The Foundations of Chemistry

The molar solubility of silver chromate is calculated first.

1.31 10 ^4 mol/L

(dissolved)

The equation for dissolution of Ag 2 CrO 4 and its molar solubility give the concentrations of
Ag
and CrO 42 ions in the saturated solution.

Ag 2 CrO 4 (s) 34 2Ag
(aq) 
 CrO 42 (aq)

1.31 10 ^4 mol/L:::F2.62 10 ^4 M 1.31 10 ^4 M

(dissolved)

Substitution into the Kspexpression for Ag 2 CrO 4 gives the value of Ksp.

Ksp[Ag
]^2 [CrO 42 ](2.62 10 ^4 )^2 (1.31 10 ^4 ) 8.99 10 ^12

The calculated Kspof Ag 2 CrO 4 is 8.99 10 ^12.

You should now work Exercise 8.

The molar solubility and Kspvalues for BaSO 4 and Ag 2 CrO 4 are compared in Table
20-1. These data show that the molar solubility of Ag 2 CrO 4 is greater than that of BaSO 4.
The Kspfor Ag 2 CrO 4 , however, is less than the Kspfor BaSO 4 because the expression for
Ag 2 CrO 4 contains a squaredterm, [Ag
]^2.
If we compare Kspvalues for two 1
1 compounds, for example, AgCl and BaSO 4 , the
compound with the larger Kspvalue has the higher molar solubility. The same is true for
any two compounds that have the same ion ratio, for example, the 1
2 compounds CaF 2
and Mg(OH) 2 and the 2
1 compound Ag 2 CO 3.

If two compounds have the same ion ratio,the one with the larger Kspwill have the

higher molarsolubility.

Appendix H lists some Kspvalues. Refer to it as needed.

1 mol Ag 2 CrO 4



332 g Ag 2 CrO 4

0.0435 g Ag 2 CrO 4



1.00 L

__? mol Ag 2 CrO 4



L

1.31 10 ^4 mole of solid Ag 2 CrO 4

dissolves to give a liter of saturated

solution.

20-2 Determination of Solubility Product Constants 827

Problem-Solving Tip:It’s Important to Know the Difference Between

Solubility, Molar Solubility, and Solubility

Product Expression

Often it is necessary that you understand which of these is given or which is asked for.

Recognition depends on a clear understanding of their definitions.

• The solubility of a compound is the amount of the compound that dissolves in a spec-
  ified volume of solution. Solubility is usually expressed as either grams per liter or
  grams per 100. mL.


• The molar solubility of a compound is the number of moles that dissolve to give one
  liter of saturated solution.


• The solubility product expression for a compound is the product of the concentra-
  tions of its constituent ions, each raised to the power that corresponds to the number
  of ions in one formula unit of the compound.

Values of solubility product constants

are usually tabulated to only two

significant figures.

Ksp(AgCl)1.8 10 ^10

Ksp(BaSO 4 )1.1 10 ^10

The molar solubility of AgCl is only

slightly higher than that of BaSO 4.