Identification of white solid lab questions 1. Why did you observe different Rf

Question

Identification of white solid lab questions

1. Why did you observe different Rf values for the three development solvents tested in your TLC analysis of your compound? Explain.

2.Why is the Rf of your compound greater in ethyl acetate than in hexane?

3. A student performs two melting-point determinations on a crystalline compound.In one determination, the capillary tube contained a sample 1-2 mm in height, and the melting point range was found to be 141-142C.In the other determination, the sample height was 4-5 mm and the melting range was 141-145C.The reported melting range is 143C.Explain what may have caused the broader range for the second sample.

4.Another student reported 136-138C for the melting range of the compound in question 3 and recorded that the rate of heating was 12C/min. Further analysis of the compound did not reveal any impurities. Explain the low melting range

5.When 2-propanol was used as the developing solvent, two substances moved with the solvent front (Rf = 1) during TLC analysis on a silica gel plate.Can you conclude that they are identical?If not, what additional TLC experiment(s) would you perform?

Recrystalliation Activity questions

1.A student attempted to perform a recrystallization but after dissolving his acetanilide in hot solvent, the sample did not recrystallize when it cooled. What is the most likely thing that the student did incorrectly and how can he correct this error (without starting over)? What should he do in the future to try to avoid this error?

2. Another student chose a recrystallizing solvent in the following way: He performed the same tests as you did above. He then chose a solvent that he observed caused the solid to dissolve right away and also dissolved when heated. After attempting to perform the recrystallization, he failed to recover any solid product. What did this student do wrong? How can he correct his error (without starting over because this is a valuable solid and he doesn’t have any more)?

3. A student performs the recrystallization but her crystals are very powdery when they should be more needle-like. What was the most likely mistake the student made? How can the student correct this error (without starting over)?

4.   A student finishes her recrystallization and finds that she has 110% recovery and the melting point is very low. Assuming that she performed her calculation and melting point correctly, what is the most likely error the student made? How can she correct this?

Explanation / Answer

Identification of white solid lab questions

Question 1 : the three solvents are different polarities hence they showed 3 different Rf values

Question 2 : Ethyl acetate is polar solvent than hexane. Hence compounds in ethyl acetate will move faster than Hexane

Question 3: When he increases height of compound (4-5mm) in capillary tube the heat will not be uniform. Compound will start to melt from down as well the same temperature will not maintain in upper part of compound. Hence it will take sometime to reach melting point temperature in upper part at the same time the temperature of whole system will increase about 20C excess.

Question 4 : heating 12C/min is very fast. He cant observe accurately in this heating speed.

Question 5 : 2 propanol is very polar solvent. hence that 2 compounds are not identical. That compounds should be eluted in various ethyl actetate and hexane mixtures. then only we can come to know whether they are identical or not. Most of organic compounds will show Rf = 1 when we use isopropanol as a solvent for TLC

Recrystalliation Activity questions

Question 1 : He may have added excess amount of solvent. He can evaporate that solvent to recrystalliation

Question 2 : He can evaporate that solvent and can be tried in other solvents

Question 3 : The solvents evaporated very fast hence they forms powder. slow evaporation only will leave crystal. hence he can do the same way with solvent system. but evaporation speed should be slow.

Question 4 : If yield comes more than 100% there must solvent or water along with compound. this should be eliminated by doing concentration or evaporation

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