```Chem 112 Class Guide: SOLUBILITY
Chapter 17, Sections 4 and 5 Learning Goals:
Upon completion of Chapter 16, Sections 6-9, you should be able to determine the following:
Calculate the Ksp of a salt solution
Calculate the solubility of a salt solution
Chapter Reading Guide: Chapter 17, Sections 4 and 5
Section 4: SOLUBILITY EQUILIBRIA
All ionic salts are somewhat soluble in water, even those termed “not soluble” by the
solubility rules in Chapter 4.
The solubility of an ionic solid in water is based on the concentration of the metal ion in
solution at equilibrium. Since the reactants for these equilibria are all solids, they do not
appear in expressions for K; K merely contains the products.
Example:
Ca3(PO4)2 (s) ↔ 3 Ca2+ (aq) + 2 PO43- (aq)
Kc=Ksp=[Ca2+]3[PO43-]2
Try Practice exercise 17.9
To solve for solubility, set up your ICE chart as we have done before, being mindful of the
stoichiometric coefficients with each product. It is not unusual to have an X3 or X4 term
once you start solving for X. Consult your calculator’s manual as to how to handle this.
Also, be mindful of your units – when solving for x, recall that your units are molarity
(mol/L). Solubility is often asked for in g/L.
Example:
Calculate the solubility of Ca3(PO4)2 in g/L if the Ksp of Ca3(PO4)2 is 2.0x10-29.
Ca3(PO4)2 (s) ↔
N/A
N/A
N/A
Initial
Change
Equilibrium
3 Ca2+ (aq) +
0
+3X
3X
2 PO43- (aq)
0
+2X
2X
Ksp=[Ca2+]3[PO43-]2
Ksp  3X  2X   9X3  4X2   36X6
3
2
Ksp = 36 X6 = 2.0x10-29
X6 =
2.0 x1029
 5.56 x103
36
X= 6 5.56 x103  9.1x106 M
6
g
 g  9.1x10 mol Ca 3 (PO4 )2 310.18 g Ca 3 (PO4 ) 2
solubility   
 2.8 x103
L
1 mol Ca 3 (PO4 )2
L
L
Try Practice exercises 17.10 and 17.11
Section 5: FACTORS THAT AFFECT SOLUBILITY
The presence of a common ion can greatly affect the solubility of a salt. Due to Le
Chatelier’s principle, adding a product ion will reduce the solubility of the slightly soluble
salt. When making an ICE chart to calculate solubility, you will have initial concentrations of
one or more of your products instead of all “X”s as in section 4 (and on the previous slide).
Example:
Calculate the solubility (in g/L) of Ca3IPO4)2 in a solution that is 0.0010 M Na3PO4. The Ksp of
Ca3(PO4)2 is 2.0x10-29.
Na3PO4 (s)  3Na+ (aq) + PO43- (aq)
Since all of the sodium phosphate will dissolve in water (check your solubility rules in
Chapter 4 if you are not sure why), we can use stoichiometry to determine how much
phosphate will be in solution.
M PO43 
0.0010 mol Na 3PO4 1 mol PO43
 0.0010 M PO43
L
1 mol Na 3PO4
This is how much phosphate we have when we start the ICE chart:
Ca3(PO4)2 (s) ↔ 3 Ca2+ (aq) + 2 PO43- (aq)
Initial
N/A
0
0.0010
Change
N/A
+3X
+2X
Equilibrium N/A
3X
0.0010 + 2X
From here, we continue normally:
Ksp=[Ca2+]3[PO43-]2 Remember that the amount that calcium phosphate is going to dissolve is
negligible compared to the amount of phosphate we already have, so we can ignore the +2X
at equilibrium. So,
K sp  3 X   0.0010  2 X  becomes: K sp  3 X   0.0010
3
2
3
2
Ksp  3 X  0.0010   9 X 3 1.0 x106   9.0 x106 X 3
3
2
Ksp  9.0 x106 X 3  2.0 x1029
X3 
2.0 x1029
 2.22 x1024
9.0 x106
X  3 2.22 x1024  1.31x108 M
8
g
 g  1.31x10 mol Ca 3 (PO4 ) 2 310.18 g Ca 3 (PO 4 ) 2
solubility   
 4.0 x106
L
1 mol Ca 3 (PO4 ) 2
L
L
Note that this solubility is much lower than what we calculated for calcium phosphate by
itself!
Try Practice exercise 17.12
Learning Resources
Chapter Learning Goals
Chapter 17, Sections 4 and 5 Learning Goals
Pre Class Assignment: This assignment must be completed prior to the next class. Check your syllabus
for the exact due date and time.
Complete to the pre class assignment
(http://berks.psu.edu/clt/chem112/Solubility_HW.docx)
Submit a copy to the dropbox located in ANGEL called “Pre Class Assignment Submission:
Solubility”
End of Chapter Problems:
Practice with these problems if you are having difficulty with any of the concepts covered in this
class guide AFTER we have met in class. If you cannot easily complete these problems, seek
Chapter 17: 51, 53, 59
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