Lab 5 - Qualitative Analysis

Lab 5 - Qualitative Analysis - Lab 5 Qualitative Analysis...

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4/15/13 Lab 5 - Qualitative Analysis www.webassign.net/ebooks/wsugencheml1/lab_5/manual.html 1/6 Contents > Lab 5 - Qualitative Analysis Lab 5 - Qualitative Analysis Purpose To develop a separation scheme and confirmatory tests for Ag 1+ , Hg 2 2+ and Pb 2+ cations, and to use it to identify the ions in a sample of unknown composition. Goals To explore solubilities and reactivities of different metal ions. To logically develop a scheme to separate and identify a mixture of ions based on observations. Introduction Qualitative analysis is the process by which components of mixtures are separated and identified. Unlike quantitative analysis, where the amount of a particular material is measured, a qualitative analysis scheme simply confirms the presence or absence of certain materials. A common analysis is the identification of aqueous ions. In such an analysis, there are two distinct phases. First, a scheme must be developed to separate the ions from each other. Then, a different test is performed to uniquely confirm the identity of each separated ion. In this lab, we develop a scheme to separate and identify the silver ion (Ag 1+ ), the mercury(I) ion (Hg 2 2+ ), and the lead(II) ion (Pb 2+ ). Mercury(I) ions exist as dimers in aqueous solution, and function as a +2 ion. Ions are often separated in solution by their different solubilities. Although most chloride salts are soluble, some ions form insoluble compounds with chloride. Among the "insoluble" compounds, some dissolve in hot water. Some insoluble compounds can be brought into solution in the presence of ammonia, forming "metal ammine complexes" of the general formula M(NH 3 ) x y+ where x and y are specific to different metals. These three solubility characteristics (solubility of chloride salt, solubility of chloride salt in hot water, solubility of chloride salt in ammonia solution) form the basis of the separation scheme for Ag 1+ , Hg 2 2+ , and Pb 2+ . If one of the metal ions remains in solution after the addition of HCl and the others form insoluble solids (precipitates) , the addition of HCl effectively separates the soluble ion from the others. The soluble ion is separated by collecting the solid at the bottom of a test tube in a centrifuge , a device that creates a centrifugal force by rotation. After the precipitate is compacted, the supernatant (the liquid solution above the solid) is decanted (carefully poured off) into a separate container. The soluble ion is in the liquid supernatant while the insoluble ion is in the solid precipitate. Thus they are physically separated from each other when the liquid is poured off, leaving the solid behind. The solid precipitate is typically washed with water to help remove any traces of the soluble ions that
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4/15/13 Lab 5 - Qualitative Analysis www.webassign.net/ebooks/wsugencheml1/lab_5/manual.html 2/6 remain. This prevents "false positive" test results later on. In order to devise a scheme for separating a mixture of the cations in this lab, each ion will be tested individually for its solubility characteristics. You must carefully observe the behavior of each ion under the test conditions described in the procedure. The presence or absence
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Lab 5 - Qualitative Analysis - Lab 5 Qualitative Analysis...

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