106: C h e m i c a l S c i e n c e C o n c e p t s
Gas Chromatography and Name
Organic Models
Partners
Date
Objectives
o To separate two different alcohols using gas chromatography
o To identify the components of a mixture by using both reference standards and retention time comparison
o Create a calibration curve on graph paper using the percentage of the ethanol peak for each of the known mixtures.
o To calculate the percentage composition of the mixture
o Build and analyze models of simple and complex organic compounds
Gas Chromatography (GC)
EXPERIMENTAL: The GC experiment consists of graphing the GC data obtained from standard solutions with known concentrations. This graph is called a calibration graph or calibration curve. It will be used to determine the ethanol concentration of an unknown solution. The instrument is a Gow Mac chromatograph equipped with a thermal conductivity detector and a chart recorder. The standard solutions have been prepared in advance (5%,10%,15%, 20%, 25% ethanol in 2-propanol). Every student will be given the chromatograms of the standard solutions so that the retention times and peak heights can be measured. The data table will be completed and a graph of peak height vs. concentration using this data will be created. Each student will inject a sample of unknown concentration. The peak height measurement from this chromatogram will enable you can determine the percentage composition of your unknown sample.
Lab report: Complete the data sheet, staple the chromatogram and calibration graph to this handout and complete the organic models report section.
GC Data Sheet
1. Measure the distance from the injection point to the middle of each peak in millimeters (mm) for the 5% solution and the 20% solution. Record this number in the tables below.
5% solution
Name of alcohol / Peak / Distance(mm) / Retention timeEthanol / 1
2-Propanol / 2
20% solution
Name of alcohol / Peak / Distance(mm) / Retention timeEthanol / 1
2-Propanol / 2
.
2. The paper was moving at a speed of 20 mm/min(chart speed). Use this value to convert the distance (mm) you measured to time (minutes). This value tells you how many minutes it took for the sample to travel through the column and it is known as the retention time. Record this value in the table above. Show your work for credit.
Conclusion:
Is the retention time of ethanol dependent upon concentration?______
Explain.
GC Data Sheet
3. Measure the height of the first peak on the chromatogram of each of the different concentration. Record the height in the table provided below.
Height(mm) / % concentration0 / 0
5
10
15
20
25
4. Create a X/Y graph using the website “Create a graph” and the data in the table above.
The X-axis is height and the y-axis is % concentration, source is your name. Attach your graph to your report.
http://nces.ed.gov/nceskids/createagraph
5. Measure the height of the ethanol peak (first peak) on the chromatogram of your unknown. Use your graph to determine the concentration of ethanol in your unknown sample.
Unknown: ______
Concentration of ethanol: ______
Organic Models
Working with molecular models is extremely helpful in visualizing organic compounds. The arrangement of atoms in space is quite different from the way we draw them. This spatial arrangement turns out to be vital (literally); atoms may have the same sequence, but not the same biological activity. Large molecules have many possible spatial arrangements. Do not regard this exercise as child’s play; Watson and Crick won a Nobel Prize for working out the structure of DNA using models. Children enjoy it too.
Experimental: The set of model atoms we are using has colored wooden balls, which are connected with pegs and springs. At the end of the period, make sure all bonds are removed and the model kit is in the same condition that you found it. A bin of extra balls is available so you can adjust the numbers of each. Pliers are available to remove pegs. Use the appropriate color ball for the elements when constructing your models.
Carbon: black Bromine: orange
Hydrogen: yellow Chlorine: green
Oxygen: red Fluorine: violet
Nitrogen: blue
Because hydrogen atoms are so small, use shorter pegs for all bonds to H atoms (yellow balls). Long pegs are used for all other single bonds. Two springs are used for a double bond, and three springs for a triple bond. Do not use a single spring to represent a single bond.
NOTE: Carbon always forms four bonds, counting double and triple bonds as two and three bonds each; hydrogen forms one bond; oxygen forms two bonds and nitrogen forms three bonds. Build the following compounds and have your instructor initial each section of your report.
1. Methane – Use short pegs to connect four atoms of H (yellow) to an atom of C (black). The actual shape of a carbon compound with only single bonds is called tetrahedral. We represent the structural formula on paper as if it were flat, square planar, with 90° angles because it is easy and convenient to draw the compounds this way.
Ethanol and 2-propanol are the alcohols used in the GC experiment. Although the compounds contain the same functional group, their biological properties are quite different. Ethanol is found in alcoholic beverages whereas 2-propanol is rubbing alcohol and is quite poisonous if ingested.
2. Ethanol – Build a model of ethanol CH3CH2OH. Connect the carbon atoms with a long peg. Draw the structural formula (not a picture of your model) of ethanol in the space provided.
3. 2-Propanol – Build a model of 2-propanol, CH3CHCH3.
Notice that the carbon chain is not straight, although we draw it as straight for clarity and the “OH” is connected to the second carbon of the chain. Draw the structural formula.
Show your models to your instructor before you dismantle them.
Does it matter which carbon has the OH bonded to it?______
Explain.
Many organic and biochemical compounds possess the property of “handedness” and are called stereoisomers. Compounds containing a carbon atom bonded to four different elements are “handed” and are easily recognized. The fact that “handed” molecules are nonsuperimposable mirror images give many of these compounds special properties. All naturally occurring carbohydrates, proteins and enzymes are handed. Many drugs such as ibuprofen, LSD, methamphetamine and antibiotics are “handed” as well.
4. Stereoisomer – Build models of the two compounds that are shown below paying particular attention to the bond orientation.
Are the compounds nonsuperimposable? ______
a. Replace a bromine atom (Br-orange ball) with a hydrogen atom (yellow ball) in both of your structures. Are the compounds nonsuperimposable? ______Explain your answer.
Show your models to your instructor for credit.
5. Build one compound from the list below (a- f) and show it to your instructor for credit.
a. Aspartame (also known as Nutrasweet) is a synthetic dipeptide that is 200 times sweeter than sucrose (table sugar).
b. d-Methamphetamine is a stimulant and resembles amphetamine, the active ingredient in prescription diet pills.The d-isomer, shown below has the stimulant effect whereas it mirror image does not.
c. Splenda(sucralose) is an artificial sweetner that is 60,000 times sweeter than sucrose
d. TNT= 2,4,6-trinitrotoluene is considered a high explosive used mostly in military and commercial blasting.
e. Procaine also known as Novacain is a local anesthetic.
f. Luminol gives off a photon of light in the presence of hydrogen peroxide, iron and blood. It is used to detect traces of blood even after the area has been cleaned.
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