneous in the ethereal layer.

The reason you need to add ether at the end is to partition the p-nitrophenol between an aqueous and organic phase.  Once you neutralize the basic solution with HCl you essentially have salt water with your greasy organic molecule in it.  But the amount of aqueous solution is so great that it overwhelms the amount of p-nitrophenol you have, so you need to add ether to help pull it out of the aqueous.

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