N OTES 17-3 Obj. 17.4, 17.5

17.4 S OLUBILITY P RODUCTS A.) Consider the equilibrium that exists in a saturated solution of BaSO 4 in water: BaSO 4 ( s ) Ba 2+ ( aq ) + SO 4 2− ( aq )

B.) The equilibrium constant expression for this equilibrium is K sp = [Ba 2+ ] [SO 4 2− ] 1.) where the equilibrium constant, K sp, is called the solubility product. 2.) solubility product of a compound equals the product of the concentration of the ions involved in the equilibrium, each raised to the power of its coefficient in the equilibrium equation.

Sample Exercise 17.9 Writing Solubility-Product (K sp ) Expressions Write the expression for the solubility-product constant for CaF 2 Give the solubility-product-constant expressions for the following compounds: (a) barium carbonate, (b) silver sulfate. Practice Exercise

K sp is not the same as solubility. Solubility is generally expressed as the mass of solute dissolved in 1 L (g/L) or 100 mL (g/mL) of solution, or in mol/L ( M ).

Sample Exercise Calculating K sp from Solubility Solid silver chromate is added to pure water at 25 ºC. Some of the solid remains undissolved at the bottom of the flask. The mixture is stirred for several days to ensure that equilibrium is achieved between the undissolved Ag 2 CrO 4 (s) and the solution. Analysis of the equilibrated solution shows that its silver ion concentration is 1.3 × M. Assuming that Ag 2 CrO 4 dissociates completely in water and that there are no other important equilibria involving the Ag + or CrO 4 2– ions in the solution, calculate K sp for this compound.

A saturated solution of Mg(OH) 2 in contact with undissolved solid is prepared at 25 ºC. The pH of the solution is found to be Assuming that Mg(OH) 2 dissociates completely in water and that there are no other simultaneous equilibria involving the Mg 2+ or OH – ions in the solution, calculate K sp for this compound. Practice Exercise

Sample Exercise Calculating Solubility from K sp The K sp for CaF 2 is 3.9 × at 25 ºC. Assuming that CaF 2 dissociates completely upon dissolving and that there are no other important equilibria affecting its solubility, calculate the solubility of CaF 2 in grams per liter.

Sample Exercise Calculating Solubility from K sp The K sp for LaF 3 is 2 × What is the solubility of LaF 3 in water in moles per liter? Practice Exercise

© 2009, Prentice-Hall, Inc F ACTORS A FFECTING S OLUBILITY A.) The Common-Ion Effect 1.) If one of the ions in a solution equilibrium is already dissolved in the solution, the equilibrium will shift to the left and the solubility of the salt will decrease. BaSO 4 ( s ) Ba 2+ ( aq ) + SO 4 2− ( aq )

Sample Exercise Calculating the Effect of a Common Ion on Solubility Calculate the molar solubility of CaF 2 at 25 °C in a solution that is (a) M in Ca(NO 3 ) 2, (b) M in NaF. (K sp of CaF 2 = 3.9 X )

Sample Exercise Calculating the Effect of a Common Ion on Solubility The value for K sp for manganese (II) hydroxide, Mn(OH) 2, is 1.6 × Calculate the molar solubility of Mn(OH) 2 in a solution that contains M NaOH. Practice Exercise

B.) pH 1.) If a substance has a basic anion, it will be more soluble in an acidic solution. 2.) Substances with acidic cations are more soluble in basic solutions.

Sample Exercise Predicting the Effect of Acid on Solubility Which of the following substances will be more soluble in acidic solution than in basic solution: (a) Ni(OH) 2 (s), (b) CaCO 3 (s), (c) BaF 2 (s), (d) AgCl(s)?

Sample Exercise Predicting the Effect of Acid on Solubility Write the net ionic equation for the reaction of the following copper(II) compounds with acid: (a) CuS, (b) Cu(N 3 ) 2. Practice Exercise

C.) Complex Ions 1.) Metal ions can act as Lewis acids and form complex ions with Lewis bases in the solvent.

Sample Exercise Evaluating an Equilibrium Involving a Complex Ion Calculate the concentration of Ag + present in solution at equilibrium when concentrated ammonia is added to a M solution of AgNO 3 to give an equilibrium concentration of [NH 3 ] = 0.20 M. Neglect the small volume change that occurs when NH 3 is added.

Sample Exercise Evaluating an Equilibrium Involving a Complex Ion Calculate [Cr 3+ ] in equilibrium with Cr(OH) 4 - when mol of Cr(NO 3 ) 3 is dissolved in a liter of solution buffered at pH Practice Exercise

2.) The formation of these complex ions increases the solubility of these salts.

D.) Amphoterism 1.) Amphoteric metal oxides and hydroxides are soluble in strong acid or base, because they can act either as acids or bases. 2.) Examples of such cations are Al 3+, Zn 2+, and Sn 2+.