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CHM 101/102 Laboratory Manual Qualitative Analysis: Group I General Chemistry 101/102 Laboratory Manual University of North Carolina Wilmington Slide 2 CHM 101/102 Laboratory Manual Qualitative Analysis: Group I Purpose Purpose To study the chemical properties of Ag +, Pb 2+, and Hg 2 2+ ions and the compounds they form. Use the chemical properties of these ions to separate and identify their presence in an unknown mixture. Safety Considerations Safety Considerations Pb 2+ and Hg 2 2+ ions are very toxic. Wash your hands thoroughly after this experiment. Place all waste solids and solutions in the appropriately labeled containers in the hood. Slide 3 CHM 101/102 Laboratory Manual Qualitative Analysis: Group I General Procedures General Procedures Use clean glassware and deionized water to avoid contaminating your sample. Perform each test to your known and unknown solutions simultaneously for comparison. Distinguish between clear, cloudy, colored, and colorless in your descriptions. cloudy yellow clear yellow clear colorless white solid Slide 4 CHM 101/102 Laboratory Manual Qualitative Analysis: Group I General Procedures General Procedures A centrifuge is used to separate heavier material, the solid precipitate, from the lighter liquid material by using centripetal force. Exercise caution when using the centrifuge. Use a counterbalance micro test tube. sample counterbalance Slide 5 CHM 101/102 Laboratory Manual Qualitative Analysis: Group I Use a water bath to heat all mixtures. General Procedures General Procedures Pay attention to whether you are to keep or discard the precipitate and/or supernatant. supernatant precipitate Slide 6 CHM 101/102 Laboratory Manual Qualitative Analysis: Group I Prepare a Known solution (you can label the test tube K) by adding 8 drops each of: Prepare a Known solution (you can label the test tube K) by adding 8 drops each of: AgNO AgNO 3 Pb(NO 3 ) 2 Hg 2 (NO 3 ) 2 Slide 7 CHM 101/102 Laboratory Manual Qualitative Analysis: Group I Prepare an Unknown solution (you can label the test tube U) by adding 15 drops of your assigned unknown mixture. Prepare an Unknown solution (you can label the test tube U) by adding 15 drops of your assigned unknown mixture. Slide 8 CHM 101/102 Laboratory Manual Qualitative Analysis: Group I Add HCl to both your known and unknown solutions and stir well with a clean and dry micro stirring rod.Add HCl to both your known and unknown solutions and stir well with a clean and dry micro stirring rod. Ag + (aq), Pb 2+ (aq), and Hg 2 2+ (aq), ions all form white precipitates with Cl - at room temperature:Ag + (aq), Pb 2+ (aq), and Hg 2 2+ (aq), ions all form white precipitates with Cl - at room temperature: AgCl, PbCl Hg 2 Cl AgCl (s), PbCl 2(s), Hg 2 Cl 2(s) Now you have a mixture of all 3 white precipitates in the test tube. The following steps will illustrate how to separate all 3 of ions, silver(I), lead(II) and mercury(I), from each other and how to positively identify them in your unknown mixture.Now you have a mixture of all 3 white precipitates in the test tube. The following steps will illustrate how to separate all 3 of ions, silver(I), lead(II) and mercury(I), from each other and how to positively identify them in your unknown mixture. Slide 9 CHM 101/102 Laboratory Manual Qualitative Analysis: Group I Place the test tubes in the centrifuge so that they counter balance one another. Place the test tubes in the centrifuge so that they counter balance one another. After centrifuging the solution for 30 seconds, the solid precipitate will fall to the bottom of the test tube. After centrifuging the solution for 30 seconds, the solid precipitate will fall to the bottom of the test tube. Slide 10 CHM 101/102 Laboratory Manual Qualitative Analysis: Group I The supernatant, the liquid above the solid precipitate, can be removed by decanting, pouring off the liquid, or by using a disposable transfer pipet. The supernatant, the liquid above the solid precipitate, can be removed by decanting, pouring off the liquid, or by using a disposable transfer pipet. Read the instructions carefully for each step to determine if the supernatant should be saved into a new clean and dry test tube, or can be discarded into the sink (or waste bottle). Read the instructions carefully for each step to determine if the supernatant should be