CH186
Solubility Equilibrium Exam Questions
From Spring 2001 Semester
1)
For each of the following, carry out the indicated calculations.
a.
b.
c.
Calculate the molar solubility of zinc oxalate, ZnC2O4 (Ksp = 2.7 x 10-8).
Calculate the molar solubility of silver carbonate, Ag2CO3
(Ksp = 8.1 x 10-12). (CO32- = carbonate ion)
Calculate the approximate Ksp value for lanthanum iodate, La(IO3)3 (FW = 664
g/mol), if a 2.0 L solution saturated at 25C with La(IO3)3 yielded 0.916 g of
La(IO3)3 upon evaporation. (IO3- = iodate ion)
2)
A 3.0 L sample of industrial wastewater solution is known to contain 0.085 M Cd2+(aq).
Because Cd2+ is toxic, it is desirable to remove this ion before disposal. If the pH of this
solution is adjusted to 8.00 via addition of solid NaOH pellets, will a precipitate of
“insoluble” Cd(OH)2 (Ksp = 2.5 x 10-14) fall out of solution? Prove your answer
quantitatively by showing your calculations. (Assume no volume change.)
3)
For each of the following “insoluble salts” state whether the solubility increases,
decreases, or remains unchanged in aqueous solution upon the indicated change.
Saturated Solution of
Change
Solubility Change
(increases, decreases, stays
the same)
4)
1
CaF2
Addition of HNO3(aq)
NiCO3
Addition of NiNO3(aq)
Co(OH)2
pH increases
Cu(OH)2
Addition of the Lewis base NH3
BaSO4
Addition of Na2SO4
Tooth enamel consists of a mineral called hydroxyapatite, Ca10(PO4)6(OH)2. Briefly
explain the chemistry behind the formation of tooth cavities.
Answers for Spring 2001 Solubility Equilibrium Exam Questions
1.6 x 10-4 M
1.3 x 10-4 M
6.1 x 10-12
1.
a.
b.
c.
2.
IP = 8.5 x 10-14. IP > Ksp, so a precipitate of Cd(OH)2 does form.
3.
Saturated Solution of
Change
Solubility Change
(increases, decreases, stays
the same)
4.
2
CaF2
Addition of HNO3(aq)
Increases
NiCO3
Addition of NiNO3(aq)
Decreases
Co(OH)2
pH increases
Decreases
Cu(OH)2
Addition of the Lewis base NH3
Increases
BaSO4
Addition of Na2SO4
decreases
The chemical basis for tooth decay results from the enamel reacting with acids produced
by bacteria that dwell in the mouth. Because it contains basic anions, the enamel
becomes more soluble in the presence of these acids due to neutralization. As a result,
cavities develop where the tooth enamel dissolves away off of the surface of the tooth.
The major chemical reaction that leads to tooth decay is:
Ca10(PO4)6(OH)2(s) + 8H+(aq)  10Ca2+(aq) + 6HPO42-(aq) + 2H2O(l)
CH186
Solubility Equilibrium Exam Questions
From Spring 2001 Semester
1)
A saturated solution of lanthanum fluoride, LaF3, has a F- ion concentration that is
approximately equal to 2.8 x 10-5 M. Calculate the solubility product, Ksp, for this ionic
compound.
2)
Calculate the solubility in g/L for nickel(II) hydroxide, Ni(OH)2 (mol. wt. = 92.70 g/mol),
if its solubility product, Ksp, is 6.0 x 10-16.
3)
Working in Dr. Wisenheimers’s lab one day, Karla Chemie adjusted the pH of a test
solution containing 0.020 M Mg2+ ion to 10.00 by addition of a small amount of base.
Assuming a negligible change in volume, did a precipitate of insoluble magnesium
hydroxide, Mg(OH)2 (Ksp = 5.6 x 10-12), form in Karla’s solution?
4)
Predict whether the solubility of lead fluoride, PbF2, will increase, decrease, or stay about
the same on addition of each of the following reagents to the saturated solution:
HNO3
NaNO3
KF
Pb(NO3)2
3
Answers for Fall 1999 Solubility Equilibrium Exam Questions
1.
2.0 x 10-19
2.
4.9 x 10-4 g/L
3.
IP = 2.0 x 10-10, IP > Ksp, so a precipitate did form
4.
increase
no change
decrease
decrease
4