CH204 (Experiment 4 - Postlab) PDF

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Experiment 4: Qualitative Analysis of Cations and Precipitation Reactions The goal or objective of this experiment was to develop a scheme for qualitatively analyzing cations in an aqueous mixture – the scheme will be used to precipitate, separate, and identify the cations, in regards to this experiment, the three cations: Ag +, Pb2+, and Hg2+. In order to separate the precipitate ions, a centrifugation was utilized, and then the scheme was resumed to identify the cations using the rules of solubility. At the beginning of the separation process, it was impertinent that HCl was added to the mixture of cations so that the cations: Ag+, Pb2+, and Hg2+, may be precipitated and separated. However adding too much HCl would be bad and should be avoided as it could tamper with the experiment because both AgCl and PbCl2 have tendencies to form soluble complexes. All of which would defeat the purpose of the experiment of trying to identify the insoluble cations. At the end of centrifugation, another drop of HCl was added to the mixture to ensure the completeness of precipitation – indicated that the mixture would need to be centrifuged again if the mixture appeared milky when the extra drop of HCl was added. In order to separate the lead (II) ion from the sample solution mixture, after the HCl was added, and the mixture was centrifuged and decanted, the precipitate was then washed with 2ml of water and centrifuged, then heated in boiling water (while being stirred.) This process dissolved the majority of the PbCl2. After separating the lead (II) ion from the mixture, to test for its presence, the hot mixture (just boiled) was centrifuged, and the liquid decanted into a test tube. In which the supernatant would be tested for presence of Pb2+ by adding a drop of 6.0M acetic acid and two drops of 1.0M K2CrO 4. After adding the two, if a yellow-orange precipitate formed, then the presence of lead (II) ions would be validated. In the case of the unknown for this particular experiment, the lead (II) ion was not present in the sample solution. Going onto the mercury (I) ion, it was separated from the mixture by adding 1ml of 6.0M NH 3 to the remaining precipitate and mixed gently, afterwards centrifuged and decanted the liquid into an empty test tube. If there was a blackish-gray precipitate remaining in the centrifuged test tube, then that indicated the presence of the mercury (I) ion – in the case of the unknown, the mercury (I) ion was present. Lastly, when testing for the presence of silver ions in the sample solution mixture, first to separate them, the remaining decanted liquid from the mixture that had contained possibly both the silver and the mercury (I) ions, had approximately 1ml of 6M HNO3 added. The presence of silver could be validated if

a white precipitate formed after the addition of the nitric acid – in the cause of the unknown sample solution mixture, the silver ion was indeed present. Throughout the experiment there were not many difficulties encountered, despite that being, several of the apparent difficulties included: the evaporation of boiling water causing the amount present to heat the solution to be lacking and the time span spent on centrifuging the mixtures could have been increased to allow for more valid results – was lacking. In order to improve the overall experimental outcome, multiple trials could always be performed respective the known/unknown solutions (staying consistent throughout the trials), cleaning of all equipment (including the newly, unpackaged test tubes) may increase the overall reliability of the experimental outcome. Despite these factors, the experiment as a whole was conducted smoothly without many bumps in the road....