Question #5dcd6

Some useful ones are:

• #pKa#
• intermolecular forces
• hydrophobic vs. hydrophilic
• boiling point
• vapor pressure

pKa, hydrophobic vs. hydrophilic, intermolecular forces
If two compounds have different pKas, then you can separate them by altering the pH so that one of them gets deprotonated or protonated, thereby dissociating in aqueous solution.

Example of altering pH to alter acid/base concentrations:

Example of actual separation:

#CH_3(CH_2)_4CH_3# vs. #HF#
#pKa = ~50# vs. #3.14#

If you change the #"pH"# to #7#, then hexane will (of course) remain protonated, while #HF# will be partially deprotonated, approximately #7244# to #1#, using the Henderson-Hasselbalch equation:

#7 = 3.14 + log([[base]]/[[acid]]) ~~ 3.14 + log (7244)#

Then, you can say that (literally) #99.99%# of the #HF# will be dissociated in an aqueous solution. To separate these two, you can add an organic solvent such as diethyl ether, and hexane, a nonpolar organic compound, will drift into diethyl ether, while #F^(-)#, an ionic inorganic compound, will drift into water, allowing you to separate them.

Adding the organic solvent makes use of knowledge of hydrophobic (water-hating) vs. hydrophilic (water-loving) conditions, as well as the difference in intermolecular forces between hexane and #HF#.

Hexane is not charged, nor is it polar, so it doesn't dissociate nearly as easily in water as it does in a nonpolar organic solvent, whereas #F^(-)# dissociates more easily in water rather than an organic solvent because it is charged, and it has no intermolecular forces that help it to dissociate in a nonpolar organic solvent other than London dispersion (which is weak due to the tiny anionic radius).

This does not always work, but it's not a bad idea to consider it.

Boiling Point
This can be used if the boiling points of two compounds are substantially different, but you need to be careful not to decompose a compound with the heat.

If you raise the temperature by heating, you can force something to vaporize away, leaving behind the compound with the higher boiling point.

This only works on compounds that do not decompose by heat and that have significantly different boiling points.

Boiling Point, Vapor Pressure
You can also do this in conjunction with altering the pressure of a solution vessel, in a process called fractional distillation.

Basically, lower the temperature until both compounds are liquid, lower the pressure until one compound vaporizes to a certain extent, and repeat until one compound is leftover. The compound with the higher vapor pressure will vaporize first each time because the higher it is, the closer it is to the current pressure as you decrease the pressure.

The only problem other than boiling points is that if the two vapor pressures are too close together, it won't work well.