posted by CM on .
what is the precipitate formed when HCl is added? I believe the answer is NaCl, but I am not sure and this precipitate only formed in the first NaOH extract, as precipitate was essentially nonexistent in the 2nd extract? Below is what was done in the experiment. Also, does the amount of precipitate in each tube indicate that all the acid impurity has been removed from the ether layer containing the unknown neutral compound? I believe precipitate formation means impurity is still present?
0.150g of an unknown mixture was placed in a centrifuge tube with 4 mL of ether. It was shook until the solid was completely dissolved. Then 2 mL of 1.0 M NaOH was added, tube shook, layers were left to separate, bottom (aqueous) layer was removed (1st NaOH extract). 2 mL more of 1.0 M NaOH were added to the tube, shook, layers separate, aqueous layer removed (2nd NaOH extract). Added 6 M HCl to each extract until each mixture was acidic. Precipitate formed at this point.
I don't think it is NaCl. If NaCl formed, wouldn't NaCl be formed each time NaOH + HCl came together. (Yes, NaCl forms but if it pptd after the first extract, it seems to me that it would ppt after all extractions.) I think this is what is happening.
Presumably you have an acidic compound and a neutral compound in the mixture.
The acid we might represent by RCOOH. So with the NaOH added, we form the sodium salt (which is soluble in water and the neutral compound is not). RCOOH goes to RCOONa. Then we separate the layers, add HCl to the RCOONa which forms the original acid and that is what is pptng. If you have all of the acid removed from the mixture, then the second extract contains no RCOONa and no ppt is formed when HCl is added.
Thank you very much.