Solubility

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Solubility Simultaneous EquilibriaPart II
Effect of pH, Complex Ion Formation Selective
Precipitation

• C. Yau
• Spring 2009

Based on Brady Senese 5/eChap. 17 Sec 3
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Effect of pH on Solubility
If the cmpd contains the anion of a weak acid,
addition of H3O (from a strong acid) increases
it solubility. CaCO3 (s) Ca2 (aq)
CO32- (aq)
The anion, CO32-, is from the weak acid H2CO3.
Why would addition of H3O affect the
solubility? CO32- (aq) H3O (aq)
HCO3- (aq) H2O (l) HCO3- (aq) H3O (aq)
H2CO3 (aq) H2O (l)
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• Addition of acid to mineral sample is a field
  test to identify presence of carbonates.
• What would one look for as a positive test for
  carbonates?
• CO32- (aq) H3O (aq) HCO3- (aq)
  H2O (l)
• HCO3- (aq) H3O (aq) H2CO3 (aq)
  H2O (l)

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Effect of Adding Strong Acid

• Write balanced eqns to explain whether addition
  of a strong acid would affect the solubility of
  the following compounds
• a) lead(II) bromide
• b) copper(II) hydroxide
• c) iron(II) sulfide

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Selective Precipitation

• Consider the Ksp of AgCl and Ag2CrO4
• Ksp Solubility
• AgCl 1.8x10-10 1.3x10-5 M
• Ag2CrO4 2.6x10-12 8.7x10-5 M
• If we add Ag to a soln containing
• 0.10 M Cl- and 0.10 M CrO42-, what will happen?
• Ans. Ag2CrO4 is more soluble. AgCl will
  precipitate out first leaving CrO42- in solution.

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• CrO42- and Cl- can be separated by adding Ag.
• Ag will precipitate out the Cl- as AgCl (s).
• The mixture is centrifuged to spin down the white
  AgCl ppt.
• The supernatant soln containing the CrO42- is
  then pipetted out.

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Complex Ion Formation

• Metal ions can combine with anions or neutral
  molecules to form complex ions
• Cu2 4 H2O Cu(H2O)42
• Fe2 6 CN? Fe(CN)64?
• The H2O and CN? in the above reactions are called
  ligands. They are acting as Lewis bases. The
  metal cations are acting as Lewis acids.

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Aqueous Metal Ions Are Complex Ions

• Water acts as a ligand, the Lewis base that forms
  a coordinate covalent bond with the metal.
• In the solvation of ionic compounds, ions are
  dissolved in water through ion-dipole
  interactions
• Complex ions are soluble, hence complexation is a
  means of dissolving some solids.

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Writing Complex Ion Formation Eqns

• Write the equation for the complex ion formation
  of the aluminum ion with 4 hydroxide as ligands.
• Al3 (aq) 4 OH- (aq)
• Write the equation for the complex ion formation
  of the tin(IV) ion with 6 fluoride ions as
  ligands.
• Write the equation for the complex ion formation
  of the mercury(II) ion with 4 chloride ions as
  ligands.

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Complex Ion Formation Increases Solubility of a
Salt

• Consider the dissolution of AgBr(s) and what
  happens when NH3 is added
• AgBr (s) Ag (aq) Br- (aq)
• Ag(NH3)2

AgBr becomes more soluble.
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Complexation Kinst

• Complexes are governed by the instability
  constant, Kinst
• Ag(NH3)2 Ag
  2NH3
• When we reverse an equation, we invert K, thus
  Kform1/Kinst.
• Ag 2NH3 Ag(NH3)2

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• What is the concentration of Cu2 available when
  10.0mL 0.1M Cu2 are combined with 10.0 mL of
  0.01M NH3?
• Kform1.1x1013 for the complex ion shown
• Cu2 (aq) 4 NH3 (aq) Cu(NH3)42 (aq)

Ans. 0.0488 M
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Classical Schemefor Identifying "Group I Cations"

• Solubility
• Hg2Cl2 1.2x10-18 M
• AgCl 1.8x10-10 M
• PbCl2 1.7x10-5 M
• Given a soln possibly containing some combination
  of Hg22, Ag and Pb2 ions, you are to determine
  which one or ones are present.

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SolubilityHg2Cl2 1.2x10-18 MAgCl 1.8x10-10
MPbCl2 1.7x10-5 M (soluble when hot)

• To unknown soln, add excess amt of 6MHCl.
• If a ppt forms, one or more of the cations are
  present.
• Heat the mixture to boiling and remove the
  supernatant. To the supernatant, add K2CrO4. If a
  yellow ppt forms, Pb2 is present.
• To the ppt, add 6 M NH3. If a ppt remains, Hg22
  is present.
• To the supernatant, neutralize the NH3 with HCl.
  If a white ppt forms, Ag is present.