WRITING A LAB REPORT
Students often have misconceptions as to what constitutes a good lab report. A good lab
report will be written in complete sentences throughout the report, except for data that is in table
format, and any pictures. A good lab report explains the experiment in detail, how you
performed a reaction, your results, and an explanation of the results.
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Use complete sentences always.
No bullets
Use the past tense, passive voice. For example: The product was filtered, washed three
times with water, and dried. NOT: I filtered the product, washed it and dried it. NOT:
The product is filtered, washed and dried. NOT: Filter the product, wash it and dry it.
Start with the Introduction. State what reaction you did, and how you did it, in a
succinct manner. Draw the reaction.
Background comes next. Explain the reaction in detail, including the mechanism and
any other pertinent facts about it.
Experimental comes next. Describe what you did in detail, including all your
observations, temperatures, masses, etc. Everything you did in the lab is described here.
Results comes next. Here you state the outcome of the lab. The results can be tabulated.
Any explanation of the results would be here also. Also any spectra, etc, would be
included here.
Conclusion is last. Here you restate what reaction you did, and what results you got, in a
succinct manner.
RESULTS:
Upon successful completion of this course the student should be able to:
1. Determine physical constants relating to intrinsic properties including melting point and
boiling point.
2. Carry out fractional distillation to achieve separation of a two-component mixture.
3. Purify a liquid or solid compound by a variety of methods, including recrystallization,
distillation or chromatography.
4. Separate a mixture of a combination of acid, base, or neutral organic compounds by a series of
solvent extractions and manipulations of pH.
5. Characterize compounds and evaluate the composition of organic mixtures by
chromatographic methods such as thin layer chromatography, column chromatography, gas
chromatography.
6. Demonstrate the appropriate safety practices for the correct usage of the equipment commonly
found in the organic lab.
Chem 2641
Experiment 4 - Fractional Distillation
Fractional distillation is a technique for separating two or more liquids in a mixture
if the components have boiling points that are fairly far apart (at least 30-40
degrees apart). By slowly allowing the vapors to pass through a tall fractionating
column, the component with the lower boiling point can pass ahead, leaving the
component with the higher boiling point behind. A perfect separation is not
possible, but by taking sequential fractions of the distillate, it is possible to obtain
an early fraction with mostly the lower boiling component, and a late fraction with
mostly the higher boiling component. In this lab you will be separating a mixture of
toluene (bp 111oC) and cyclohexane (bp 81oC) using a fractionating column, and
taking four fractions. Each of the fractions will be analyzed by gas
chromatography in the next laboratory session.
Read pp 91-93 in Williamson, 5th edition, for the theory of separation.
The main apparatus is shown below. It will be assembled in the fume hood.
All ground-glass joint are dabbed with a bit of grease before joining. Failure to do
this can result in stuck-together joints.
Keck clips will be used to hold the column to the distilling head, the distilling head
to the condenser, the condenser to the take-off adaptor, and the take-off adaptor
to the conical side-arm receiving flask. No Keck clip is used on the distilling flask
itself! This is held to the rack with a firmly secured clamp. You will need a second
clamp to loosely hold the condenser.
The heat will be applied using a heating mantle. The heating mantle is plugged into
the variable voltage regulator, which is plugged into the wall socket.
You will need four collection vials, labeled with your name and the numbers 1-4,
that will hold approximately 4 mL of distillate each.
Procedure
Obtain 20 mL of the 50:50 cyclohexane:toluene mixture. Pour it into a 100 mL
round-bottom flask. Add a boiling chip, then clamp the flask to the ring stand.
Assemble the remaining parts of the distillation setup according to the picture,
and to the instructions given above. Make sure your distilling head has an O-ring;
this holds the thermometer tightly without leakage.
Connect the water hoses to the condenser. Water must enter the lower part of
the condenser, and move upwards against gravity. The higher hose goes to the
drain.
Have your instructor check your apparatus before you turn on the heat!
