Equilibria of Slightly Soluble Ionic Compounds: We lied to you in chem 115

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1 Precipitation Reactions Predicting Whether a Precipitate Will Form Note the ions present in the reactants Consider all possible cation-anion combinations Use the solubility rules to decide whether any of the ion combinations is insoluble An insoluble combination identifies the precipitate that will form Aqueous solution of AgNO3 and Na2CrO4 are mixed. Identify the precipitates (if any) that form Equilibria of Slightly Soluble Ionic Compounds: We lied to you in chem 115 y Ag2CrO4(s) will ppt For a slightly soluble ionic compound in water, equilibrium exists between solid solute and aqueous ions Lab Application: Selective precipitation method of removing one ion type while leaving others dissolved in solution Ions added to form an insoluble product(precipitate) with ion(s) remaining in supernatant When both products are insoluble, their relative Ksp values can be used for separation We are what we repeatedly do. Excellence, then, is not an act, but a habit. Aristotle Page 1 of 12

2 Qsp and Ksp Compare IP to KSP to determine if solution is at equilibrium When IP < KSP; solution is unsaturated When IP = KSP ; solution is saturated (at equilibrium) When IP > KSP ; solution is super-saturated (unstable situation) Molar solubility, s Calculate the molar solubility (in mole/l) of the following compounds in pure water BaCrO4, KSP = Hg(CN)2, KSP = s = mole/l Au(OH)3, KSP = s = mole/l s =0.018 mole/l We are what we repeatedly do. Excellence, then, is not an act, but a habit. Aristotle Page 2 of 12

3 Concept Check Which of the following substances has the greatest molar solubility in water? a. PbI2, KSP = b. BaF2, KSP = c. Ca(OH)2, KSP = d. Zn(IO3)2, KSP = e. Ag2SO4, KSP = Give the Ksp expression in terms of molar solubility, s, for Ca3(PO4)2(s) in pure water KSP = 108 s 5 Calculate the the ph of a sat d Mg(OH)2 solution. KSP Mg(OH)2 = ph = We are what we repeatedly do. Excellence, then, is not an act, but a habit. Aristotle Page 3 of 12

4 Factors Affecting Solubility the common ion effect Thought experiment: Consider the addition of solid Na2CrO4 (s) saturated PbCrO4(aq) Na2CrO4(s) 100 % 2Na+(aq) + CrO4 2 (aq) This provides the common ion CrO4 2 Ca(OH)2 is only sparingly soluble with a solubility product of KSP is Calculate the molar solubility of Ca(OH)2 in pure water Calculate the molar solubility of Ca(OH)2 in 0.10 M Ca(NO3)2? s = mole/l Calculate the molar solubility of Ca(OH)2 in 0.10 M NaCl? s = mole/l s = mole/l We are what we repeatedly do. Excellence, then, is not an act, but a habit. Aristotle Page 4 of 12

5 Factors Affecting Solubility ph another type of common ion effect Low ph lots of H3O +, high ph lots of OH solubility of neutral salts is unaffected by ph Anions of strong acids (Cl, Br, I, NO3, HSO4, BrO3, ClO3, and ClO4 ) have no base strength and do not react with H3O + Solubility of metal hydroxides: Concept Check In which of the following solutions will CaCO3 be most soluble? a. Pure water b. 0.1 M Na2CO3 c. 0.1 M CaCl2 d. 0.1 M HCl e. 0.1 M NaCl Concept Check A change in ph will significantly affect the solubility of which, if any, of the following compounds? a. CuCl b. AgNO2 c. BaF2 d. AgI e. Both B and C We are what we repeatedly do. Excellence, then, is not an act, but a habit. Aristotle Page 5 of 12

6 Concept Check Which of the following salts are more soluble in acidic solutions than pure water? a. AgNO2 b. PbCl2 c. MgCO3 d. Fe(OH)2 e. All of the above but B Predicting precipitation reactions For a saturated solution of a slightly soluble salt, IP = Ksp Consider solution made when two mixtures of slightly soluble salts are combined, If IP KSP, no precipitate will form If IP > KSP, a precipitate will form o As ppt forms ions are removed from solution and [ ] and IP o Precipitate will continue to form until IP = KSP Does a precipitate form when L of 0.30 M Ca(NO3)2 is mixed with L of M NaF? KSP of CaF2 = Since IP > Ksp, CaF2 will precipitate until IP = Concept Check Solid Na2C2O4 is gradually added to a solution containing 0.1 M Ba 2+, 0.1 M Ca 2+, and 0.1 M Sr 2+. What will be the first salt to precipitate? a. BaC2O4 Ksp = b. CaC2O4 Ksp = c. SrC2O4 Ksp = We are what we repeatedly do. Excellence, then, is not an act, but a habit. Aristotle Page 6 of 12

