tlc - Suggest a good method to confirm that the two samples...

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E XPE RI M E N T : T H I N L A Y E R C H R O M A T O G R A P H Y Name _______________________________________ Instructor ____________________________________ Chem ____________ Section/Color ______________ Date ________________________________________ Laboratory Questions These questions are to be completed and turned in at the beginning of the pre-lab period. PR E-L A B Q U EST I O NS 1. Why should the jar used to develop the TLC plates be covered? 2. Suggest a reason that TLC plates that are stored in a highly humid room result in different R f values than plates that are stored in a dehumidified environment. Hint: Consider the structure of the layered material on the TLC plates and how that material might interact with water molecules in the air. 3. Why should the 'spots' applied to a TLC plate be kept as small as possible? 4. Even if two spots have the same R f value, we cannot be certain that the two samples are identical compounds.
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Unformatted text preview: Suggest a good method to confirm that the two samples are the same or not the same. 5. Why is TLC a poor technique for the analysis of low-boiling liquids? (Hint: low-boiling means high vapor pressure at room temperature) Because there is an equilibrium for the vapor pressure of the solvent. If the jar is left open, the solvent won't elute all the way up the plate, but the spots will continue to travel up. The water in the atmosphere will adsorb onto the plates and change the degree of polarity of the substrate. Large initial spots will remain large after the plate is developed, which will reduce the accuracy of the measurement. Cospotting The liquid will vaporize o f of the plate easily (and possibly completely), depending on the initial quantity of sample and the vapor pressure. 1 pt 1 pt 1 pt 1 pt 1 pt...
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This note was uploaded on 07/17/2011 for the course CHEM 237 taught by Professor Yu-chinli during the Spring '07 term at Texas A&M.

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