Equilibrium Applications - Solubility
Equilibria In Saturated Solutions of Slightly Soluble Substances
This unit does not deal with substances that dissolve readily in water or other solvents. Anything that dissolves at least 1 g/100 mL of solvent is considered to be soluble. This unit deals with substances that are slightly soluble to those that are considered to be insoluble (unable to dissolve easily in a solvent). There are also some questions related to precipitates (slightly soluble substances) that are of interest to us.
 
Some of these questions are:
1) How can the approximate solubility of a precipitate be calculated?
2) How can we predict whether or not a precipitate forms when two solutions are mixed?
3) Which reagents or methods can be used to dissolve precipitates?
 
The Solubility Product and Precipitate Formation
Consider a saturated solution of silver chloride.
The equilibrium can be represented by:   AgCl(s)  <=====> Ag+(aq) + Cl-(aq)
 
The equilibrium law expression is:   Ksp = [Ag+][Cl-]
 
The Ksp equation resembles an equilibrium expression of a heterogeneous equation. The Keq value is changed to a Ksp because we are now dealing with solubilities and precipitates. The 'sp' means solubility product constant and you can see that the [Ag+] and [Cl-] are multiplied together to create a product.

The Ksp equation does not contain a term concerning the solid AgCl(s). Once a solution has reached saturation, the addition of more solid will not change the [] of the ions already in solution.

The Ksp value for AgCl is 1.8 x 10-10. This shows that the [Ag+] and [Cl-] are very low in a saturated solution of AgCl. Equilibrium is established before a significant amount of AgCl dissolves.
 
 

Another way of saying this would be that the Ag+ ions and the Cl- ions can exist together at Equilibrium in the same solution only when their concentrations are low enough so that the product of their concentrations does not exceed the Ksp value.
 
eg. Calculate the [Ag+] and [Cl-] in a saturated solution of the salt at 25oC. What is the approximate solubility of AgCl in mol/L at this temperature?
 
The ions Ag+ and Cl- react together on a 1:1 basis therefore any AgCl that does dissolve will product 'x' amount of each of the ions.
 
AgCl(s) <======>  Ag+(aq) + Cl-(aq)
                                   x              x
 
Ksp = [Ag+][Cl-]

1.8 x 10-10 = (x)(x)

1.8 x 10-10 = x2

x = 1.3 x 10-5 mol/L
 

Therefore 1.3 x 10-5 moles of AgCl dissolves in 1 L of H2O. The equilibrium []'s of Ag+ and Cl- are each 1.3 x 10-5 mol/L. Since we are dealing with concentrations we can automatically substitute in the units of concentration, moles/Litre.
 
Follow-up Problems
Determine the solubility of AgI, Ag2CrO4, and Zn2[Fe(CN)6] in
a) moles/L and b) g/L at 25oC. You will have to refer to the Ksp tables in your databook.