E6-Pigments.pdf - Experiment#6 – Inorganic Synthesis and Nomenclature Precipitation of Ancient Mineral Pigments OBJECTIVES In successfully completing

E6-Pigments.pdf - Experiment#6 – Inorganic Synthesis and...

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6 – 1 Experiment#6 – Inorganic Synthesis and Nomenclature: Precipitation of Ancient Mineral Pigments OBJECTIVES In successfully completing this lab you will: §੿ characterize inorganic, ionic compounds as soluble or insoluble; §੿ assess a set of solubility rules and a system of nomenclature for combinations of commonly encountered ions; §੿ predict formulae of precipitates based on observations of reactions; and §੿ write balanced, net ionic equations to describe precipitation reactions. INTRODUCTION In this experiment you will mix a number of different pairs of reagents, in the form of aqueous solutions. You will observe the system after mixing and look for signs of a chemical reaction. Given that this experiment is centered on the synthesis of ancient pigments, you can probably guess that vibrant colors are likely to be produced. If a reaction has occurred, you should write the formula and name of any new product that is obtained as well as a balanced net ionic equation to describe its formation. Species (typically ions) that are present in solution, but do not form a precipitate, are known as spectator ions and need not be included in the balanced equation. To decide what chemical species actually exist in aqueous solution, it is necessary to understand and recognize strong and weak electrolytes. Strong electrolytes are essentially completely dissociated in solution (no neutral molecules, only ions). Weak electrolytes are very slightly dissociated as ions in aqueous solution. If a weak electrolyte is uncharged, the result is a very low concentration of ions, while neutral molecules are present in significant concentration. There is a third category, non- electrolytes, which do not dissociate at all in aqueous solution. (Note: although they are an important class of compounds, non-electrolytes will not be encountered in this experiment.) Appendix A of this experiment entitled Strengths of Electrolytes provides information to enable you to categorize the electrolytes you encounter in this experiment as either strong or weak. Note that the label on a bottle containing a solution does not necessarily indicate the chemical species as it exist in solution. A label, such as NaCl (aq), indicates that the solution was prepared by dissolving solid NaCl in water. Since NaCl (like almost all soluble salts) is a strong electrolyte, the solution contains no “NaCl” molecules; it contains only Na + and Cl , along with large amounts of H 2 O. In fact, a saturated NaCl (aq) solution has approximately 10 times more H 2 O than either Na + or Cl , while seawater has about 1000 times more H 2 O than ions. Because NaCl does not exist in solution as molecules, “NaCl” cannot possibly be a product of a reaction mixture prepared by pouring a solution containing Na + into a container holding a solution of Cl . In fact, the primary way to isolate solid NaCl from solutions is to evaporate nearly all the water, eventually precipitating solid NaCl.
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