Solubility Solubility EquilibriaEquilibria

Will it all dissolve, and if not, how much will?

SOLUBILITY EQUILIBRIASOLUBILITY EQUILIBRIA• Solubility: Relative term used to describe how

much of a particular substance dissolves in a

certain amount of solvent.

• Substances that dissolve very well are said to

be soluble

• Insoluble species don’t dissolve well.

• All substances are “soluble” to some extent

• We will look at slightly soluble substances

• All dissolving is an equilibrium.• If there is not much solid it will

all dissolve.• As more solid is added the

solution will become saturated.• Solid ↔ dissolved• The solid will precipitate as fast

as it dissolves, forming an equilibrium.

SOLUBILITY EQUILIBRIASOLUBILITY EQUILIBRIA

Watch out• Solubility is not the same

as solubility product.• Solubility product is an

equilibrium constant.• It doesn’t change except

with temperature.• Solubility is an

equilibrium position for how much can dissolve.

• A common ion can change this.

KKspsp Values for Some Salts at Values for Some Salts at 2525CC

Name Formula Ksp

 Barium carbonate   BaCO3   2.6 x 10-9 

 Barium chromate   BaCrO4   1.2 x 10-10 

 Barium sulfate   BaSO4   1.1 x 10-10 

 Calcium carbonate   CaCO3   5.0 x 10-9 

 Calcium oxalate   CaC2O4   2.3 x 10-9 

 Calcium sulfate   CaSO4   7.1 x 10-5 

 Copper(I) iodide   CuI   1.3 x 10-12 

 Copper(II) iodate   Cu(IO3)2   6.9 x 10-8 

 Copper(II) sulfide   CuS   6.0 x 10-37 

 Iron(II) hydroxide   Fe(OH)2   4.9 x 10-17 

 Iron(II) sulfide   FeS   6.0 x 10-19 

 Iron(III) hydroxide   Fe(OH)3   2.6 x 10-39 

 Lead(II) bromide   PbBr2   6.6 x 10-6 

 Lead(II) chloride   PbCl2   1.2 x 10-5 

 Lead(II) iodate   Pb(IO3)2   3.7 x 10-13 

Name Formula Ksp

 Lead(II) iodide   PbI2   8.5 x 10-9 

 Lead(II) sulfate   PbSO4   1.8 x 10-8 

 Magnesium hydroxide   Mg(OH)2   5.6 x 10-12 

 Silver bromate   AgBrO3   5.3 x 10-5 

 Silver bromide   AgBr   5.4 x 10-13 

 Silver carbonate   Ag2CO3   8.5 x 10-12 

 Silver chloride   AgCl   1.8 x 10-10 

 Silver chromate   Ag2CrO4   1.1 x 10-12 

 Silver iodate   AgIO3   3.2 x 10-8 

 Silver iodide   AgI   8.5 x 10-17 

 Strontium carbonate   SrCO3   5.6 x 10-10 

 Strontium fluoride   SrF2   4.3 x 10-9 

 Strontium sulfate   SrSO4   3.4 x 10-7 

 Zinc sulfide   ZnS   2.0 x 10-25 

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 (next page)

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 @ 25C Ksp AgI = 1.5 x 10-16

Solving Solubility ProblemsSolving Solubility Problems

For the salt AgI at 25C, Ksp = 1.5 x 10-16

AgI(s) Ag+(aq) + I-(aq)

I

C

E

OO

+x +x

x x

1.5 x 10-16 = x2

x = solubility of AgI in mol/L = 1.2 x 10-8 M

Solving Solubility ProblemsSolving Solubility Problems

For the salt PbCl2 at 25C, Ksp = 1.6 x 10-

5 PbCl2(s) Pb2+(aq) + 2Cl-(aq)

I

C

E

OO

+x +2x

x 2x

1.6 x 10-5 = (x)(2x)2 = 4x3

x = solubility of PbCl2 in mol/L = 1.6 x 10-2 M

Try Problem 81Try Problem 81

Relative Solubilities• Ksp will only allow us to compare the

solubility of solids the that fall apart into the same number of ions.

• The bigger the Ksp of those the more soluble.

• If they fall apart into different number of pieces you have to do the math.

• Which one is most soluble?

Name Formula Ksp

 Iron(II) hydroxide   Fe(OH)2   4.9 x 10-17

 Iron(II) sulfide   FeS   6.0 x 10-14 

 Iron(III) hydroxide   Fe(OH)3   2.6 x 10-39 

Try Problem 86Try Problem 86

The Common Ion Effect

• When the salt with the anion of a weak acid is added to that acid:– it reverses the dissociation of the acid.– lowers the percent dissociation of the

acid.

• The same principle applies to salts with the cation of a weak base..

• The calculations are the same as with acid base equilibrium.

Solving Solubility with a Common Solving Solubility with a Common IonIon

For the salt AgI at 25C, Ksp = 1.5 x 10-16

What is its solubility in 0.05 M NaI?

AgI(s) Ag+(aq) + I-(aq)

I

C

E

0.05O

+x +x

x 0.05+x

1.5 x 10-16 = (x)(0.05+x) (x)(0.05)

x = solubility of AgI in mol/L = 3.0 x 10-15 M

Try Problem 93Try Problem 93

pH and solubility

• OH- can be a common ion.• More soluble in acid.• For other anions if they come from a

weak acid they are more soluble in a acidic solution than in water.

• CaC2O4 ↔ Ca+2 + C2O4-2

• H+ + C2O4-2 ↔ HC2O4

-

• Reduces C2O4-2 in acidic solution.

Precipitation• The reaction quotient (called ion

product) may be applied to solubility equilibria - determines if a substance will precipitate from solution

• Ion Product, Q =[M+]a[Nm-]b • If Ksp<Q a precipitate forms, reverse

process occurs • If Ksp=Q equilibrium solution is just

saturated• If Ksp>Q No precipitate, forward process

occurs

Precipitation Example• A solution of 75.0 mL of 0.020 M BaCl2 is

added to 125.0 mL of 0.040 M Na2SO4. Will a precipitate form? (Ksp= 1.5 x 10-9M BaSO4)

BaSO4 could form if Ksp<Q.

For Q you need initial concentrations:[Ba2+] = mmol Ba2+ / total mL = (0.0750L)(0.020 M)/(0.0750L + 0.125L) =

0.0075 M[SO4

2-] = mmol SO42- / total mL

= (0.1250L)(0.040 M)/(0.0750L + 0.125L) = 0.025 M

Q = [Ba2+] [SO42-] = (0.0075 M)(0.025 M) = 1.9 x 10-4

Ksp<Q so BaSO4 will form.

To figure out concentrations set up an ice table.

Try Problem 98Try Problem 98