Practice: What is the molar solubility of Fe(OH) 3 (s) in a solution that is buffered at pH 3.50 at 25°C? The Ksp of Fe(OH)3 is 6.3 x 10–38 at 25°C.
The solubility product constant represents the equilibrium value for an ionic solid and its ions in a given solution.
Solubility Product Constant
Concept #1: The solubility product constant is a measure of how much an ionic solid will dissolve within a solvent.
Example #1: A hypothetical compound MX3 has a molar solubility of 0.00562 M. What is the value of Ksp for MX3?
a) 3.16 x 10-5
b) 2.99 x 10-9
c) 9.48 x 10-5
d) 2.69 x 10-8
Example #2: Which of the following compounds will have the highest molar solubility in pure water?
a) Co(OH)2 Ksp = 1.3 x 10-15
b) Sr3(PO4)2 Ksp = 4.0 x 10-28
c) PbCl2 Ksp = 1.60 x 10-5
d) AgCN Ksp = 5.97 x 10-17
e) PbSO4 Ksp = 1.82 x 10-8
Solubility Product Constant Calculations
Example #3: La(OH)3 has a Ksp of 2.0 x 1021. How many grams of La(OH)3 (MW: 189.93 g/mol) are dissolved as hydroxide ions in 2.5 liter of a saturated solution of La(OH)3?
Example #4: Find the pH of a saturated solution of Aluminum hydroxide, Al(OH)3. The Ksp of Al(OH)3 is 1.9 x 10-10.
Example #5: Calculate the molar solubility of FeCO3 in a solution of 0.00167 M Na2CO3. Ksp for FeCO3 is 2.1 x 10-11.
The Reaction Quotient & Precipitation
Concept #2: In addition to determining if a chemical reaction is at equilibrium, the reaction quotient can also determine if a precipitate will form.
Concept #3: Comparing the reaction quotient Q to the equilibrium constant K will determine if a precipitate will form or not.
When Q is equal to Ksp the solution is saturated and there is no shifting in the equilibrium position.
When Q is greater than Ksp the solution is supersaturated and the reaction shifts in reverse to form more precipitate.
When Q is less than Ksp the solution is unsaturated and the reaction shifts forward to form more ions.
The Reaction Quotient & Precipitation Calculations
Example #6: Will a precipitate form when 0.150 L of 0.100 M Pb(C2H3O2)2 and 0.100 L of 0.20 M NaCl are mixed? The Ksp value of PbCl2 is 1.2 x 10-5.
Example #7: What is the minimum pH at which Fe(OH)2 will precipitate if the solution has [Fe2+] = 0.0583 M? Ksp of Fe(OH)2 is 4.87 x 10-17?
