Salt Solubility

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# Salt Solubility - PowerPoint PPT Presentation

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

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