The Foundations of Chemistry

OUTLINE

20-1 Solubility Product Constants

20-2 Determination of Solubility

Product Constants

20-3 Uses of Solubility Product

Constants

20-4 Fractional Precipitation

20-5 Simultaneous Equilibria

Involving Slightly Soluble

Compounds

20-6 Dissolving Precipitates

OBJECTIVES

After you have studied this chapter, you should be able to

• Write solubility product constant expressions


• Explain how Ksp’s are determined


• Use Ksp’s in chemical calculations


• Recognize some common, slightly soluble compounds


• Describe fractional precipitation and how it can be used to separate ions


• Explain how simultaneous equilibria can be used to control solubility


• Describe some methods for dissolving precipitates

SS

o far we have discussed mainly compounds that are quite soluble in water. Although

most compounds dissolve in water to some extent, many are so slightly soluble that

they are called “insoluble compounds.” We shall now consider those that are only

very slightly soluble. As a rough rule of thumb, compounds that dissolve in water to the

extent of 0.020 mole/liter or more are classified as soluble. Refer to the solubility guide-

lines (Table 4-8) as necessary.

Slightly soluble compounds are important in many natural phenomena. Our bones and

teeth are mostly calcium phosphate, Ca 3 (PO 4 ) 2 , a slightly soluble compound. Also, many

natural deposits of Ca 3 (PO 4 ) 2 rock are mined and converted into agricultural fertilizer.

Limestone caves have been formed by acidic water slowly dissolving away calcium

carbonate, CaCO 3. Sinkholes are created when acidic water dissolves away most of the

underlying CaCO 3. The remaining limestone can no longer support the weight above it,

so it collapses, and a sinkhole is formed.

SOLUBILITY PRODUCT CONSTANTS

Suppose we add one gram of solid barium sulfate, BaSO 4 , to 1.0 liter of water at 25°C

and stir until the solution is saturated.Very little BaSO 4 dissolves. Careful measurements

of conductivity show that one liter of a saturated solution of barium sulfate contains only

0.0025 gram of BaSO 4 , no matter how much more BaSO 4 is added. The BaSO 4 that does

dissolve is completely dissociated into its constituent ions.

20-1

In a process known as biomineral-

ization, some organisms produce

solids that also occur naturally as

minerals. The calcium carbonate,

CaCO 3 , in an eggshell has the same

crystal structure as in the mineral

calcite.

Pouring ammonium sulfide solution

into a solution of cadmium nitrate

gives a precipitate of cadmium sulfide.

(NH 4 ) 2 S
Cd(NO 3 ) 2 88nCdS(s)


2NH 4 NO 3

Cadmium sulfide is used as a pigment

in artists’ oil-based paints.