1. Solubility Equilibria of “Insoluble” Ionic Compounds
Concept, Dissolution Equation and Ksp
And definition of “solubility” as “x”
Remember (relearn?) your PAI’s and formulas of ionic compounds!
CHM 121 topic
Problem types (relate to prior types)
Q vs K (becomes “Will a ppt form?”)
Plug-in (often in a “common ion” situation)
Find K from [ ]0’s and one [ ]eq (Exp 26!)
Find [ ]eq’s from [ ]0’s and K (“solubility”!)
*Read the blue summary outine on this material! Has some examples, too.
PS05-Ksp

2. Concept: Dissolution Equation and Ksp
The reaction equation that “goes” with Ksp is always:
Ionic Solid Aqueous Cation + Aqueous Anion
MxAy x Mm+(aq) y An-(aq)
Ksp thus always has the same “form”:
Denominator is always “1” because there’s always only a single reactant, which is a solid!
K for dissolution equation is a “product” of ions. So…it’s called the “solubility product”, Ksp
PS05-Ksp

3. Must learn how to write Ionic Compound Dissolution Equations!!
Write the Dissolution Equation and Corresponding Ksp Expression (Law of Mass Action) for the following solids:
Ru2(CO3)3(s)
Ag3PO4(s)
CoO3(s)
2 Ru3+(aq) CO32-(aq) ;
3 Ag+(aq) + PO43-(aq) ;
Co6+(aq) O2-(aq) ;
PS05-Ksp

4. Molar Solubility (is what it “says it is”)
The number of moles of a solid that dissolve per L of solution (formed) to reach equilibrium (i.e., a saturated solution).
KEY IDEA: Think of this as a “Type 4” equil. problem!
Initial situation is given
We have “some solid”, and, if present, some of the product ions).
Want: how much “rxn” occurs till the system reaches equilibrium. (Determine the equilibrium state).
Defining x in the usual way…it is the “molar solubility”!
Let x = “the number of moles of solid that dissolve per liter of solution to reach equilibrium”
Because of this, Tro just calls “x” “S”!!? (I don’t like this because it makes it seem “different” from a regular equilibrium problem.)
PS05-Ksp

5. Molar Solubility (example 1, in pure water)
Find the molar solubility of Cu(OH)2(s) in pure water at 25C. Ksp = 2.2 x 10-20
Cu(OH)2(s) Cu2+(aq) OH-(aq) ;
(M)
Cu(OH)2(s)
[Cu2+]
[OH-] I C E
“some”
- “x”
+ x
+ 2x
Still some left x 2x
*Substitute into Ksp, and solve for x, (See board)
*Also see Example 17.8 in Tro!
PS05-Ksp

6. Molar Solubility (example 2, in a solution with a “common ion”)
Find the molar solubility of Cu(OH)2(s) in 0.10 M Cu(NO3)2(aq) at 25C. Ksp = 2.2 x 10-20
Cu(OH)2(s) Cu2+(aq) OH-(aq) ;
(M)
Cu(OH)2(s)
[Cu2+]
[OH-] I C E
“some”
0.10
- “x”
+ x
+ 2x
Still some left x 2x
*After substitution into Ksp, use the “small x” approximation [usually] and then solve for x. (See board)
*Also see Example in Tro!
PS05-Ksp

7. Will a precipitate form?
For a Ksp type of equilibrium / problem, “precipitate forming” means “reverse reaction occurs”!
Since reverse rxn occurs only with Q > K, the question simply becomes:
Is Q > K ? [“Is Q > Ksp?”]
See Example in Tro
NOTE: Must consider mixing, if applicable (see tan handout sheet for Exp 26 and).
PS05-Ksp

8. “Type 3” Kind of Problem (Experiment 26!)
Know initial state info, and one [ion]eq, find K.
See tan sheet from Exp 26
PS05-Ksp