7 Extra Practice questions: Qualitative Common ion effect Which of the following compounds are more soluble in acidic solution than in pure water? AgCN PbI2 Al(OH)3 ZnS no CaCO3 Ni(OH)2 Ca3(PO4)2 Mg(OH)2 Consider a saturated solution of BaF2 (KSP = ). When added to the saturated barium fluoride solution, which of the following salts will affect the solubility of BaF2 in water? NaF BaCl2 KNO3 NaCl decrease KF decrease Ba(NO3)2 No effect No effect decreae decrease 116_PSV_16_solubility_07 Write balanced equations to explain whether or not the addition of H3O + from a strong acid affects the solubility of each of the following ionic compounds lead(ii) bromide copper(ii) hydroxide no effect iron(ii) sulfide increase solubility increase solubility We are what we repeatedly do. Excellence, then, is not an act, but a habit. Aristotle Page 7 of 12

8 Extra Practice Quantitative Solubility Problems Calculate the molar solubility, s, of Mg(OH)2 in pure water. KSP of Mg(OH)2 = Calculate the molar solubility of Mg(OH)2 in water buffered at a ph of s = mole/l s = mole/l Lead(II) chromate, PbCrO4, is only sparingly soluble with a solubility product KSP = Calculate the molar solubility of PbCrO4 in pure water Calculate the molar solubility of PbCrO4 in M K2CrO4 s = mole/l s = mole/l We are what we repeatedly do. Excellence, then, is not an act, but a habit. Aristotle Page 8 of 12

9 MgF2 is only sparingly soluble with a solubility product KSP = Calculate the molar solubility of MgF2 in 0.10 M NaF Calculate the molar solubility in 0.10 M MgCl2 s = mole/l s = mole/l A saturated solution of CuCl in water was prepared and filtered. From the filtrate, 1.0 L was measured out into a beaker and evaporated to dryness. The solid CuCl residue recovered in the beaker was found to weigh 0.041grams Calculate the solubility product constant, Ksp, for CuCl. KSP = We are what we repeatedly do. Excellence, then, is not an act, but a habit. Aristotle Page 9 of 12

10 Extra Practice Problems: Predicting Precipitation Will a precipitate form of BaCO3 on mixing equal volumes (200. ml) of the following solutions? Ksp (BaCO3) = M BaCl2 and M Na2CO M Ba(NO3)2 and M Na2CO3 IP > KSP for BaCO3 so BaCO3 will ppt No. IP < KSP for BaCO3 so no BaCO3 will ppt Potassium chromate, K2CrO4, is slowly added to a solution containing 0.20 M AgNO3 and 0.20 M Ba(NO3)2. Describe what happens if the KSP for Ag2CrO4 is and the KSP of BaCrO4 is a. The BaCrO4 precipitates first out of solution b. The Ag2CrO4 precipitates first out of solution and then BaCrO4 precipitates c. Both BaCrO4 and Ag2CrO4 precipitate simultaneously out of solution d. Neither BaCrO4 nor Ag2CrO4 precipitates out of solution b We are what we repeatedly do. Excellence, then, is not an act, but a habit. Aristotle Page 10 of 12

11 Extra Challenge Problem Assuming that the total volume does not change after g of KCl is added to 1.00 L of a saturated aqueous solution of AgCl, calculate the number of moles of Ag + ion in the solution after equilibrium has been reestablished. For AgCl, Ksp = mole Ag + We are what we repeatedly do. Excellence, then, is not an act, but a habit. Aristotle Page 11 of 12

12 Calculate the value of Ksp for BaSO4 given [Ba 2+ ]= M in a saturated solution of BaSO4 : What is the ph of a saturated solution of AgOH? KSP is We are what we repeatedly do. Excellence, then, is not an act, but a habit. Aristotle Page 12 of 12