When the apparatus has been approved, turn on the heat on the variable voltage
regulator, and set it to about 4. As the distillation proceeds, you will be turning up
the setting as needed (whenever the temperature stops rising, or even drops). You
may want to wrap the distilling column loosely with aluminum foil to help insulate it
against cold air currents in the hood. When the distillate begins to come off, fill
the conical flask then transfer the contents to a labeled vial. You will collect 4 full
vials of distillate. Place a small piece of foil between the vial and the cap. When all
four vials have been collected, rubber band them together and give them to your
instructor. They will be kept in the refrigerator until the next lab.
Remove the heating source from the apparatus, and let the flask cool to room
temperature. Then begin to disassemble the apparatus, starting with the take-off
adaptor and moving backwards to the distilling flask. Wipe the grease from all
greased surfaces, using Kimwipes or paper towels. All of the glassware is then
given a quick acetone rinse and put away, except for the distilling flask, which must
be emptied into the non-chlorinated waste container, and washed well with acetone.
Chem 2641
Experiment 5 – Gas Chromatography
In this lab you will determine the percent composition of each of the fractional
distillation fractions that you obtained in the last lab.
Gas chromatography is a technique that allows the separation of gaseous
components using a specific type of column packing, in this case Carbowax 20 M
(Column A). Typically the components emerge the column, and enter the detector,
in the order of their boiling points. An internal standard, m-xylene, is used to allow
the calculation of the composition.
Read Williamson, 5th edition, pp 215-220 for more information.
I.
Preparation of the standard mixture:
This mixture will be used to determine the response factors of toluene and
cyclohexane. It should be made out as follows: 800 mg m-xylene
600 mg toluene
200 mg cyclohexane
II.
Preparation of the mixtures from the fractions:
For Fraction 1:
600 mg m-xylene
200 mg Fraction 1
For Fraction 2:
600 mg m-xylene
400 mg Fraction 2
For Fraction 3:
600 mg m-xylene
400 mg Fraction 3
For Fraction 4:
600 mg m-xylene
400 mg Fraction 4
III.
You will be using a 10 L syringe to insert the samples into the gc. The
syringe must be cleaned between uses with pentane. The injection
technique will be demonstrated by your instructor. You will typically be
injecting between 1-3 L of sample, so that the largest peak does not run
off the page.
You will first inject the standard mixture; then each of the Fraction mixtures.
The peaks emerge in the following order: pentane first (if any), cyclohexane,
toluene, then m-xylene last.
IV.
Calculations: You will measure the peak height of the cyclohexane,
toluene and m-xylene in the standard mixture gc, then each of the
Fractions’ gc.
First you calculate the Response Factor (RF) from the standard mixture:
RFcyclohexane =
peak height cyclohexane
mass cyclohexane
________________________
peak height xylene
mass xylene
RFtoluene =
peak height toluene
mass toluene
________________________
peak height xylene
mass xylene
Then you use the data from each of the Fractions’ gc’s to solve for the mass
cyclohexane X and mass toluene Y in EACH fraction as follows:
RFcyclohexane (from above) =
RFtoluene (from above) =
peak height cyclohexane
X
________________________
peak height xylene
mass xylene
peak height toluene
Y
________________________
peak height xylene
mass xylene
You solve for X and for Y for each fraction. The you determine the % cyclohexane
and the % toluene in each fraction.
Experiment 4- Fractional Distillation
Data:
Temperature
70C
80
85
92
99
Distillate
First drop
Vial1
Vial2
Vial3
Vial4
Experiment 5- Gas chromatography
Standard mixture - 801.0mg xylene
597.0mg toluene
202.0mg cyclohexene
Fraction 1 – 596.0mg m-xylene
203.0mg fraction 1
Fraction 2 – 604.0mg m-xylene
403.0mg fraction 2
Fraction 3 – 602.0mg m-xylene
401.0mg fraction 3
Fraction 4 – 597.0mg m-xylene
406.0mg fraction 4