Solubility ProductsConsider the equilibrium that exists in a

saturated solution of BaSO4 in water:

BaSO4(s) Ba2+(aq) + SO42−(aq)

Solubility ProductsThe equilibrium constant expression for this equilibrium is

Ksp = [Ba2+] [SO42−]

where the equilibrium constant, Ksp, is called the solubility product.

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).

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.

BaSO4(s) Ba2+(aq) + SO42−(aq)

PracticeCalculate 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)

PRACTICE EXERCISE

The value for Ksp for manganese(II) hydroxide, Mn(OH)2 , is 1.6 10–13. Calculate the molar solubility of Mn(OH)2 in a solution that contains 0.020 M NaOH.

Answer:  4.0 10–10 M

Factors Affecting Solubility pH

If a substance has a basic anion (CO3

2-), 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.

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

Factors Affecting Solubility

Mg(OH)2(s) Mg2+(aq) + 2OH-

(aq)

[OH-] = 1.0 x 10-5 Insert [OH-] into the solubility productKsp = [Mg2+][OH-]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

Factors Affecting Solubility

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

Factors Affecting Solubility Other salts with basic anions, such as

CO32-, PO4

3-, 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

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)?

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.

SAMPLE EXERCISE Predicting the Effect of Acid on Solubility

(b) Similarly, CaCO3(s) dissolves in acid solutions because CO3

2– 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.

(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.

SAMPLE EXERCISE continued

(c) The solubility of BaF2 is also enhanced by lowering the pH, because F– is a basic anion.

Factors Affecting SolubilityComplex Ions

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

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

Factors Affecting Solubility

AgClKsp = 1.8 x

10-10

will dissolve in NH3, a Lewis base

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

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

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

Kf = [Ag(NH3)2+]

[Ag+][NH3]2= 1.7 x 107

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+.

Will a Precipitate Form? In a solution,

If Q = Ksp the system is at equilibrium and the solution is saturated.

If Q < Ksp more solid will dissolve until Q = Ksp

If Q > Ksp the salt will precipitate until Q = Ksp

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

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

Selective Precipitation of Ions One can

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