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.