Exercise 4: Can Pattems of Re be Used to ldentify an lon? 1 Imagine that you are
ID: 1087063 • Letter: E
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Exercise 4: Can Pattems of Re be Used to ldentify an lon? 1 Imagine that you are a supervisor of a laboratory. Your group is analyzing a sample of stream water that has high levels of an anion contaminant. You know that the sample contains either chloride, sulfate or carbonate You have a new student intern and you have them do the analysis of the sample using solutions of Sr2+ Pb2+ andZn2+.The intern carries out a similar investigation as the one you did in this lab. The results of the reactions between each of the cations with each of the anions is the same as your results. The intern then goes on to do the test on the stream sample. Here is an excerpt from the intern's report about the test on the stre am sample. 15 drops of stream water was placed in a test tube using a 1 mL plastic pipet. 5 drops of 0.10 M Sr2+solution was added to the stream water, one drop at a time, with shaking between each drop to mix the solutions. I observed a white, cloudy precipitate form in the solution. This reaction was the same as the reaction I saw when I did a control reaction between a solution of Sr2+ with a solution of SO42-. Therefore, I conclude the anion in the stream sample is SO42 Do you agree or disagree with the intern's conclusion about the identity of the anion in the stream? Provide some constructive criticism of the intern's work, explaining why you either agree or disagree with the intern's conclusion. Explain what further tests would be needed to convince you of the identity of the anion, or, explain why you are convinced that the intern is correct. 2. Why was it important to use an expe rimental control and to use the same volume of cation and anion solution during the investigation? You wrote down some initial ideas about the research question before you planned your experiment. What were those initial ideas? Did they change over the course of the experiment? Explain what changed and why it changed. 3.Explanation / Answer
Identification of an ion
1. Stream of water is tested.
the intern concludes SO4^2- present
SrCO3 also forms a white precipitate. Since the stream may have CO3^2- or SO4^2-, it is unwise to conclude the ion present is SO4^2- and not CO3^2- without any further tests.
2. It is important for the intern to test for all the ions at the same concentrations so that the most insoluble precipitate does not form first and give a false test for the other insoluble anion in solution.
3. The concentration of anion in the stream would affect the extent of precipitate formation in the acutal experiment which was not anticipated when initial ideas about the tests were written down. Therefore proper blank measurements or tests are necessary to avoid any such inconclusive and wrong results.
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