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Solubility & Solubility & SOLUBILITY SOLUBILITY PRODUCT CONSTANTSPRODUCT CONSTANTS
Solubility Rules
All Group 1 (alkali metals) and NH4+
compounds are water soluble.All nitrate, acetate, and chlorate compounds are water soluble.All Br-, Cl-, I- (except mercury (I), Pb+2, and Ag+) compounds are water soluble.All sulfate (except Pb+2, Ba+2, Sr+2; Ag+ and Ca+2 are sparingly soluble) compounds are water soluble.
Solubility Rules
All oxides, hydroxides, phosphates, carbonates, and sulfides form water INSOLUBLE compounds except Group 1 (alkali metals) and ammonium compounds.
Solubility Products
Copper (I) bromide has a solubility of 0.02869 g/L at 25°C. What is the equilibrium constant for this reaction?The equation for the solubility of this compound is written:
CuBr(s) Cu + Br(aq)
(aq)
Solubility Products
The equilibrium constant is written:
Ksp = ][Br ][Cu -
Solubility Products
Convert mass to moles:
0.02869 g ( ) = 2 x 10-4 moles
[Cu+] = 2 x 10-4 moles/L
[Br-] = 2 x 10-4 moles/L
g 143.45mole 1
Solubility Products
Calculate the Ksp
Ksp = (2 x 10-4 moles/L) (2 x 10-4 moles/L)
Ksp = 4 x 10-8
SOLUBILITY PRODUCT CONSTANTSSOLUBILITY PRODUCT CONSTANTS
Consider the following reaction
The equilibrium constant expression is
Ksp = [Pb2+][Cl-]2
Ksp is called the solubility product constant or
simply solubility product
For a compound of general formula, MyXz
PbCl2(s) Pb2+(aq) + 2Cl- (aq)
Ksp = [Mz+]y[Xy-]z
Ksp = [Mg2+][NH4+][PO4
3-]
Ksp = [Zn2+][OH-]2
Ksp = [Ca2+]3[PO43-]2
MyXz(s) yMz+(aq) + zXy- (aq)
Ca3(PO4)2(s) 3Ca2+(aq) + 2PO43- (aq)
Zn(OH)2(s) Zn2+(aq) + 2OH- (aq)
MgNH4PO4(s) Mg2+(aq) + NH4+(aq) + PO4
3- (aq)
Molar solubility: the number of moles that
dissolve to give 1 liter of saturated solution
As with any equilibrium constant the numerical
value must be determined from experiment
The Ksp expression is useful because it applies
to all saturated solutions
- the origins of the ions are not relevant
Consider that Ksp BaSO4 = 1.1 x 10-10
FACTORS AFFECTING SOLUBILITY
The common ion effect
This effect affects solubility equilibria as it
does other ionic equilibria
The solubility of a compound is less in a solution
that contains a common ion than in pure water
Consider again the reaction
Adding Pb2+ or Cl- precipitates PbCl2
PbCl2(s) Pb2+(aq) + 2Cl-(aq)
Can be explained using LeChatelier’s principle
Likewise adding PbCl2 to aqueous NaCl results
in [Pb2+][Cl-]2 > Ksp
So for instance MgF2 is less soluble in 0.10 M
NaF than in pure water by a factor of 35000
pH of the solution
Solubility of some solids in water may be
increased by changing the pH
e.g. Al(OH)3 is more soluble in acidic solutions
than in pure water
Salts that contain basic anions such as CN-, F-,
and PO43- will react in this fashion
Salts that contains anions of strong acids do
not show this reactivity
Al(OH)3(s) Al3+(aq) + 3OH- (aq)
3H+(aq) + 3OH- (aq) 3H2O(l)
Al(OH)3(s) + 3H+(aq) Al3+(aq) + 3H2O(l)
Complex ion formation
Solubility of an ionic compound increases in a
solution that contains a coordinating Lewis
base
Typical examples are NH3, CN-, OH- and Cl-
Cu(OH)2(s) Cu2+(aq) + 2OH- (aq)
Cu2+(aq) + 4NH3(aq) [Cu(NH3)4]2+(aq)
Cu(OH)2(s) + 4NH3(aq)
[Cu(NH3)4]2+(aq) + 2OH- (aq)
Similarly,
The reaction decrease the metal ion
concentration so that [M2+][OH-]2 < Ksp and
the M(OH)2 dissolves
Zn(OH)2(s) + 4NH3(aq)
[Zn(NH3)4]2- (aq) + 2OH- (aq)
THE REACTION QUOTIENTTHE REACTION QUOTIENT
The reaction quotient (called ion product) may
be applied to solubility equilibria - determines
if a substance will precipitate from solution
Qsp < Ksp Forward process occurs
No precipitation occurs
Qsp = Ksp Solution is just saturated
Qsp > Ksp Reverse process occurs
Precipitation occurs
SELECTIVE PRECIPITATION
This is a method which allows separation of
metal ions or anions based on solubility
e.g. Cl-, Br-, and I- can be separated by
selective precipitation with silver ions
Ksp AgCl = 1.8 x 10-10
AgBr = 3.3 x 10-13
AgI = 1.5 x 10-16
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