June 20, 2005
Dr. Nash
85 Points Possible
Chemistry 115
Exam 2 (redo)
NAME:__________________
Potentially useful constant: R= 8.314 J/Mol K= 0.08206 Latm/mol K
1. At 25 C, the pH of a 0.097 M solution of cyanic acid HSCN is 3.88.
(a) What is the percent dissociation of this 0.097 M solution?
(b) What is the Ka of HSCN
(c) What is the percent dissociation of HSCN is a 0.194 M solution?
(d) What is the pH of a 0.194 M solution of HSCN
(e) What is the Kb of KSCN?
(f) What is the pH of a 0.097 M solution of KSCN?
2. A buffer solution is prepared by dissolving .125 moles of HSCN and 0.100 moles of
NaSCN in 1.2 L of solution.
(a) What is the pH of this buffer solution?
(b) What is the pH of the buffer solution after 0.45 moles of HCl(g) is dissolved in
this 1.2 L of buffer? (assume the volume of the solution does not change)
(c) What would the pH of the buffer solution have been in 1.00 moles of NaOH
was dissolved in 1.2 L of the buffer (again, no volume change)
(d) The pH of a buffer solution prepared using the same two species (HSCN and SCN- is
5.02. What is the ratio of [HSCN] to [SCN-] in this solution?
3. Consider the data below for the following equilibrium at 25 C
Np(OH)3  Np3+ + 3 OHName
Np(OH)3 (s)
Np3+(aq)
OH-(aq)
Gof (kJ/mol)
-835.0
-245.0
-157.0
(a) What is Go for this process?
(b) What is Ksp for this process?
(c) What is the solubility of Np(OH)3 in water?
(d) What is the solubility of Np(OH)3 in a solution buffered to pH 11.00?
4. Consider the following data for a reaction taking place in an acidic solution phase at
25C.
CH3I(aq) + H2O(l)  CH3OH(aq) + HI(aq)
Experiment
1
2
3
4
[CH3I]
0.100
0.200
0.200
0.400
[H+]
0.01
0.10
0.01
0.10
rate Ms-1
0.0137
0.0274
0.0274
0.0548
(a) What is the rate law and overall order of this reaction?
(b) What is the value of the rate constant for this reaction?
(c) What would the rate of the reaction be if the concentration of CH3I in acid solution is
0.05 M?
5. Consider the following reaction which takes place in the gas phase
CH3Br(g) + H2O(g)  CH3OH(g) + HBr(g).
with the following data:
Note: it is observed that in this case, H2O does not explicitly appear in the rate law:
25C
time(s)
[CH3I]
0
10
20
30
40
50
60
80
90
100
110
120
130
140
150
160
170
180
190
200
37C
[CH3I]
3.0000
1.1321
0.6977
0.5042
0.3947
0.3243
0.2752
0.2113
0.1893
0.1714
0.1567
0.1442
0.1336
0.1245
0.1165
0.1095
0.1033
0.0977
0.0927
0.0882
3.0000
0.9677
0.5769
0.4110
0.3191
0.2609
0.2206
0.1685
0.1508
0.1364
0.1245
0.1145
0.1060
0.0987
0.0923
0.0867
0.0817
0.0773
0.0733
0.0698
(a) What is the order of this reaction? Show your work.
(b) What is the rate constant at 25 C?
(c) What is the (first) half-life of CH3I at 25 C?
(d) How long will it take at 25 C for [CH3I] to reach 0.0010 M
(e) What is the rate constant at 37 C
(f) What is the activation energy of this reaction?
(g) What would the rate constant be for this reaction at 40 C?
Bonus: Tritium is a radioactive isotope of hydrogen (3H) used in triggers for
thermonuclear weapons. It decays by beta decay (emission of a nuclear electron) with a
half-life of 8.25 years. A trigger for a thermonuclear device can function when at least
33% of its original supply of tritium remains in the ‘device’. So, how often does the
tritium in a thermonuclear device need to be changed?
Potentially Useful Equations:
 K   H o
ln  2  
R
 K1 
1 1
  
 T2 T1 
 k   Ea  1 1 
  
ln  2  
R  T2 T1 
 k1 
 Ea
k  Ae
RT
1
1

 kt
 At A 0
ln A t  ln A0  kt
G  G  RT ln Q
o
 At  A0 e kt