Chem 12
NOTES – 3.6 Ksp
In the last unit, we learned about Keq (the equilibrium constant). The Ksp is simply
the Keq for a reaction involving the dissociation of a salt!
Ksp is the ____________________________________ constant.
Ex: Write the solubility product expression for a saturated solution of CaF2
CaF2 (s)
Ca2+ (aq) + 2F- (aq)
Remember, we do not include the solid in the expression (we never include solids
or pure liquids in equilibrium or solubility product expressions).
Ex: Write the solubility product expression for a saturated solution of CaCO3
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CALCULATING SOLUBILITY or THE Ksp
There are basically TWO situations we will encounter:
#1 – You know the solubility (concentrations) and you need to calculate the Ksp
#2- You have the Ksp and you need to calculate solubility (concentration)
Situation #1: Calculate the Ksp
Ex: The solubility of Ag2CrO4 in water is 1.31 x 10-4 moles/L. Calculate the Ksp .
Ex: At a certain temperature, the solubility of SrCO3 is 7.5 x 10-5 M. Calculate the
Ksp for SrCO3.
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Ex: (Tricky!) Calculate the Ksp for ZnCO3 if 150.0 g is required to saturate 200.0 mL
of solution.
HOW TO SOLVE FOR Ksp:
 Write the equil’m equation (showing the salt breaking into ions)
 Write the [ ]’s in mol/L under the species in the reaction
 Write the Ksp expression and “plug in” the concentrations
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Situation #2: Calculate the solubility
For these types of problems (you don’t know the concentrations), you must use
the Ksp TABLE in your DATA BOOKLET!
These types of questions are not tricky so don’t panic! You just need to work
backwards 
Ex: Calculate the solubility of CaSO4 in mol/L and in g/L.
HOW TO SOLVE FOR SOLUBILITY
 Write the equil’m equation (showing the salt breaking into ions)
 Write the Ksp expression and look up the Ksp (in table)
 Substitute ‘x’ for concentration and solve using Ksp expression!
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Comparing Ksp values:
Note: the Ksp is directly proportional to the [ions]!
- This tells us a few things….
o When you have a LARGE value for Ksp…many ions have dissolved!
o LARGE Ksp means “ ____________________”
o SMALL Ksp means “ ____________________”
Q: Which substance is most soluble at 25⁰C ? Which is least soluble?
AgCl
AgI
AgBr
Do Ksp Worksheet (all Q’s except Trial Ksp)
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NOTES – 3.7 Trial Ksp
In Unit 2, we calculated a “trial Keq”. We can do the same thing for solubility
reactions!
When two solutions containing ions are mixed, we can ask whether a PPT will
form. We have to consider if there is sufficient concentration of both ions to
establish the equilibrium.
 Solubility table: tells you which ions are soluble together and which form
precipitates. BUT…it is possible to mix ions in low enough concentrations
that a precipitate will not form.
 Ksp table: the Ksp tells you the “minimum” requirements to form a PPT. In
other words the concentrations when multiplied together must AT LEAST
EQUAL the Ksp for a PPT to form!
Example 1:
We mix solutions of Ca2+ and CO32-, so that [Ca2+] = 2.3 x 10-4 M and the
[CO32-] = 8.8 x 10-2 M. The Ksp for CaCO3 is 5.0x 10-9. Will a precipitate form?
For a PPT to FORM….
TRIAL Ksp must be ____________________ to the Ksp
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Example 2:
50.0 mL of 0.0035 M Ca(NO3)2 solution is mixed with 150 mL of 2.0 x 10-5 M
Na2CO3 solution. Will a precipitate form?
150 mL of
-5
2.0 x 10 M
Na2 CO 3
50.0 mL of
0.0035 M
Ca(NO 3 ) 2
2+
Ca
Na+
NO -3
CO 3
2-
The Calcium and Nitrate ions from
the calcium nitrate and the Sodium
and Carbonate ions from the sodium
carbonate are all present in the
mixture immediately after mixing.
Do Ksp Worksheet (Trial Ksp Questions)
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