2. Extraction of caffeine from tea: a. What is the advantages of adding CaCO, be

Question

2. Extraction of caffeine from tea: a. What is the advantages of adding CaCO, before the extraction b. If we extract with dichloromethane we have 2 phases. Which phase will the lower phase? Why? c. If we extract with ethylacetate which phase will be the lower phase? Why? 3. Uncertainty in measuring volumes. a. We are using automatic pipettes and we have to be sure that the volume dispensed is the one indicated on the pipette 0.5, 1 mL, ). A pipette is calibrated by weighing. Last year a group reported the following values (in g) for a 500 HL pipette (0.4935, 0.4931, 0.930, 0.4993, 0.4898, 0.4976, 0.4971, 0.4974, 0.4952, 0.4972, 0.4970. Calculate the mean value, the standard deviation and the relative accuracy of the pipette. b. If we use that pipette to prepare 5 mL of the 50 mg/L solution by diluting 10 times the stock solution what is the relative uncertainty of the diluted (50 mg/mL) solution.

Explanation / Answer

2

a. Tannins are phenolic compounds of high molecular weight that are slightly soluble in dichloromethane can be eliminated by converting it to their salts ie. phenolic anions by adding sodium carbonate (being acidic in nature can be converted to salts by deprotonation of the -OH group) which remain in the water.

b. Dichloromethane because dichloromethane (1.33 g/cm³) is much denser than water (1.0cg/cm³) and insoluble in it.

c. In this case, water will be the lower phase and ethyl acetate (0.902 g/cm³) the top layer containing caffeine as water is denser.

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