For ionic species the value of k sp depends of the

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sulfur and borax surround once-filled lake beds. For ionic species the value of Kspdepends of the concentration of each of the ions present in solution. Consider the dissolution reaction of magnesium hydroxide, Mg(OH)2(s), described by, Mg(OH)2(s)####Mg2+(aq)+2OH(aq)s2s(3) where sis the solubility of Mg(OH)2(s) in moles per liter. The solubility product expression depends on the concentrations of both the Mg2+(aq) and the OH(aq) ions formed and is given by: Ksp=[Mg2+][OH]2=(s)(2s)2=4s3(4) To determine the value of Kspfor a saturated Mg(OH)2solution, you can titrate an aliquot of the solution using a strong acid to measure the concentration of OH(aq). You can then determine the value of sfrom the relationship 2s= [OH] in Equation (3). Finally, you can substitute this value into Equation (4) to obtain the value of Kspfor magnesium hydroxide at the observed temperature. Thus, one method of determining the value of Kspis to titrate one of the species present in solution at equilibrium. In this experiment you will use a similar method to measure the value of Kspat various temperatures for the dissolution of borax. Borax is a boron-containing mineral that occurs in nature as a white powder composed of soft highly soluble crystals. Borax was first discovered by silk-road travelers in dry lake beds in Tibet and later found in the California and Nevada deserts of America. It is the active ingredient in some soaps and detergents, a flux used in soldering, an anti-fungal agent, and is even used as a texturizer in some prepared foods. It is also used in the manufacture of the thermoresistant borosilicate glass commonly marketed as, "Pyrex." You may be most familiar with borax because of the laundry additive brand, “20 Mule Team Borax.” The empirical formula of the natural form of the mineral borax is Na2B4O7•10H2O and its IUPAC name is sodiumtetraborate decahydrate, but here we shall simply refer to it by its common name, “borax.” The structure of the tetraborate anion, B4O5(OH)4 2–, is shown in Figure 1. From this figure you can see that a more structurally accurate description of the borax molecule is given by the chemical formula, Na2B4O5(OH)4•8H2O, which is the one we shall use here. OBOBOBOBOOHOOHOHHFigure 1: The Tetraborate Anion, B4O5(OH)4 2–
Santa Monica College Chemistry 12 Minerals, Dry Lakes, and Borax The chemical equation for the dissolution of borax may be written as: Na2[B4O5(OH)4]8H2O(s)####2 Na+(aq)+B4O5(OH)42(aq)+8H2O(l)(5)borax tetraborate anion The tetraborate anion formed in solution is a weak base and reacts with water according to: B4O5(OH)42(aq)+5H2O(aq)$$$$4H3BO3(aq)+2OH(aq)(6) The products of the reaction are boric acid, H3BO3(aq), a weak acid, and the hydroxide ion, OH(aq), a strong base. We can determine the concentration of the tetraborate anion in solution (and thus the amount of dissolved borax) in the following way: First, we collect a sample of the saturated solution at equilibrium – being careful only to collect the liquid portion and not any of the solid borax in equilibrium with it. Next we titrate the hydroxide ions using a strong acid, say Page 3 of 14

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