CHM120H5 Study Guide - Quiz Guide: Centrifugal Force, Pasteur Pipette, Ph Indicator

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19 Apr 2015

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In this experiment, all tests were performed concurrently on the known and unknown solutions and all work except the centrifugation was carried out in the fume hood. Firstly, to about 2ml of the unknown and known samples in test tubes, 0. 2 ml (approximately 5 drops) of. These solutions were then mixed well and centrifuged for about 2 minutes. The clear liquid from the solutions was decanted into a clean test tube using a pasteur pipette and saved up for a later step. To the precipitate formed, 2. 5 ml of distilled water was added and the solutions were heated in a boiling water bath for about 4 minutes, while stirring constantly. To confirm the presence of lead, about 5 drops of 1m k2cro4 (aq) to the clear liquid, which resulted in the formation of a deep yellow precipitate of pbcro4(s), confirming the presence of lead.

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Please put the following steps in order for the general procedurefor testing anions. Be sure to clean and then rinse your teststubes with distilled water before starting, when you get a freshsample of anion solution or are done for the day. To determinewhich of the three possible cations (sodium, potassium, or lithium)are in the anion unknown you must do a flame test as was done inexperiment 2.

1 2 3 4 5 Finally dispose of all used chemicals in the waste bottlefor exp. 5 in the fume hood and wash glassware.

1 2 3 4 5 To anion solutions that form a precipitate with 0.1Msilver nitrate add 0.5 mL of 6M nitric acid, observe and mixwell.

1 2 3 4 5 Test the unknown for the anion analysis (5B-xxxx) usingthe same tests and logic used for the known anion solutions.

1 2 3 4 5 Test fresh samples of anion solutions that did not reactwith 6M HCl or 0.1M barium nitrate by adding 0.5 mL of 0.1M silvernitrate, observe and mix.

1 2 3 4 5 Observe while adding 0.5 mL of 6M HCl to 1 mL samples ofthe anion solutions and then add 5 drops of 0.1M barium nitrate andobserve again.

salmonwombat608

If you were to run the experiment without using any celite, what would happen to the purity of your sample and why?

What practical purpose does the distillation serve in this experiment?

The experiment is below. Just answer the above questions.

Introduction Caffeine is one of three legal, mind-altering drugs available in the U.S without a prescription. Two common sources of this drug are tea and coffee, both of which are prepared via extraction of the solid foodstuff with liquid water. In this lab you will extract caffeine from food samples with liquid solvents and quantify the amount obtained. Laborator Hypothesis: We can rank various tea and coffee samples by caffeine content by performing a solid-liquid extraction from the foodstuff, followed by analysis via thin-layer chromatography. Experiment: In a 50 mL Erlenmeyer flask were placed 2.0 g of black tea. Dichloromethane (15 mL) and 0.2 M NaOH (5 mL) were added successively, and the stoppered flask was swirled gently by hand for 10 minutes (note: avoid vigorous shaking, as it will cause an emulsion to form). The dicloromethane layer was separated from the leaves and the aqueous layer by vacuum filtration through 7 g Celite 545, and the leaves were washed with a further 15 mL dichloromethane. The combined dichloromethane solution was placed in a 50 mL round bottom flask and concentrated to a minimal volume (ca. 1 mL) via simple distillation. The remaining dichloromethane solutions was transferred to a watch glass via pipet, and the solvent was evaporated in a fume hood. The sample was evaluated by TLC (a mixture of methanol and ethyl acetate as the mobile phase, co-spotted with authentic caffeine). Data: What was your crude yield of caffeine? What was the mass percentage of caffeine in your food source'? Given that the materials we are interested in are UV active, compared to the TLC standard how would you characterize the purity of your caffeine extracts? Conclusions: Can you comment on the purity of your caffeine sample? For all samples your class worked with where does your foodstuff rank in terms of caffeine content? How much caffeine is reported for the typical serving of coffee or tea? How did your extraction and isolation compare with the typical serving?

To test for Ag⁺

1. add 2 drops 6M HCl to 1 mL of unknown sample. Centrifuge (3-4 minutes). Add 1 more drop of 6M HCl and centrifuge (if necessary). A precipitate will form; decant (pour off) solution to another test tube (save solution)

2. wash precipitate from step 1 with 1 or 2 mL of distilled water. Stir, centrifuge, and decant the liquid (can be discarded). Then add 1 mL of distilled water to precipitate and place the test tube in boiling water. Let the test tube boil, and stir occasionally. Centrifuge the hot test tube (solution) and then decant after centrifuging (save the decant solution). Save the precipitate.

3. Add 10 drops of 6M HNO₃ to the precipitation from step 2 and stir it. Centrifuge and decant the solution (save it).

4. Add 6M HNO₃ to the solution from step 3 until it is acidic toward litmus paper. If Ag⁺ is present in the acidic solution, a white precipitate of AgCl will form.

To test for Cu²⁺ and Sb³⁺

1. Add 15M NH₃ to 1 mL of unknown sample until the solution is basic to litmus paper. A precipitate may form at this point due to the formation of insoluble oxychlorides of bismuth, tin, or antimony. Add 1 drop 6M HCL for each mL of solution. Verify the pH level by using methyl violet paper, the paper should turn blue-green. Adjustment may be necessary to achieve the blue-green color. Add HCl or NH₃ until it is almost the same blue-green color.

