Question

It is important to think through the reactions that are occurring (or not occurring) and determine how you can rule out the presence of a certain metal ion or how you can posi- tively identify its presence. Use the Solubility Table 1 and Appendix 1 of this manual to understand the precipitation réactions that are observed. You need to consider both the anion and the cation that react to form a given precipitate. The color and the general ap- pearance of the precipitate are important to observe and record. Even when two precipitates are both white, one may be a thick, dense, white precipitate whereas the other may form a cloudy white solution. Recording detailed information is crucial in identifying the ions in your unknown salt. For emission spectra from alkali metals and alkaline earth metals, see Part 4 of Week 1. a) After dissolving the unknown in 20 mL of DI water, you add 0.5 mL of 0.05 M AgNO3 and observe a precipitate. What are the possible precipitates that are formed? Write the net ionic equations for the reactions. b) When HCl is added to your unknown, bubbles are observed. What can you conclude from this observation and from your observation in part (a) of this question? Identify the anion in this unknown (only one anion). What is the gas produced that causes the bubbles to form? c) To identify the two cations in your unknown, you do a flame test, and to confirm the identity, you use precipitation reactions. i) By adding H,SO, to your unknown, you observe no precipitate. What can you conclude regarding possible cations in your unknown? ii) In a flame test of your unknown salt without the didymium glass filter, you observe a bright yellow flame, and looking through the glass color filter you observe a light purple flame. Identify the two cations in your unknown.

Answer 1

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