Factors Affecting Solubility

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Factors Affecting Solubility
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Solubility Products

• Consider the equilibrium that exists in a
  saturated solution of BaSO4 in water

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Solubility Products

• The equilibrium constant expression for this
  equilibrium is
• Ksp Ba2 SO42-
• where the equilibrium constant, Ksp, is called
  the solubility product.

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Solubility Products

• Ksp is not the same as solubility.
• Solubility is generally expressed as the mass of
  solute dissolved in 1 L (g/L) or 100 mL (g/mL) of
  solution, or in mol/L (M).

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Factors Affecting Solubility

• The Common-Ion Effect
• If one of the ions in a solution equilibrium is
  already dissolved in the solution, the
  equilibrium will shift to the left and the
  solubility of the salt will decrease.

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Practice

• Calculate the molar solubility of CaF2 at 25oC in
  a solution that is a) 0.010 M Ca(NO3)2, b) 0.010
  M NaF
• Ksp 3.9x 10-11 (from Appendix D)

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PRACTICE EXERCISE The value for Ksp for
manganese(II) hydroxide, Mn(OH)2 , is 1.6 ?
1013. Calculate the molar solubility of Mn(OH)2
in a solution that contains 0.020 M NaOH.
Answer  4.0 ? 1010 M
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Factors Affecting Solubility

• pH
• If a substance has a basic anion (CO32-), PO43-,
  CN-, S2-), it will be more soluble in an acidic
  solution.
• Substances with acidic cations (like metal
  hydroxides) are more soluble in basic solutions.

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Factors Affecting Solubility

• Mg(OH)2(s) Mg2(aq) 2OH-(aq)
• Ksp 1.8 x 10-11 sat soltn pH 10.52
• Mg2 1.7 x 10-4 M
• Suppose solid Mg(OH)2 is equilibriated with a
  solution buffered at pH 9.0
• The pOH is 5.0,so OH- 1.0 x 10-5

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Factors Affecting Solubility

• Mg(OH)2(s) Mg2(aq) 2OH-(aq)
• OH- 1.0 x 10-5
• Insert OH- into the solubility product
• Ksp Mg2OH-2 1.8 x 10-11
• Mg2(1.0 x 10-5)2 1.8 x 10-11
• Mg2 0.18 M
• Mg(OH)2 will dissolve until Mg2 0.18M

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Factors Affecting Solubility

• The solubility of almost any ionic compound is
  affected by a sufficiently acidic or basic
  solution
• The affects are very noticeable when one or both
  of the ions are at least moderately acidic or
  basic
• Metal hydroxides, like Mg(OH)2, are prime examples

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Factors Affecting Solubility

• Other salts with basic anions, such as CO32-,
  PO43-, CN-, and S2- behave similarly
• The solubulity of slightly soluble salts
  containing basic ions increases as H increases
  and pH is lowered.
• Salts with anions negligible basicity (anions of
  strong acids) are unaffected by pH

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SAMPLE EXERCISE Predicting the Effect of Acid on
Solubility
Which of the following substances will be more
soluble in acidic solution than in basic
solution (a) Ni(OH)2(s), (b) CaCO3(s), (c)
BaF2(s), (d) AgCl(s)?
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SAMPLE EXERCISE Predicting the Effect of Acid on
Solubility
Solve (a)  Ni(OH)2(s) will be more soluble in
acidic solution because of the basicity of OH
the H ion reacts with the OH ion, forming water.
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SAMPLE EXERCISE Predicting the Effect of Acid on
Solubility
(b) Similarly, CaCO3(s) dissolves in acid
solutions because CO32 is a basic anion.
The reaction between CO32 and H occurs in a
stepwise fashion, first forming HCO3. H2CO3
forms in appreciable amounts only when the
concentration of H is sufficiently high.
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SAMPLE EXERCISE continued
(c) The solubility of BaF2 is also enhanced by
lowering the pH, because F is a basic anion.
(d) The solubility of AgCl is unaffected by
changes in pH because Cl is the anion of a
strong acid and therefore has negligible basicity.
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Factors Affecting Solubility

• Complex Ions
• Metal ions can act as Lewis acids (e- pair
  acceptors) and form complex ions with Lewis bases
  (e- pair donors) in the solvent.

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Factors Affecting Solubility

• Lewis bases, other than water, can also interact
  with metal ions, particularly transition metals.
• The formation of complex ions can affect the
  solubility of a metal salt

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Factors Affecting Solubility

• AgCl
• Ksp 1.8 x 10-10
• will dissolve in NH3, a Lewis base

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Factors Affecting Solubility

• AgCl(s) Ag (aq) Cl- (aq)
• Ag(aq) 2NH3(aq) Ag(NH3)2(aq)
• AgCl(s) 2NH3(aq) Ag(NH3)2(aq) Cl-
  (aq)
• NH3 causes AgCl(s) to dissolve, moving the rxn to
  the right

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Factors Affecting Solubility

• For a Lewis base, like NH3, to increase
  solubility of a metal salt, it must interact
  greater with the metal ion than water does
• Assembly of a metal-ion Lewis base is called a
  complex ion

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Factors Affecting Solubility

• The stability of a complex ion in an aqueous
  solution is determined by the size of the
  equilibrium constant for its formation from the
  hydrated metal ion

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Factors Affecting Solubility

• Amphoterism
• Amphoteric metal oxides and hydroxides are
  soluble in strong acid or base, because they can
  act either as acids or bases.
• Examples of such cations are Al3, Zn2, and Sn2.

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Will a Precipitate Form?

• In a solution,
• If Q Ksp the system is at equilibrium and the
  solution is saturated.
• If Q lt Ksp more solid will dissolve until Q Ksp
• If Q gt Ksp the salt will precipitate until Q Ksp

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SAMPLE EXERCISE 17.15 Predicting whether a
Precipitate will Form
Will a precipitate form when 0.10 L of 8.0 x 10-3
M Pb(NO3)2 is added to 0.40 L of 5.0 x 10-3 M
Na2SO4? The Ksp for PbSO4 6.3 x 10-7
Q 6.4 x 10-6 it will precipitate
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PRACTICE EXERCISE Will a precipitate form when
0.050 L of 2.0 x10-2 M NaF is mixed with 0.010 L
of 1.0 x 10-2 M Ca(NO3)2? The Ksp of CaF2 is 3.9
x 10-11
Q 4.6 x 10-7 it will precipitate
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Selective Precipitation of Ions

• One can use differences in solubilities of salts
  to separate ions in a mixture.