2. Add 15-20 drops of 1M thioacetamide to solution and heat in boiling water for 5 minutes. Centrifuge and decant to another test tube (may contain Fe and Cr). Test solution for completeness of precipitation by adding 2 drops of thioacetamide and letting it sit for 1 minute. If precipitation forms, add a few more drops of thioacetamide solution and heat again. Combine the two batches of precipitate. Wash precipitate with 1 M NH₄Cl solution, stir, and warm in bath water; centrifuge, and discard the wash solution.

3. To sulfide precipitate from step 2 add 15 drops of 6M KOH, 2mL of water, and 3 drops 1M thioacetamide. Heat solution in boiling water for 5 minutes and stir occasionally. Centrifuge and decant the solution. Wash any precipitate remaining twice with 5 drops of water and combine the washings with the solution. Tin and antimony will be present in the solution as complex ions, and copper and bismuth will remain as undissolved sulfide precipitate. Save precipitate for later.

4. Add 12 M HCL to the solution from step 3 until it is acidic toward litmus paper. When acidified, some tin and antimony will precipitate as orange sulfides. Add 5 drops of 1M thioacetamide and heat. Centrifuge and decant solution (this solution can be discarded)

5. Add 10-15 drops of 12M HCl to the precipitate from step 4 and heat for 5 minutes to dissolve precipitate. Continue heating until H₂S is gone. Centrifuge out any insoluble sulfur residue.

6. Add 0.5 grams of oxalate acid and 2 mL of water to remainder of solution from step 5. Heat the solution, if necessary, to dissolve the oxalate acid. This reagent forms a stable complex ion with Sn⁴⁺. Add 10 drops of 1M thioacetamide and heat. Formation of an orange precipitate indicates the presence of antimony.

6. Add 30 drops of 6M HNO₃ to remaining sulfide precipitate from step 3. Heat solution gently in boiling water to complete dissolution of the sulfides. Centrifuge the solution to remove sulfur and decant it into a small beaker. Place beaker on hot plate and evaporate the liquid to a volume of about one-half mL. DO NOT LET SAMPLE DRY OUT.

7. Add 15M NH₃ dropwise to the solution obtained in step 8 until it is basic toward litmus paper. Add a few more drops. The formation of a deep blue solution indicates the presence of copper.

Testing for Fe³⁺ and Cr³⁺

1. [If a solution is being used from which group II cations have been precipitated, remove the H₂S and excess acid by boiling the solution in a small beaker until a volume of about 1 mL. Remove any sulfur residue by centrifuging the solution.] [if a solution being used contains only group II ion, then use 1 mL of this solution.]

2. Add 3 drops of 3% H₂O₂ to 1 mL solution from step 1. Add 15 drops of 6M NaOH to solution to make it strongly alkaline (basic). Stir for 1 minute and boil to remove all excess H₂O₂. After H₂O₂ is decomposed, solution will âbumbâ. If oxidation of chromium (II) has occurred, the solution will be yellow. If not, add 3 more drops of H₂O₂ and boil again. Centrifuge and decant. Save precipitate (contains Fe³⁺) and solution (contains Cr³⁺).

3. Acidify solution from step 2 by adding drops of 15 M HNO₃(about 5 drops) and test with litmus paper. Add 6M NH₃ drop by drop with mixing until solution is basic. This treatment will precipitate Al³⁺, will be gelatinous white. May appear yellow because of the presence of CrO₄^2-. Centrifuge and decant the solution, it contains CrO₄²^-. Add 1 M BaCl₂ to solution from step 3 to precipitate yellow BaCrO₄. If precipitate is slow to form, heat in boiling water. Centrifuge the solution and decant. Dissolve the precipitate in 3 drops of 6M HNO₃; heat and stir. Add 4 drops of water and cool it down. Add 6 drops of diethyl ether followed by 1 drop of 3% H₂O₂. Shake gently, and allow layers to settle. A blue color in ether (top) later confirm test for chromium.

4. Add 8 drops of 15M HNO₃ to precipitate from step 2. Heat until precipitate is dissolved. Allow to cool and add 10 drops of 1M NH₄Cl and then add 15 M NH₃ until solution is alkaline to litmus paper. This will precipitate iron as brown. Then add 4-5 drops of NH₃ solution and stir. Centrifuge and decant solution (this solution may be discarded).

5. Dissolve the precipitate from step 5 by adding 10 drops of 6M HCl. Dilute with 2mL of distilled water and add 2 drops of 0.5M KSCN. A blood-red color indicates presence of Fe³⁺.

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I will be given 3 mL of an unknown sample and I will have 75 minutes to do a qualitative analysis for Ag, Cu, Sb, Fe, Cr.

I need help coming up with a flow chart based on the above steps so that it is makes this class assignment smoother since I will only be given 75 minutes to complete it.

Thank you!

CHM120H5 Study Guide - Quiz Guide: Centrifugal Force, Pasteur Pipette, Ph Indicator

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