analytical chemistry ?

[Mr. Z]

Try these on for size...


1. In order for two compounds to be separated by NaHCO3 extraction, they must have:

a) different dipole moments
b) different acid base properties
c) different boiling points
d) different molecular weights


2. Which pair of compounds could be separated most readily by solvent extraction.

a) aminocycylohexane and chlorocyclohexane
b) cyclohexanol and cyclohexanone
c) phenol and p-cresol
d) butanoic acid and 2 methylpropanoic acid

3. Carboxylic acids are generally separated as their methyl esters in gas chromatography. Why are the carboxylic acids themselves difficult to separate by this technique?

a) carboxylic acids all have the same nominal polarity
b) carboxylic acids are insoluble in most organic solvents while
their methyl esters are soluble.
c) carboxylic acids decompose readily in a gas chromatographic
column
d) carboxylic acids do not vaporize readily but their methyl
esters do so easily

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[freakazoid]

1)B
2)B
3)D

The last one I wasn't sure about, but I believe gas chromatography works by separating compounds of different volatilities . . .

 

[rCubed]

hmm..i thought that #1 was a.
since NaHCO3 is a salt, it comprises the aqueous layer, and for a compound to be soluble in it, it must contain a dipole moment, otherwise it would be soluble in the organic layer

 

[Mr. Z]

freakazoid is on the money, nice job.

 

[limit]

Originally posted by rCubed
hmm..i thought that #1 was a.
since NaHCO3 is a salt, it comprises the aqueous layer, and for a compound to be soluble in it, it must contain a dipole moment, otherwise it would be soluble in the organic layer

Close. NaHCO3 is used in extraction to deprotonate strong acids in order to make them more polar, hence allowing them to dissolved and be removed with the aqueous layer. This makes the answer B (acid base characteristics)

 

[rCubed]

thanks limit, i appreciate the clarification

seperation is by far the most difficult subject for me on this exam, followed by....this is sad: translational motion