Metal Ions And Inorganic Compounds Lab Report

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Experiment 8: Identification of metal ions and inorganic compounds in aqueous solution

Introduction:

Qualitative analysis is the identification a sample's component(s). Unlike a quantitative analysis, we are not concerned with the amount of a substance present in a sample but only with its identity. In this exercise we will focus on identifying the cations and anions that make up ionic compounds, both solid and in solution. Ideally there would be chemical tests that could be used to identify individual ions without interference by any other ions. Unfortunately, there are often complications. For example, the formation of a yellow precipitate upon addition of aqueous S2- confirms the presence of Cd2+ in a solution. The color of this compound, however, will be hidden if any Pb2+ or Cu2+ are present in solution since they will form a black precipitate with added
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The most common are precipitation and complexation. In a precipitation reaction, an ion in solution reacts with an added reagent to form a solid. Whether a solid will form from a given reaction can be predicted by the solubility product constant (Ksp) of the solid under the given conditions. Solubility product constants are the equilibrium constants for the dissolution of an "insoluble" ionic solid in water. A low Ksp implies that the compound does not dissolve to an appreciable degree in water. If the two ions are mixed in solution, a precipitate will tend to form. If steps have been taken to remove ions that form competing precipitates, the presence of a properly colored solid can be used to confirm the presence of a given ion. If several different precipitates remain, the conditions of the solution can be manipulated to selectively redissolve one or more of the solids. When the equilibria involved are well understood, selective precipitation can be a powerful tool in the identification of unknown

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