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.
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.
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).
4) Perform a “hot filtration” over a steamed fluted piece of filter paper.
5) If you need to, this is the point where you will boil off excess solvent.
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.
7) The recrystallized unknown solution is filtered with a 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).
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.
10) Obtain the mixed melting points for both of your samples. / 1) The solvent started to boil at 2:25 p.m.
2) Added 2.3756 grams of unknown M to a flask.
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) Hot filtration performed.
5) At TA’s suggestion, boiled off 20 mL of water.
6) Started cooling solution at 3:05 p.m. At 3:22, placed flask into ice water.
7) Collected crystals with Büchner funnel.
8) Started drying crystals in oven at 3:35 p.m.
The total mass recovered was 1.9342 grams, or 81.4%
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) 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.