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Qualitative Analysis: Group I. General Chemistry 101/102 Laboratory Manual University of North Carolina Wilmington. Safety Considerations. Pb 2+ and Hg 2 2+ ions are very toxic. Wash your hands thoroughly after this experiment.
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Qualitative Analysis: Group I General Chemistry 101/102Laboratory ManualUniversity of North Carolina Wilmington
Safety Considerations • Pb2+ and Hg22+ 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. Qualitative Analysis: Group I • Purpose • To study the chemical properties of Ag+, Pb2+, and Hg22+ ions and the compounds they form. • Use the chemical properties of these ions to separate and identify their presence in an unknown mixture.
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 Qualitative Analysis: Group I
Qualitative Analysis: Group I • 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. counterbalance sample
General Procedures • Pay attention to whether you are to keep or discard the precipitate and/or supernatant. supernatant precipitate • Use a water bath to heat all mixtures. Qualitative Analysis: Group I
Qualitative Analysis: Group I • Prepare a Known solution (you can label the test tube “K”) by adding 8 drops each of: AgNO3 Pb(NO3)2 Hg2(NO3)2
Qualitative Analysis: Group I • Prepare an Unknown solution (you can label the test tube “U”) by adding 15 drops of your assigned unknown mixture.
Qualitative Analysis: Group I • Add HCl to both your known and unknown solutions and stir well with a clean and dry micro stirring rod. • Ag+(aq), Pb2+(aq), and Hg22+(aq), ions all form white precipitates with Cl - at room temperature: AgCl(s), PbCl2(s), Hg2Cl2(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.
Qualitative Analysis: Group I • 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.
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. • 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).
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. • 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.
Qualitative Analysis: Group I • While the solutions are still hot, remove using test tube holders and place in the centrifuge. • PbCl2(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.
Qualitative Analysis: Group I Con • Confirm that the supernatant contained Pb2+(aq) ions by adding drops of K2CrO4. • 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.
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. • 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 (NH4OH) and changing the basicity (or pH), the mercury will precipitate as a grey solid.
Qualitative Analysis: Group I • Remove the supernatant from step 4 and place in a new clean and dry test tube. • Adding HNO3, 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.
