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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

Chapter 18


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


We have problems

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

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


Solution


Problem

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


Solution


Solubility and the Common Ion Effect

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


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


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)


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?


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?


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?


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


Check out the heating pads & video

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


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)


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


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


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


Example

  • What is Knet for the reaction below?


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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