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Chapter 18: Solubility and Complex-Ion Equilibria

Petrucci • Harwood • Herring • Madura. GENERAL. Ninth Edition. CHEMISTRY. Principles and Modern Applications. Chapter 18: Solubility and Complex-Ion Equilibria. Juana Mendenhall, Ph.D. Assistant Professor Morehouse College Lecture 2 Chapter 18. Objectives.

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Chapter 18: Solubility and Complex-Ion Equilibria

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  1. Petrucci • Harwood • Herring • Madura GENERAL Ninth Edition CHEMISTRY Principles and Modern Applications Chapter 18: Solubility and Complex-Ion Equilibria Juana Mendenhall, Ph.D. Assistant Professor Morehouse College Lecture 2 Chapter 18 General Chemistry: Chapter 18

  2. Objectives • Determine if a salt will precipitate from solution based on concentration of its ions. • Determine the concentration of ions remaining in solution after precipitation and predict whether precipitation will be complete • Calculate the separation of ions using fractional precipitation • Calculate the effect of pH on the precipitation and dissolving of certain substances General Chemistry: Chapter 18

  3. Sequence of steps for calculating: Calculating Ksp from solubility data Calculating solubility from Ksp data General Chemistry: Chapter 18

  4. Definition of Terms • Molar solubility (@ 25ºC): the number of moles of solute in one liter of a saturated solution (mol/L) • Solubility (@ 25ºC): the number of grams of solute in one liter of a saturated solution (g/L) General Chemistry: Chapter 18

  5. The Ksp Concept • Ksp is usually limited to slightly soluble solutes. • For more soluble solutes we must use ion activities • Activities (effective concentrations) become smaller than the measured concentrations. • The Salt Effect (or diverse ion effect). • Ionic interactions are important even when an ion is not apparently participating in the equilibrium. • Uncommon ions tend to increase solubility. • Review Solubility Rules • Ksp indicates how soluble an ionic compound is in H20; the larger the Ksp the more soluble the compound General Chemistry: Chapter 18

  6. Incomplete Dissociation • Assumption that all ions in solution are completely dissociated is not valid. • Ion Pair formation occurs. • Some solute “molecules” are present in solution. • Increasingly likely as charges on ions increase. Ksp (CaSO4) = 2.310-4 by considering solubility in g/100 mL Table 19: Ksp = 9.110-6 Activities take into account ion pair formation and must be used. General Chemistry: Chapter 18

  7. 18-5 Criteria for Precipitation and Its Completeness AgI(s) Ag+(aq) + I-(aq) Ksp = [Ag+][Cl-] = 8.510-17 Mix AgNO3(aq) and KI(aq) to obtain a solution that is 0.010 M in Ag+ and 0.015 M in I-. Saturated, supersaturated or unsaturated? Q = [Ag+][Cl-] = (0.010)(0.015) = 1.10-4 > Ksp General Chemistry: Chapter 18

  8. The Ion Product Q is generally called the ion product. Q > Ksp Precipitation should occur. Q = Ksp The solution is just saturated. Q < Ksp Precipitation cannot occur. General Chemistry: Chapter 18

  9. Example • Calculate the solubility of copper hydroxide, Cu(OH)2 in g/L. Ksp = 2.2 x 10-20. • Step 1: ICE chart General Chemistry: Chapter 18

  10. 18-6 Fractional Precipitation • A technique in which two or more ions in solution are separated by the proper use of one reagent that can cause precipitation of both ions. • Significant differences in solubility's are necessary. • Key to the technique isslow addition of the reagent. General Chemistry: Chapter 18

  11. Example of fractional ppt • Silver nitrate is slowly added to a solution that is 0.020 M in Cl- ions and 0.020 M in Br- ions. Calculate the concentration of Ag+ ions (in mol/L) required to initiate a) ppt of AgBr (Ksp 7.7 x 10-13) and b) ppt of AgCl (Ksp = 1.6 x 10-10) General Chemistry: Chapter 18

  12. 18-7 Solubility and pH • pH can affect the solubility of a salt. • Especially when the anion of the salt is the conjugate base of a weak acid. • Mg(OH)2 Milk of Magnesia. Adding OH- ions (increasing pH shifts) equilibrium from right to left decreases the solubility of Mg(OH)2 Adding H+ ions (decreasing the pH) shifts equilibrium from left to right increases the solubility of Mg(OH)2 General Chemistry: Chapter 18

  13. Example of solubility and pH • At 25 ºC the molar solubility of Mg(OH)2 in pure H20 is 1.4 x 10-4M. Calculate its molar solubility in a buffer medium whose pH is a) 12.00 and b) 9.00 General Chemistry: Chapter 18

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