Experiment 4: Qualitative Analysis of Cations and Precipitation Reactions
The overall goal of experiment four was to determine the identity of unknown cations presented to the student. But in order to know the identity of these unknowns, in part 1, Ag+, Pb+, and Hg22+ were presented to the student in aqueous solutions and then precipitated through experimentation. In part 2, the same procedure was enacted to determine which substances precipitated through qualitative analysis. Solubility rules were also a major theme as solubility is important in determining whether a reaction will produce a precipitate. Starting out the experiment, HCl was added to the solution in the test-tube in order to form a reaction between the HCl, Ag, Pb, and Hg cations. The products of this addition of HCl were PbCl2, AgCl, and Hg2Cl2. After the HCl was added, the solution turned a milky white color. It was important to not add too much HCl because excess HCl would have caused an aqueous complex of PbCl2 and AgCl to form instead of the desired solid PbCl2. The mixture was then centrifuged in order to let the solid particles of the three ions to fall to the bottom. Another drop of HCl was added to test if the reaction had been completed. If the solution were to turn milky white again then it would’ve signaled an incomplete reaction between the cations and HCl. The next objective was to separate the lead(II) ion from the mixture. The test tube with all three solids precipitated at the bottom was heated, which allowed the PbCl2 to dissolve. The supernatant fluid containing the Pb(II) ions was then separated from the solid mercury(I) ions and solid silver ions decanting, adding a drop of acetic acid to the supernatant fluid, and also adding two drops of K2CrO4 to the test tube. A milky yellow mixture was observed as this indicated the presence of the lead (II) ion. This insoluble yellow precipitate was the insoluble compound of PbCrO4. In part 2 of the experiment, after the supernatant...
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