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Salt Solubility. Chapter 18. Solubility product constant. K sp Unitless CaF 2(s)  Ca 2+ (aq) + 2F - (aq) K sp = [Ca 2+ ][F - ] 2 [Ca 2+ ] = molar solubility (in M) Get K sp values from Appendix J, pages A-24-25. We have problems. K sp for BaSO 4 = 1.1 x 10 -10 @ 25 °C

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Salt Solubility

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Salt solubility l.jpg

Salt Solubility

Chapter 18


Solubility product constant l.jpg

Solubility product constant

  • Ksp

  • Unitless

  • CaF2(s) Ca2+(aq) + 2F-(aq)

  • Ksp = [Ca2+][F-]2

  • [Ca2+] = molar solubility (in M)

  • Get Ksp values from Appendix J, pages A-24-25


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We have problems

  • Ksp for BaSO4 = 1.1 x 10-10 @ 25°C

    • Calculate this salt’s solubility in water in g/L.


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Solution


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Problem

  • If [Ca2+] = 2.4 x 10-4 M, what is the Ksp value of CaF2?


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Solution


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Solubility and the Common Ion Effect

  • Problem: How much AgCl (g/L) would dissolve in water given Ksp= 1.8 x 10-10?


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Solubility and the Common Ion Effect

  • If solid AgCl is placed in 1.00L of 0.55M NaCl, what mass of AgCl (g/L) would dissolve?

  • Before solving this, would you expect it to be lesser or greater than in pure water?

    • Think back to Le Châtelier’s Principle and the pH of buffers


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Basic anions and salt solubility

  • Do we recall what a conjugate acid/conjugate base is?

  • If salt has conjugate base of weak acid, then salt more soluble than value given by Ksp

    • Why?

      PbS(s) Pb2+(aq) + S2-(aq)

      S2-(aq) + H2O(l) HS-(aq) + OH-(aq);

      Where net = Ksp  Kb (>Ksp)


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Effect of pH on solubility

  • Mg(OH)2 (s) Mg2+(aq) + 2OH-(aq)

  • If one adds base, OH-, which way will the equilibrium shift?

    • Will more solid form or dissolve?

  • If one adds acid, H3O+, which way will the equilibrium shift?

    • Will more solid form or dissolve?


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Effect of pH on solubility

  • Mg3(PO4)2 (s) 3Mg2+(aq) + 2PO4-3(aq)

  • If one adds base, OH-, which way will the equilibrium shift?

    • Will more solid form or dissolve?

  • If one adds acid, H3O+, which way will the equilibrium shift?

    • Will more solid form or dissolve?


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Effect of pH on solubility

  • MgCl2 (s) Mg2+(aq) + 2Cl-(aq)

  • If one adds base, OH-, which way will the equilibrium shift?

    • Will more solid form or dissolve?

  • If one adds acid, H3O+, which way will the equilibrium shift?

    • Will more solid form or dissolve?


So what can we say about the conjugate base and solubility l.jpg

So what can we say about the conjugate base and solubility?


The reaction quotient q l.jpg

Q tells us:

Whether it’s at equilibrium

If not, which way it’ll shift

CaF2(s) Ca2+(aq)+ 2F-(aq)

Ksp = [Ca2+][F-]2

Q = [Ca2+][F-]2

If Q = Ksp, then @ eq.& soln is saturated

If Q < Ksp, then will shift to right (dissolve more) & soln is unsaturated

If Q > Ksp, then will shift to left (not dissolve anymore, ppt out) & soln is supersaturated (will ppt out)

The reaction quotient, Q


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Check out the heating pads & video

  • http://genchem.chem.wisc.edu/demonstrations/Gen_Chem_Pages/11solutionspage/crystallization_from_super.htm


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Problem

  • AgCl placed in water. After a certain amount of time, concentration= 1.2 x 10-5 M.

  • Has the system reached eq.?

  • If not, will more solid dissolve?

  • How much more?

  • (Ksp = 1.8 x 10-10)


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Selective precipitation

  • Will a precipitate form, given the ion concentrations?

  • What concentrations of ions are needed to bring about precipitation?

  • Basically, compare Q to Ksp


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Problem

  • If [Mg2+] = 1.5x10-6 M, and enough hydroxide ions are added to make the solution 1.0 x 10-4 M of hydroxide ions, will Mg(OH)2 precipitate?

  • If not, will it occur if the concentration of hydroxide ions is increased to 1.0 x 10-2 M?

  • Ksp = 5.6 x 10-12


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Complex ion equilibria

  • Complex ion = a central metal ion (e- acceptor = Lewis acid) bound to 1 or more ligands

  • Ligand = neutral molecule or ion that acts as Lewis base (e- donor) with the central metal ion

  • Kf values on page A-26, Appendix K


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Example

  • What is Knet for the reaction below?


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Problem

  • What is the net rxn and value of Knet for dissolving AgBr in a solution containing S2O32-?

  • Ksp = 5.4 x 10-13

  • Kf = 2.0 x 1013


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