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Unformatted text preview: 1 Experiment 4: Qualitative Analysis of Cations and Precipitation Reactions PURPOSE Qualitative analysis in inorganic chemistry is concerned with detecting and identifying elements that are present in a sample of material. In this experiment, you will develop a scheme for the qualitative analysis of three cations, using a schematic approach. The procedures in the qualitative analysis of this kind involve precipitation reactions to sequentially remove cations from a mixture of soluble salts. Along each step of the separation, the precipitated cation will be centrifuged and removed from the remaining soluble cations. The cations that we will study are Ag + , Pb 2+ , and Hg 2 2+ . The cations will be tested with reagents that will furnish anions or polyatomic that will precipitate the cations. The common test reagents will have a different effect on the behavior of one cation to another and will provide the basis for their separation. During the first part of this experiment you will be given a solution that contains all three cations. You will separate the three cations and on the basis of your observations, you will analyze an unknown solution that contains at least two of the three cations. Additionally, you will learn the solubility rules to determine the solubility of salts. CONCEPTS You should review the following concepts prior to the lab: 1) Naming and identifying salt compounds. 2) Formula unit equation, total ionic equations and net equations for precipitation reactions. 3) Defining qualitative and quantitative analsis. PRINCIPLES Solubility of Salts The solubility of a compound is the amount of the compound that dissolves in a specified volume of water to form an aqueous solution. Solubility is expressed as either grams/liter or grams/100 mL. Compounds whose solubility in water is less than 0.1 g/L are usually classified as insoluble compounds. There is no sharp boundary between soluble and insoluble compounds. Compounds whose solubilities fall near the arbitrary division are called moderately soluble compounds. Ionic compounds (salts) that dissolve in water are represented by (aq). In order to determine which of the salts will dissociate in dilute aqueous solutions, it is necessary to know a few of the solubility rules. Generally, salts composed of the following ions are completely soluble in dilute aqueous solution. Generally, the ions below are almost always spectator ions in reactions that occur in dilute aqueous solutions. i. Group IA metals: Li + , Na + , K + , Rb + , Cs + ii. Nitrate ion and chlorate ion : NO 3- & ClO 3- iii. Ammonium ion: NH 4 + A table of solubility guidelines for common ionic compounds in water is located below in Table 1. 2 Table 1 Solubility Guidelines for Common Ionic Compounds in Water Ion Soluble (>10g/L) Slightly soluble (0.1 10g/L) Insoluble < 0.1 g/L: NO 3- All CH 3 COO- Most AgCH 3 COO BeCH 3 COO Cl- , Br- , I- Most PbCl 2 , PbBr 2 , HgBr 2 AgCl, AgBr, AgI, Hg 2 Cl 2 , Hg 2 Br 2 , Hg 2 I 2 , PbI 2 , HgI...
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