Today:
• Quiz Comments
• Practice for Melting Point Ranges and
Recrystallization
• Introduction to Molecular Modeling
(Lab today is in SEGA lab Science249)
Melting point ranges for mixtures of compounds A and B
Phase Diagram
What is the mp. and mol% composition of the eutectic mixture?
What is the mp. range of mixture X? o
C
oC
140
Pure B
Pure A
X
100
80
60
60
40
E
20
20
Solid A and B
mol%A 100%A
75%A
50%A
25%A
mol%B
25%B
50%B
75%B
100%B
The following solubility data were collected before
recrystallizing a sample of caffeine (C):
Solvent
Sol. Of C at 20o
Sol of C in hot solvent
Water
Ethanol
Acetone
Benzene
Pet. Ether
slightly soluble
soluble
soluble
slightly soluble
insoluble
soluble
soluble
soluble
soluble
insoluble
1) Which single solvent(s) could you use for
recrystallization?
2) Suggest a mixed solvent pair that you could use instead.
Solubility Data for Caffeine
1.000g of caffeine completely dissolves in a minimum of
– 45.0ml of water at 14oC
– 5.5 ml of water at 80oC
– 1.5 mL of boiling water
Assume that you are recrystallizing 2.000g of impure
caffeine.
After one recrystallization from 30.0 ml of water you ended
up with 1.200 g of purified caffeine. How many grams of
caffeine were left in the cooled water? (careful …)
If you reduced the volume of the mother liquor to 5 ml, how
many grams of caffeine would be left in solution in the
cooled solution?
Molecular Modeling
Remember these:
Hybridization of C
Bond length, angle, dihedral angle; staggered and
eclipsed conformations
Conformations of cyclohexane; axial , equatorial
substituents
We’ll do MM Exps. 1, 2, 4, only
Next time:
Distillation/Gas
Chromatography (Exp.3)
(Notebook preparation please…
Preview/prestudy experiment)