Give reasons for each of the following experimental techniques ...

[Lisaclaire]

Give reasons for each of the following experimental techniques used in crystallization.
  (a) A hot crystallization solution is not filtered unless absolutely necessary.
  (b) An Erlenmeyer flask containing a hot solution is not tightly stoppered to prevent solvent
  loss during cooling.
  (c) The suction of a vacuum filtration apparatus is broken before the vacuum is turned off.
  (d) Vacuum filtration is avoided when crystals are isolated from a very volatile solvent.

Question 2

Which of the following solvents could not be used as solvent pairs for crystallization? Explain.
  (Hint: A solvent pair must be two miscible liquids; see Table 3.1, p. 51.)
  (a) Hexanes and water
  (b) Chloroform and diethyl ether
  (c) Acetone and methanol

[sylvia]

Answer to Question 1

(a) Solvent is lost by evaporation and premature crystallization can occur. In addition, some
of the compound wil be also lost by adsorption on the filter paper.
(b) A vacuum will be created in a tightly stoppered flask as the solvent vapors cool.
(c) If the aspirator is turned off while the system is still under vacuum, water will be sucked
into the filter assembly.
(d) A very volatile solvent will vaporize during the vacuum filtration causing the compound
to precipitate. Also the rapid evaporation of the solvent on the filtered crystals prior to
their being washed wil result in impurities in the solvent being deposited on them.

Answer to Question 2

(a) yes; this solvent pair is immiscible.
(b) no; this solvent pair is miscible
(c) no; this solvent pair is miscible

[Lisaclaire]

Wow, this really help

[jackie]

Excellent