Gas chromatographic analysis of a mixture of organic compounds gave the followin
ID: 1016847 • Letter: G
Question
Gas chromatographic analysis of a mixture of organic compounds gave the following peak areas (cm^2): hexane = 2.4; heptane = 1.7; hexanol = 1.2; toluene = 0.6 a. Compute the mole percent composition of the mixture (Assume that the response of the detector is directly proportional to the number of vapor molecules crossing over the detector and the unit peak area per mole is the same for each component.) b. Compute the mass percent composition of the mixture making the same assumptions as in part a. Why is sulfuric acid (H_2S0_4), rather than hydrochloric acid (HCI) used to catalyze the dehydration of alcohols? An air peak was observed in your chromatogram. What is the source of the air? What are two techniques that you could use to improve the resolution of a GC chromatogram without changing the column? Which typically has more theoretical plates, a gas chromatography column or a fractional distillation column? Why?Explanation / Answer
1.
a. moles percent
moles of hexane = 2.4/86.2 = 0.03 mol
moles of heptane = 1.7/100.2 = 0.017 mol
moles of hexanol = 1.2/116.2 = 0.01 mol
moles of toluene = 0.6/92.1 = 0.006 mol
Total = 0.063 mol
mole% hexane = 0.03 x 100/0.063 = 48.2 mol
mole% heptane = 0.017 x 100/0.063 = 26.6 mol
mole% hexanol = 0.01 x 100/0.063 = 15.7 mol
mole% toluene = 0.006 x 100/0.063 = 9.5 mol
b. Total mass = 5.9
Mass % of hexane = 2.4 x 100/5.9 = 40.7 mol
Mass % of heptane = 1.7 x 100/5.9 = 28.6 mol
Mass % of hexanol = 1.2 x 100/5.9 = 20.3 mol
Mass % of toluene = 0.6 x 100/5.9 = 10.4 mol
2. Cl- is a strong nuclophile which may attack the carbocation formed after loss of H2O group. The conjugate base of H2SO4 is a weaker nucelophile and thus does not attack the carbocation. Thus H2SO4 is preferred as catalyst instead of HCl for dehydration reaction.
3. The air peak observed in the chromatogram is due to H2O in the dehdyration reaction.
4. A temperature program ramp would increase the resolution in gas chromatography. Also injecting smaller dose of sample may also improve resolution in gas chromatography.
5. A gas chromatography column has higher number of theoretical plates when compared to a fractionating column as the theoretical plates can be increased or decreases in case of gas chromatography column by the flow of carrier gas.
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