40 - A P CHEMISTRY Lab 15-2 The Solubility Product of...

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A P CHEMISTRY Lab 15-2 The Solubility Product of Silver Acetate Pre-Lab Questions - (Must be completed before lab work begins.) 1. Write the specific definition of K sp found in your textbook. 2. Using a handbook of chemical data, look up the values of K sp for the following substances at 25 0 C. Silver chloride _______________ Calcium carbonate __________________ Silver bromide _______________ Silver acetate ______________________ Barium sulfate _______________ Calcium hydroxide __________________ 3. The solubility product of lead chromate is 2.0 x 10 -16 . Calculate the solubility in moles per liter of lead chromate in each of the following solutions: (a) saturated lead chromate in water (b) saturated lead chromate in 0.10 M Na 2 CrO 4 solution (c) saturated lead chromate in 0.001 M Pb(NO 3 ) 2 solution INTRODUCTION - Solubility is defined to be the number of grams of a substance that will dissolve in 100 mL of solvent. For substances that dissolve to a reasonable extent in a solvent, the solubility is a useful method of describing how much solute is present in a solution. However, for substances that are only very sparingly soluble in a solvent, the solubility of the salt is not a very convenient means for describing a saturated solution. When the solute is only very sparingly soluble in the solvent, the solution is more conveniently (and correctly) described by the equilibrium constant for the dissolving process. Consider the salt silver chloride, AgCl, which is very sparingly soluble in water. When a portion of solid silver chloride is placed in a quantity of pure water, Ag +1 ions and Cl -1 ions begin to dissolve from the crystals of solid and enter the water. As the number of Ag +1 and Cl -1 ions present in the water increases, the likelihood of the Ag +1 and Cl -1 ions reentering the solid increases. Eventually, an equilibrium is established between the solution and the undissolved solute crystals remaining: Ag +1 and Cl -1 ions will be dissolving from the crystals, while elsewhere Ag +1 and Cl -1 ions will "undissolve" at the same rate. Beyond this point, there will be no net increase in the number of Ag +1 and Cl -1 ions to be found in the solution at any future time. The solution has become saturated, which means that the ions that are dissolved have reached a state of equilibrium with the remaining undissolved solute. AgCl(l) = Ag +1 (aq) + Cl -1 (aq) In writing the equilibrium constant expression for this equilibrium, we might at first be tempted to write K =[Ag +1 (aq)] [Cl -1 (aq)]/[AgCl(s)] in which square brackets [ ] refer as usual to concentrations in moles per liter. However, because AgCl(s) is a pure solid, its concentration is a constant value (a function of the density of the solid) and is included in the measured value of the equilibrium constant. For this reason, a new equilibrium constant, K sp , called the solubility product constant is defined for the dissolving process outlined earlier: K sp = [Ag +1 (aq)][Cl -1 (aq)] The fact that the concentration of the remaining undissolved solid AgCl does not enter into this equilibrium is easily
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This note was uploaded on 08/31/2010 for the course SCI 56-1120 taught by Professor Unni during the Spring '09 term at Columbia College.

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40 - A P CHEMISTRY Lab 15-2 The Solubility Product of...

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