ionic concentration

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STATE UNIVERSITY OF NEW YORK AT OSWEGO
DEPARTMENT OF CHEMISTRY
ANALYTICAL CHEMISTRY/ CHE322/ FALL 2006
11/ 29/ 06
EXAM 3
Instructor: Dr. Kadima
Last name__________________________First________________IDN__ __ __ __ __ __ __ __
______________________________________________________________________________
1. (20 points) Circle the correct answer
1.1 A solution of 0.10 M disodium hydrogen phosphate (Na2HPO4) is
a) basic
b) acidic
c) neutral
The acid dissociation constants for phosphoric acid species are: Ka1= 7.11x10-3,
Ka2=
6.32x10-8, Ka3= 4.5x10-13
1.2 As the ionic strength increases, the activity coefficient
a) increases
b) decreases
c) remains the same
1.3 Cu2+ reacts spontaneously with Zn(s) to produce Zn2+ and Cu(s).
a) true
b) false
The standard reduction potentials for the reduction of Cu2+ to Cu(s) and Zn2+ to Zn(s) are:
Cu 2  2e   Cu( s)
0.3419 V
Zn 2  2e   Zn( s)
-0.7618V
1.4 Adding HNO3 to a saturated barium sulfate (BaSO4) which is in contact with solid BaSO4
will….
a) decrease the solubility
b) increase the solubility
1.5 Oxalic acid reacts spontaneously with Cerium (IV) to produce CO2 and Cerium (III).
a) true
b) false
The standard reduction potentials for Ce (IV) and CO2 are:
Ce4+ + e  Ce3+
E = 1.72
2 CO2 + 2H+ + 2e  H2C2O4
E = -0.481
2. (5 points) Calculate the ionic strength of a solution that is 0.060 M La(NO3)3 and 0.030
Fe(NO3)2.
3.
(10 points) Consider the following equilibria , in which all ions are aqueous:
(1) Ag+ + Cl

AgCl(aq)
K = 2.0 x 103
(2) AgCl(aq) + Cl  AgCl2
K = 9.3 x 101
(3) AgCl(s)  Ag+ + Cl
K = 1.8 x 10-9
(a) Calculate the numerical value of the equilibrium constant for the reaction
AgCl(s)  AgCl(aq)
(b) Find the numerical value of K for the reaction
AgCl2  AgCl(s) + Cl
2
4. (10 points) Calculate E, G, and K at 25C for the reaction
4 Co3+ + 2H2O

4Co2+ + O2(g) + 4H+
3
5. (10 points)

a) Use activities to calculate the hydronium ( H 3 O ) concentration in 0.120 M
solution of HNO 2 that is 0.050 M in NaCl .
Ka 
aH
3O

aNO 
aHNO2
2
 7.1  10  4
 H O 0.85

3
 NO 0.81

2
 HNO 1
2
b) Calculate the concentration of hydronium assuming all activity coefficients are
equal to 1.
c) What is the % relative error introduced by the assumption that all activity coefficients
are equal to 1?
4
6.
(15 points) The pK a of chloroacetic acid (ClCH2CO2H) is 2.865.
a) Calculate the pH of a 0.100 M chloroacetic acid solution. The error on the hydronium
concentration may not exceed 1%.
b) Which approximation(s) have you used ?
c) Which approximation didn’t use? Explain why.
5
7.
(10 points) Calculate the pH and concentrations of (CH3)3N and (CH3)3NH+ in a 0.060 M
solution of trimethylammonium chloride [ (CH3)3NHCl]. The pKa of trimethylammonium is
9.800 (Ka = 1.58 10-10).
6
8. (10 points) Calculate the pH and the buffer capacity of a solution that is 0.200 M in NH 3
and 0.300 M in NH 4 Cl .
10
The acid dissociation constant of the ammonium ion is 5.70  10
.
7
9. (10 points) Find the solubility (grams per liter) of CaSO4 (formula mass 136.14) in (a)
distilled water and (b) 0.50 M CaCl2.
K sp  2.4  105
8
BONUS
10. (10 points) Calculate the electrochemical potential (of the platinium electrode) after adding
2

5 mL of 0.0200 MnO4 , when titrating 50.00 ml of 0.050 M Fe
prepared in 1.00 M
HCl.
The formal potentials for the two half reactions are:
Fe3  e   Fe 2
E o'  0.700V
MnO4  8H   5e   Mn 2  4 H 2 0
E o'  1.51V
9
Appendix: some useful relationships, equations etc..
a A    A

(3)
1
ci z i2

2 i
 log  A 
(4)
0.51  z 2A  
1  3.3   A  
G  G o  RT ln Q
(6)
G o   RT ln K
(7)
E  Eo 
RT
ln Q
nF
E  Eo 
0.05916
log Q
n
Eo 
RT
log K
nF
Eo 
0.05916
log K
n
o
o
o
E reac
 E red
 Eox
H O  
K aCHA
H O  
K a1K a 2

3

3
at 25C and   0.1
(5)
(8)
at 25C
(9)
at 25C
(11)
(10)
(12)
10
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