At the same time you will have to keep in mind what

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Unformatted text preview: action? Dakota State University page 191 of 232 Experiment 19: Qualitative Analysis General Chemistry I and II Lab Manual Experiment 19: qualitative analysis Purpose: To determine the cations present in an unknown solution Pernoid: Not a real word Background: See “Basic Laboratory Procedures”; Litmus paper, Bunsen burner Introduction: One of the oldest questions asked by chemists is “what is it?” As I understand it, the Native Americans would chew a particular type of leaf to relieve headaches; chemists wanted to know what was in it that did so. The result is aspirin. The concept of extracting organic materials and analyzing them to determine their chemical composition is beyond the scope of this course (in fact, these topics are covered in Organic Chemistry). However, in this experiment, we will be performing the General Chemistry equivalent; qualitative analysis. In qualitative analysis, we ask simply “what is in it.” We are not worried about how much there is (this is “quantitative analysis”), just, yeah or nay, is this metal present? It will take you several weeks to complete this procedure; this experiment replaces a “lab practical,” because in it, you will have to be particularly wary of your technique, observation skills, note taking skills, and labeling skills. Your technique will be tested because cross contamination can cause “false positives” (that is, you will think ions are present that really are not). What’s more, if you fail to clean your equipment carefully, you can cause contamination that will result in the same problem. Also, be careful of the equipment you choose; we are looking for metal ions, so choosing things such as metal scoopulas can cause problems. At the same time, you will have to keep in mind what ions are (or might be present) in each container at all times. You will not be able to keep all solutions you make, but you must be very careful not to discard a solution you will need later on. Remember that we are beginning with a mixture of all possible ions; do not throw away solutions if you are not sure that it is no longer needed. You observation skills will be critical. Often it is challenging to tell if a positive is sufficient to call it positive, or determine if it is simply a contaminant. We will have ways to help you differentiate, but you will have to always be wary of what you add, and what happens. Note taking is of critical importance. The smallest observation, which does not seem to be significant when you first write them down, could be the determining factor when deciding what is present and what is not. All too often, a student will ask me a question, such as, “what does this mean?” I usually cannot help, not because I am not willing, but because I was not present to witness the entire procedure, so I often do not know where the student is or what might have happened to get them there. The only witness is the analyst; any lost notes, then, are simply lost forever. Dakota State University page 192 of 232 Experiment 19: Qualitative Analysis General Chem...
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