Unformatted text preview: C343/S2009 Lab Quiz #2 February 19, 2009 KEY 1. Consider the following silica gel TLC plate of compounds A, B, C and D developed in hexanes. Indicate the relative polarities of A‐D based on the given TLC plate. (2 pts) Most polar A > B > C > D least polar 2. Which of the following solvent mixtures is more polar (1 pt)? 20:80 CH2Cl2/hexanes OR 80:20 CH2Cl2/hexanes 3. If two substances are run on the same TLC plate and have the same Rf value, can you conclude that they are the same compound? Why or why not? (2 pts) No, you can conclude that they have the same or similar affinity for the stationary phase versus the mobile phase, but TLC does not PROVE that two spots are the same compound. One can infer when compounds are NOT the same compound, but you cannot prove that they are the same compound. 4. You are trying to determine a TLC solvent system which will separate the compounds X, Y, and Z. You ran the compounds on a TLC plate using hexanes/ethyl acetate 95:5 as the eluting solvent and obtained the chromatogram below. How could you change the solvent system to give better separation of these three compounds? (3 pts) One should raise the percentage of the polar solvent until the spots separate with distinctively different Rf values. Usually, one would increase the polarity of the solvent by 5% until the separation is good. 5. If the carotene pigments of spinach eluted before the xanthophylls during your TLC experiment, what does that tell you about carotenes compared to xanthophylls? (3 pts) One can deduce that carotenes have less affinity for the stationary phase, and therefore, are less polar. 6. Another common organic lab is the separation of analgesics (pain killers) using column chromatography (CC). If you had to separate ibuprofen and aspirin using CC, which compound would you expect to elute from the column first using hexanes as your solvent? (3 pts) Comparing the structures, you can deduce that ibuprofen is a less polar compound with less binding sites between the structure and the stationary phase. It would be less polar, more soluble in hexanes and would elute first. TURN OVER FOR MORE FUN! C343/S2009 Lab Quiz #2 February 19, 2009 7. Among the following drawings below, which one would be the best placement of the thermometer to give the correct boiling point of the vapor? For the “wrong” drawings, explain why these are incorrect and what your expected temperatures readings would be. (3 pts) A B C B is the correct placement of the thermometer. “A” would yield a temperature that would be too low, and “C” would demonstrate a temperature that would be too high. 8. Place the following compounds in order of boiling point. Disregard any differences in molecular weight. (2 pts) O O A C B E < C < B < A < D Highest O HO D E 9. Explain the purpose of using a boiling chip during distillation. (3 pts) Boiling chips are small, insoluble, porous stones made of calcium carbonate or silicon carbide. These stones have pores inside which provide cavities both to trap air and to provide surface area where bubbles of solvent vapor can form and provide bubbles to form and be released. Boiling chips are used to prevent solvent from “bumping”. 10. What is the purpose of the packing material in fractional distillation? (3 pts) Fractionating columns help to separate the mixture by allowing the mixed vapors to cool, condense, and vaporize again multiple times. The packing material increases the surface area, allowing the distilling liquid to recondense and redistill multiple times, which increases the ability to separate liquids with closer boiling points. The performance of many separation processes depends on having a series of equilibrium stages and is enhanced by providing more such stages (more packing material or higher surface area). In other words, having more theoretical plates increases the efficacy of the separation process. 11. The figure below contains the vapor pressure curves for two compounds A and B. (5 pts) a. What is the normal boiling point of compound A? ca. 70‐75oC b. What is the normal boiling point of compound B? ca. 115oC c. Which compound has a higher vapor pressure at room temperature? A . d. What is the boiling point of compound A at 400 mmHg? e. Which compound would evaporate fastest? TURN OVER FOR MORE FUN! 50oC A ...
View Full Document
- Spring '11
- pts, TLC plate, normal boiling point