A-27 Pre-lab Questions: TLC Name: Section: Experiment 13: How can a chemist use

Question

A-27 Pre-lab Questions: TLC Name: Section: Experiment 13: How can a chemist use Thin Layer Chromatography to determine the polarity of a molecule? To answer these questions you will need to conduct some additional research on the TLC method. A solution of the sample to be analyzed has been made in a suitable solvent and will be provided for you. With this solution, a small spot is applied at a distance near the bottom of the edge of a TLC plate using an applicator. (see Figure 1) This spot must be above the level of solvent in the TLC chamber. Why? 1. The plate is placed in a developing chamber containing the appropriate solvent (eluent), covered, and removed when the solvent has traveled near the top of the plate. Why should you NOT allow the solvent to run all the way off the top of the plate? 2. The solvent front is marked with pencil upon removal from the solvent and the spot is illuminated under UV light and circled with pencil. Why is the spot illuminated with UV light. [Reminder: look at structure of molecules. Refer to a textbook or online resource for necessary information.] 3· If more than one compound is present, all spots are circled with pencil. Why circle with pencil and not pen?

Explanation / Answer

5) M1V1 = M2V2

(0.805*50) = (M2*23.55)

M2 = molarity of NaOH = 1.71 M

6) heat liberated(q) = m*s*DT

                     = (50+23.55)*4.184*(29.4-22.1)

                     = 2.25 kj

7) no of mol of aceticacid reacted = 50*0.805/1000 = 0.04 mol

   enthalpy of reaction (DHrxn) = -q/n

                  = -2.25/0.04

                  = -56.25 kj/mol

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