Chem 2000
Spring 2004
Assignment 5
Friday Feb 26,
2004
1) The solubility of silver carbonate is 0.032 M at 20oC. Calculate the Ksp for silver
carbonate (Ag2CO3).
I
∆
E
Ag2CO3 (s) U 2 Ag+ (aq) + CO32- (aq)
?
0
0
-x
2x
x
x = solubility = 0.032 M
?
2x
x
Ksp = [Ag+]2[CO32-] = (2x)2x = 4x3 = 4*(0.032)3 = 1.3 * 10-4
2) The solubility in water of silver dichromate (Ag2Cr2O7) at 15 oC is 8.3 *10-3 g
per100 ml. Calculate the Ksp for silver dichromate.
I
∆
E
Ag2Cr2O7 (s) U 2 Ag+(aq) + Cr2O72-(aq)
?
0
0
-x
2x
x
?
2x
x
MW(Ag2Cr2O7) = 431.8 g/mol
x = (8.3*10-3 g)/((0.100 l)*(431.6 g/mol) = 1.92*10-4 M
Ksp = [Ag+]2[Cr2O72-] = (2x)2x = 4x3 =4*(1.92*10-4)3 = 2.8*10-11
3) Find the molar solubility of SrCO3 (Ksp = 5.4*10-10) in:
a) pure water
I
∆
E
SrCO3(s)
?
-x
?
U
Ksp = [Sr2-][CO32-] = x2
Sr2-(aq)
0
x
x
+ CO32-(aq)
0
x
x
x = (Ksp)0.5 = (5.4*10-10)0.5 = 2.3*10-5 M
b) in 0.13 M Sr(NO3)2
I
∆
E
SrCO3(s)
?
-x
?
U
Sr2-(aq)
0.13
x
0.13 + x
+ CO32-(aq)
0
x
x
Ksp = [Sr2-][CO32-] = (0.13 + x) x = 5.4*10-10
assume x<< 0.13
(0.13) x 5.4*10-10 x = 4.2*10-9 M
4) Write the balanced equation for the dissociation of the following compounds in
water indicating whether a change in pH affects its solubility.
a) AgCl
AgCl(s) U Ag+(aq) + Cl-(aq)
Neither Ag+ nor Cl- undergo hydrolysis reactions with water and therefore
it solubility will be unaffected by pH.
b) SrCO3
SrCO3(s) U Sr2-(aq) + CO32-(aq)
CO32-(aq) + H2O(aq) U HCO3-(aq) + OH-(aq)
An increase in pH will decrease the solubility of SrCO3, since it leads to
an increase in CO32-.
c) Fe(OH)2
Fe(OH)2 U Fe+ + OHAn increase in pH will decrease the solubility of Fe(OH)2.
d) CuS
CuS(s) U Cu2+(aq) + S2-(aq)
S2-(aq) + H2O(l) U HS-(aq) + OH-(aq)
Solubility decreases with increasing pH.
5) Does any solid Cu(OH)2 form when 0.075 g of KOH is dissolved in 1.0 L of
1.0*10-3 M Cu(NO3)2? (Ksp = 2.2*10-20)
[OH-]o = (0.075 g)/(56.105 g/mol)/(1.0 l) = 1.34*10-3 M
I
Cu(OH)2(s) U Cu2+ (aq)
?
0.0010
+ 2 OH-(aq)
1.34*10-3
Q = [Cu2+][OH-]2 = 0.0010*(1.34*10-3)2 = 1.8*10-9 > Ksp = 2.2*10-20
this means that the equilibrium will shift to the left, ie to reactants, which
in turn implies that solid Cu(OH)2 will form.
6) Find the solubility of AgI in 2.5 M NH3 where:
AgI (s) U Ag+(aq) + I –(aq)
Ksp = 8.3 * 10-17
Ag+ + 2NH3 U Ag(NH3)2+
K = 1.7 * 10 +7
(hint: determine the total amount of silver in the dissolved species)
AgI (s) U Ag+(aq) + I –(aq)
Ksp = 8.3 * 10-17
Ag+ + 2NH3 U Ag(NH3)2+
K = 1.7 * 10 +7
_______________________________________
AgI (s) + 2NH3 U I –(aq) + Ag(NH3)2+ KTotal = KspK = 1.41*10-9
I ?
∆ -x
E ?
2.5 M
-2x
2.5 –2x
0
x
x
0
x
x
Ktotal = [I –][ Ag(NH3)2+]/[NH3]2 = x2/(2.5 –2x)2 = 1.41*10-9
taking the square root of both sides x/(2.5 –2x) = 3.75*10-5
assume x<<2.5
x/2.5 3.75*10-5
x = 9.4 * 10-5 M = solubility