Sample Post Lab Exercise for the recrystallization experiment
Post lab write ups include Observations (what I actually did/saw/performed in lab) as well as answering
any postlab questions. As it turns out, this lab has no postlab questions. All you have to do is identify
your unknown and explain WHY in the Data Analysis and Discussion/Conclusion.
Procedures:
1) Heat your solvent close to the boiling point.
1) The solvent started to boil at 2:25 p.m.
2) Add your impure unknown to an 250 ml
Erlenmeyer flask and heat this mixture with your
solvent on hot plate. Add just enough hot water
until all your unknown has dissolved in hot water.
2) Added 2.3756 grams of unknown M to a flask.
3) Continue heating the water/unknown to
continue heating for 3 to 5 minutes. Try not to add
more than 20 mL of solvent. Remove the mixture
from the hot plate and carefully add a spatula tip
full of decolorizing carbon (charcoal).
3) Added about 40 mL of hot water to the
unknown solid to get in to dissolve. Added a
spatula tip of charcoal to the flask.
4) Perform a “hot filtration” over a steamed fluted
piece of filter paper.
4) Hot filtration performed.
5) If you need to, this is the point where you will
boil off excess solvent.
5) At TA’s suggestion, boiled off 20 mL of water.
6) Allow your solution to cool slowly. Once this
flask has cooled off (cool to your touch!) , you
should see crystals form. This material will then
get chilled in an ice bath for ~ 10 mins.
6) Started cooling solution at 3:05 p.m. At 3:22,
placed flask into ice water.
7) The recrystallized unknown solution is filtered
with a Büchner funnel.
7) Collected crystals with Büchner funnel.
8) Allow your crystals at least 30-40 minutes to dry
in the oven before you attempt to perform a
melting point/mixed melting point. The oven
should be at least 80 OC to drive off as much water
vapor as possible (at least 20 minutes).
8) Started drying crystals in oven at 3:35 p.m.
The total mass recovered was 1.9342 grams, or
81.4%
9) You will take the melting point of your
unknown. Once you have the temperature that the
melt occurs, mix a few milligrams of your unknown
with the suspected known material. Also, to see
what a melting point depression looks like, mix a
few milligrams of your unknown with a known
material with a very different melting
temperature.
9) Performed melting point at 4:08. Unknown M
has a melting point of 124 – 126 oC.
Suspected unknown material is succinimide.
Performed 2 mixed melting points: Unknown M
with succinimide and Unknown M with
benzamide.
10) Obtain the mixed melting points for both of
your samples.
10) Performed 2 mixed melting points:
Unknown M with succinimide.
Melting Point 103-110
Unknown M with benzamide.
Melting Point 124 – 126 oC
Unknown M is Benzamide
Data Analysis:
The proof of identification of an unknown solid is easily accomplished by a mixture of known
materials with unknown materials (mixed 1:1) and melted. If the unknown has the same identity and the
known standard have the same identity, melting points will occur sharply at the predicted temperature.
Since the melting point was depressed and broad when the unknown was mixed with succinimide, it is
obvious that the unknown was NOT succinimide.
Discussion/Conclusion:
For this lab, I would mention and discuss some of the factors that go into WHY the melting point
would be lowered due to the presence of an impurity (think colligative properties and the eutectic
point!). I’ll leave this for you to do a little looking up in the general chemistry books or on-line: please
remember to cite ALL references. Using a reference is a good thing to do, but get into the habit of
properly using your citations! Leaving them off is NEVER smart.