saved into a new clean and dry test tube, or can be discarded into the sink (or waste bottle). Slide 11 CHM 101/102 Laboratory Manual Qualitative Analysis: Group I After the supernatant has been removed, add 1 mL DI water to the precipitate in both your known and unknown test tubes. After the supernatant has been removed, add 1 mL DI water to the precipitate in both your known and unknown test tubes. For 3 minutes, heat the test tubes in a water bath by placing them in a small beaker half filled with water and stir with a clean stirring rod every 30 seconds. For 3 minutes, heat the test tubes in a water bath by placing them in a small beaker half filled with water and stir with a clean stirring rod every 30 seconds. While the solutions are still hot, remove using test tube holders and place in the centrifuge. While the solutions are still hot, remove using test tube holders and place in the centrifuge. Slide 12 CHM 101/102 Laboratory Manual Qualitative Analysis: Group I While the solutions are still hot, remove using test tube holders and place in the centrifuge. While the solutions are still hot, remove using test tube holders and place in the centrifuge. PbCl 2(s) is unique in that it dissolves into lead and chloride ions when heated (therefore it is soluble in hot water). PbCl 2(s) is unique in that it dissolves into lead and chloride ions when heated (therefore it is soluble in hot water). After the test tubes have been centrifuged (while still hot!!) remove and save the supernatant into a new clean and dry test tube. After the test tubes have been centrifuged (while still hot!!) remove and save the supernatant into a new clean and dry test tube. Slide 13 CHM 101/102 Laboratory Manual Qualitative Analysis: Group I Con Confirm that the supernatant contained Pb 2+ (aq) ions by adding drops of K 2 CrO 4. Confirm that the supernatant contained Pb 2+ (aq) ions by adding drops of K 2 CrO 4. Solutions that contain lead ions will form a bright yellow precipitate and will look cloudy. Solutions that contain lead ions will form a bright yellow precipitate and will look cloudy. Solutions that do not contain lead ions will not form turbid or cloudy solutions upon adding potassium chromate, but will remain translucent and colored yellow. Solutions that do not contain lead ions will not form turbid or cloudy solutions upon adding potassium chromate, but will remain translucent and colored yellow. Slide 14 CHM 101/102 Laboratory Manual Qualitative Analysis: Group I Return your attention to the white precipitate saved from step 2. We have removed lead ions from this precipitate using heat. The white precipitate that remains theoretically contains silver and mercury(I) as solid chloride precipitates. Return your attention to the white precipitate saved from step 2. We have removed lead ions from this precipitate using heat. The white precipitate that remains theoretically contains silver and mercury(I) as solid chloride precipitates. The AgCl (s) will dissolve into Ag + (aq) and Cl - (aq) ions with the addition of ammonia. Now the silver ions will be dissolved in the supernatant, or liquid portion. The AgCl (s) will dissolve into Ag + (aq) and Cl - (aq) ions with the addition of ammonia. Now the silver ions will be dissolved in the supernatant, or liquid portion. By adding ammonia (NH 4 OH) and changing the basicity (or pH), the mercury will precipitate as a grey solid. By adding ammonia (NH 4 OH) and changing the basicity (or pH), the mercury will precipitate as a grey solid. Slide 15 CHM 101/102 Laboratory Manual Qualitative Analysis: Group I Remove the supernatant from step 4 and place in a new clean and dry test tube. Remove the supernatant from step 4 and place in a new clean and dry test tube. Adding HNO 3, nitric acid, to the solution will cause the soluble silver and chloride ions to reform a white AgCl (s) precipitate, and fall out of solution. Adding HNO 3, nitric acid, to the solution will cause the soluble silver and chloride ions to reform a white AgCl (s) precipitate, and fall out of solution. Note that by adding acid to the solution, again a pH change (blue litmus paper turns red), the solubility of silver ions was altered. Note that by adding acid to the solution, again a pH change (blue litmus paper turns red), the solubility of silver ions was altered. Slide 16 CHM 101/102 Laboratory Manual Qualitative Analysis: Group I