Solubility and Complex Ion Equilibria

The Solubility Product Constant (Ksp) is the equilibrium constant for the solubility

equilibrium of a slightly soluble (or nearly insoluble) ionic compound.

CaC2O4(s) H2O

Ca+2

(aq) + C2O4-2

(aq)

Ksp = [Ca+2

] [C2O4-2

]

Like other equilibrium constants, Ksp depends on the temperature.

Examples:

A liter of a solution saturated at 25ºC with calcium oxalate, CaC2O4, is evaporated to dryness,

giving a 0.0061 g residue of CaC2O4. Calculate the solubility product constant for the salt at

25ºC.

By experiment it is found that 1.2 x 10-3

mol of lead(II) iodide, PbI2, dissolves in 1 L of aqueous

solution at 25ºC. What is the solubility product constant at this temperature?

The mineral fluorite is calcium fluorite, CaF2. Calculate the solubility (in grams per liter) of

calcium fluorite in water from the solubility product constant (3.4 x 10-11

).

In the previous examples the hydrolysis of an anion is not important. If the anion is

fairly basic, you need to take in to consideration the possibility of hydrolysis.

The importance of the Ksp becomes apparent when you consider the solubility of

one salt in the solution with another salt having the same cation or anion.

Example:

What is the solubility of calcium oxalate in 0.15 M calcium chloride? Compare this molar

solubility with that found in the previous example for CaC2O4 in pure water.

CaC2O4(s) H2O

Ca+2

(aq) + C2O4-2

(aq)

Precipitation Calculations

Precipitation is another way of looking at a solubility equilibrium. Rather than ask

how much of a substance will dissolve in solution, you ask: Will precipitation

occur for given starting ion concentrations?

Will the reaction go in the forward or reverse direction?

To answer this you need to evaluate the reaction quotient, Qc and compare it to the

Kc.

If Qc < Ksp, the reaction should go in the forward direction

If Qc >Ksp, the reaction should go in the reverse direction

If Qc = Ksp, the reaction mixture is at equilibrium

Here the Qc is considered the Ion product because it is the product of the ion

concentration in a solution, each concentration raised to a power equal to the

number of ions in the formula of the ionic compound.

Example:

The concentration of calcium ion in blood plasma is 0.0025 M. If the concentration of oxalate

ion is 1.0 x 10-7

M, do you expect calcium oxalate to precipitate?

A solution of 0.00016 M lead (II nitrate, Pb(NO3)2 was poured into 456 mL of 0.00023 M

sodium sulfate, Na2SO4. Would a precipitate of lead(II) sulfate be expected to form if 255 mL of

the lead nitrate solution were added?

Fractional Precipitation is the technique of separating two or more ions from a

solution by adding a reactant that precipitates first ion, then another ion and so

forth. Suppose a solution is 0.10 M Ba+2

and 0.10 M Sr+2

. When you slowly add a

concentrated solution of K2CrO4 to the solution the BaCrO4 precipitates out first.

To understand why you need to:

1. Calculate the concentration of CrO4-2

to begin to precipitate out the Ba2+

2. Calculate the concentration of CrO4-2

to begin to precipitate out the Sr2+

Effect of pH on Solubility

CaC2O4(s) H2O

Ca+2

(aq) + C2O4-2

(aq)

C2O4-2

(aq) + H+

(aq) HC2O4-(aq) + H2O(l)

In general:

Salts of weak acids should be expected to be more soluble in acidic solutions

Consider two slightly soluble salts, calcium carbonate and calcium sulfate. which

of these would have its solubility ore effected by the addition of HCl? Would the

solubility of the salt increase or decrease?

Many metal sulfides are insoluble in water, but dissolve in acidic solutions.

Complex Ion Formation, or stability constant, Kf, of a complex ion is the

equilibrium constant for the formation of the complex ion from the aqueous metal

ion and the ligands.

Ag+

(aq) + NH3(aq) Ag(NH)3+

(aq)

Ag(NH)3+

(aq) + NH3(aq) Ag(NH3)2+

(aq)

The dissociation constant, Kd, for a complex ion is the reciprocal, or inverse value

of Kf

Kd = �

�� =

What is the concentration of Ag+

(aq) ion in 0.010 M AgNO3, that is also 1.00 M

NH3?

Kf =

[Ag+]

[NH3]

2

[Ag(NH3)2+]

[Ag(NH3)2+]

[Ag+] [NH3]

2

Amphoteric hydroxides are metal hydroxides that react with both bases and acids.

Example: Zn is a metal that forms insoluble hydroxides

Zinc hydroxide reacts with an acid:

Zn(OH)2(s) + 2H3O+

(aq) Zn+2

(aq) + 4H2O(l)

But also reacts with a base:

Zn(OH)2(s) + OH-(aq) Zn(OH)4

2-(aq)

Complex ions and Solubility

Predicting whether a precipitate will form in the presence of a complex ion

Will silver chloride precipitate from a solution that is 0.010M AgNO3 and 0.010M

NaCl?

Will silver chloride precipitate from this solution if it is also 1.00 M NH3?

Qualitative Analysis is the determination of the identity of the substances present

in a mixture.