1 Salt Solubility Chapter 18

2 Solubility product constant K sp K sp Unitless Unitless CaF 2(s)  Ca 2+ (aq) + 2F - (aq) CaF 2(s)  Ca 2+ (aq) + 2F - (aq) K sp = [Ca 2+ ][F - ] 2 K sp = [Ca 2+ ][F - ] 2 [Ca 2+ ] = molar solubility (in M) [Ca 2+ ] = molar solubility (in M) Get K sp values from Appendix J, pages A Get K sp values from Appendix J, pages A

3 We have problems K sp for BaSO 4 = 1.1 x 10 25°C K sp for BaSO 4 = 1.1 x 10 25°C –Calculate this salt’s solubility in water in g/L.

4 Solution

5 Problem If [Ca 2+ ] = 2.4 x M, what is the K sp value of CaF 2 ? If [Ca 2+ ] = 2.4 x M, what is the K sp value of CaF 2 ?

6 Solution

7 Solubility and the Common Ion Effect Problem: How much AgCl (g/L) would dissolve in water given K sp = 1.8 x ? Problem: How much AgCl (g/L) would dissolve in water given K sp = 1.8 x ?

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9 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? 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? 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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11 Basic anions and salt solubility Do we recall what a conjugate acid/conjugate base is? 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 K sp If salt has conjugate base of weak acid, then salt more soluble than value given by K sp –Why? PbS (s)  Pb 2+ (aq) + S 2- (aq) S 2- (aq) + H 2 O (l)  HS - (aq) + OH - (aq) ; Where net = K sp  K b (>K sp )

12 Effect of pH on solubility Mg(OH) 2 (s)  Mg 2+ (aq) + 2OH - (aq) Mg(OH) 2 (s)  Mg 2+ (aq) + 2OH - (aq) If one adds base, OH -, which way will the equilibrium shift? If one adds base, OH -, which way will the equilibrium shift? –Will more solid form or dissolve? If one adds acid, H 3 O +, which way will the equilibrium shift? If one adds acid, H 3 O +, which way will the equilibrium shift? –Will more solid form or dissolve?

13 Effect of pH on solubility Mg 3 (PO 4 ) 2 (s)  3Mg 2+ (aq) + 2PO 4 -3 (aq) Mg 3 (PO 4 ) 2 (s)  3Mg 2+ (aq) + 2PO 4 -3 (aq) If one adds base, OH -, which way will the equilibrium shift? If one adds base, OH -, which way will the equilibrium shift? –Will more solid form or dissolve? If one adds acid, H 3 O +, which way will the equilibrium shift? If one adds acid, H 3 O +, which way will the equilibrium shift? –Will more solid form or dissolve?

14 Effect of pH on solubility MgCl 2 (s)  Mg 2+ (aq) + 2Cl - (aq) MgCl 2 (s)  Mg 2+ (aq) + 2Cl - (aq) If one adds base, OH -, which way will the equilibrium shift? If one adds base, OH -, which way will the equilibrium shift? –Will more solid form or dissolve? If one adds acid, H 3 O +, which way will the equilibrium shift? If one adds acid, H 3 O +, which way will the equilibrium shift? –Will more solid form or dissolve?

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

16 The reaction quotient, Q Q tells us: Q tells us: –Whether it’s at equilibrium If not, which way it’ll shift If not, which way it’ll shift CaF 2(s)  Ca 2+ (aq) + 2F - (aq) CaF 2(s)  Ca 2+ (aq) + 2F - (aq) K sp = [Ca 2+ ][F - ] 2 K sp = [Ca 2+ ][F - ] 2 Q = [Ca 2+ ][F - ] 2 Q = [Ca 2+ ][F - ] 2 If Q = K sp, eq. & soln is saturated If Q = K sp, eq. & soln is saturated If Q < K sp, then will shift to right (dissolve more) & soln is unsaturated If Q < K sp, then will shift to right (dissolve more) & soln is unsaturated If Q > K sp, then will shift to left (not dissolve anymore, ppt out) & soln is supersaturated (will ppt out) If Q > K sp, then will shift to left (not dissolve anymore, ppt out) & soln is supersaturated (will ppt out)

17 Check out the heating pads & video tions/Gen_Chem_Pages/11solutionspage/c rystallization_from_super.htm tions/Gen_Chem_Pages/11solutionspage/c rystallization_from_super.htm tions/Gen_Chem_Pages/11solutionspage/c rystallization_from_super.htm tions/Gen_Chem_Pages/11solutionspage/c rystallization_from_super.htm

18 Problem AgCl placed in water. After a certain amount of time, concentration= 1.2 x M. AgCl placed in water. After a certain amount of time, concentration= 1.2 x M. Has the system reached eq.? Has the system reached eq.? If not, will more solid dissolve? If not, will more solid dissolve? How much more? How much more? (K sp = 1.8 x ) (K sp = 1.8 x )

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20 Selective precipitation Will a precipitate form, given the ion concentrations? Will a precipitate form, given the ion concentrations? What concentrations of ions are needed to bring about precipitation? What concentrations of ions are needed to bring about precipitation? Basically, compare Q to K sp Basically, compare Q to K sp

21 Problem If [Mg 2+ ] = 1.5x10 -6 M, and enough hydroxide ions are added to make the solution 1.0 x M of hydroxide ions, will Mg(OH) 2 precipitate? If [Mg 2+ ] = 1.5x10 -6 M, and enough hydroxide ions are added to make the solution 1.0 x 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 M? If not, will it occur if the concentration of hydroxide ions is increased to 1.0 x M? K sp = 5.6 x K sp = 5.6 x

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23 Complex ion equilibria Complex ion = a central metal ion (e - acceptor = Lewis acid) bound to 1 or more ligands 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 Ligand = neutral molecule or ion that acts as Lewis base (e - donor) with the central metal ion K f values on page A-26, Appendix K K f values on page A-26, Appendix K

24 Example What is K net for the reaction below? What is K net for the reaction below?

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26 Problem What is the net rxn and value of K net for dissolving AgBr in a solution containing S 2 O 3 2- ? What is the net rxn and value of K net for dissolving AgBr in a solution containing S 2 O 3 2- ? K sp = 5.4 x K sp = 5.4 x K f = 2.0 x K f = 2.0 x 10 13

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