16.6 Solubility Equilibria
Solubility Equilbria
• Many ionic cmpds are very soluble in water (as NaCl) but others have limited solubility--p 109 table 4.2
• Let’s envision what happens when make a saturated solution of some ionic cmpd of limited solubility, as CaCO3. Assume that the solvent is water and the temp is 25oC.
• CaCO3(s) Ca2+(aq) + CO32-(aq)
CaCO3(aq)
• A certain quantity of CaCO3 goes into solution and immediately dissociates into ions. _______________
• A saturated solution has ___________
• Remember in writing equil expressions that pure solids, pure liquids--______.
Solubility Product
• CaCO3(s) Ca2+(aq) + CO32-(aq)
• Ksp = _____________
• Ksp is called the _______________ (equil constant) (Implies ideal behavior)
• [Ca2+] implies conc units in _________
• Ksp values tabulated in book, p 672, table 16.2
Relationship btn molar sol’y (s) and Ksp
• Define molar solubility, s ,as the ________
• Let us examine the stoichiometrically different salts and their relationship to molar solubility.
• 1:1 salt as AgCl, CaCO3
• AgCl(s) Ag+(aq) + Cl-(aq)
• Ksp = ____________
• [Ag+] = [Cl-] = ____
• 2:1 or 1:2 salt as CaF2 or Ag2SO4
• CaF2(s)
• Ksp = ________
• [F-] = __ [Ca2+] = __
• 1:3 or 3:1 salt as Fe(OH)3 or Ag3PO4
•
• Ag3PO4(s)
• Ksp =
• [Ag+] = ___ [PO43-]= ___
• 2:3 or 3:2 salt as Bi2S3 or Ca3(PO4)2
• Bi2S3(s)
• Ksp =
• [Bi3+] = ___ [S2-] = _____ • • Remember this assumes ideal behavior (as
complete ionization--no ion pair formation, no hydrolysis of ions as Al3+ )
Ksp and Q (ion-product)
• Remember Ksp refers to a saturated sol’d, Q refers to any sol’n, not just an equilibrium system.
• Ag3PO4(s) 3Ag+(aq) + PO43-(aq)
• Ksp = [Ag+]3[PO43-]
• Q = [Ag+]3[PO43-] : these need not be equil
concs
• If
• Q < Ksp ______
• Q = Ksp ______
• Q > Ksp _______
Problems: Ksp to s and s to Ksp
• What are the molar solubilities of CaCO3(Ksp = 8.7 x 10-9) and Ag2CO3 (Ksp = 8.1 x 10-12)
• Calc Ksp of Ag3PO4, given the sol’y of Ag3PO4 is 6.7 x 10-3 g/L
• 16.45: The sol’y of an ionic cmpd, M2X3
(molar mass=288g) , is 3.6 x 10-17 g/L. What’s the Ksp?
• 16.47:What is the pH of a saturated zinc hydroxide sol’n?
• 16.48: The pH of a sat’d sol’n of a metal hydroxide, MOH, is 9.68. Calc. the Ksp.
Mix two sol’s together, do you get a ppt (predicting ppt rxns)
• Do you get a ppt if mix
• 10 mL of 0.0010M AgNO3 and 10 ml of 0.0010M Na2SO4
• 10 mL of 1.0 x 10-6M iron(II) chloride and 20 ml of 3.0 x 10-4M barium hydroxide
• 16.50: A volume of 75 mL of 0.060 M NaF is mixed with 25 mL of 0.15 M Sr(NO3)2. Calc the concs in the final soln of NO3
-, Na+, Sr2+, and F-. Ksp for SrF2 is 2.0 x 10-10.
16.8: The common ion effect and solubility
• What does Le Chatelier say about the presence of a common ion and solubility
• AgI(s) Ag+(aq) + I-(aq)
add Ag+ from AgNO3
• ___________
• Calculate the molar sol’y of SrF2 (Ksp = 2.0 x 10-10) in
• water
• in 0.010 M Sr(NO3)2
• in 0.010 M NaF
• _________
• 16.56 similar
• 16.55: How many grams of CaCO3 will dissolve in 300 mL of 0.050 M Ca(NO3)2?
• What’s the solubility of Fe(OH)3 in HOH? Ksp for Fe(OH)3 =1.1 x 10-36.
• So does changing pH affect solubility of insoluble hydroxides?
•
• 16.62: Calc the pH of Fe(OH)2 in water and at a pH of 7.00, at a pH of 8.00 and at a pH of 10.00.
• 16.61: Compare the molar soly of Mg(OH)2 in water and in a soln buffered at a pH of 9.00 .
Factors that affect solubility
• 1. _______• 2. common ion ____________• 3. pH: salt contains anion (conj base of
WA): ____________
• CaF2(s) Ca2+(aq) + 2F-(aq)
H+
• 4. hydroxides--pH: ___
• Except for amphoteric hydroxides as Al(OH)3, Pb(OH)2, Cr(OH)3, Zn(OH)2, Cd(OH)2
• Al(OH)3 + OH- Al(OH)4-(aq)
• As pH increases (add more OH-), tie up the hydroxide salt as a soluble complex,
• 5.Complex ion formation
• Cu2+(aq) +2OH- Cu(OH)2(s)
• Cu(OH)2(s) + 4NH3(aq) Cu(NH3)42+(aq)
complex ion
• Cu2+(aq) + 4NH3(aq) Cu(NH3)42+(aq)
• Kf = [Cu(NH3)42+] = 5.0 x 1013 [Cu]
[NH3]4
• Table of formation constants Kf, p 685, table 16.4
• 16.67: If 2.50 g of CuSO4 are dissolved in 900 mL of 0.30 M NH3, what sre the concs of Cu2+, Cu(NH3)4
2+ and NH3 at equilibrium?
• 16.70: Calc the molar soly of AgI in a 1.0 M NH3 soln.
• Which of the following will be more soluble in acid solution than in water?
• BaSO4
• PbCl2
• Fe(OH)3
• CaCO3
• Ca3(PO4)2
• AgBr
• 16.54: The molar soly of AgCl in 6.5x10-3M AgNO3 is 2.5 x 10-8 M. In deriving Ksp from these data which of the following assumptions are reasonable?
• Ksp is the same as soly.
• Ksp of AgCl is the same in 6.5x10-3M AgNO3 as in pure water.
• Soly of AgCl is independent of the conc of AgNO3.
• [Ag+] in soln does not change significantly upon the addition of AgCl to 6.5x10-3M AgNO3.
• [Ag+] in soln after the addition of AgCl to 6.5x10-3M AgNO3 is the same as it would be in pure water.