Chemistry 115
Practice Exam 1
May 25, 2005
Dr. Nash
100 Points Possible
NAME:__________________
Potentially useful constant: R = 0.08206 L atm/mol K = 8.314 J/Mol K
At wt O=16.00 g/mol, N=14.01 g/mol, C=12.01 g/mol, H=1.00 g/mol,
kB=1.38 x 10-23 J/K, 760 mmHg = 1atm, 4.184 J = 1 cal.
T(C)
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
v.p. Hg(l) torr
v.p. Hg(s) torr
0.0014
0.0016
0.0021
0.0026
0.0031
0.0040
0.0045
0.0061
0.0065
0.0092
0.0093
0.0137
0.0132
0.0201
0.0186
0.0293
0.0258
0.0420
0.0354
0.0598
0.0483
0.0841
0.0652
0.1172
0.0874
0.1619
0.1160
0.2215
0.1530
0.3005
0.2001
0.4044
0.2600
0.5400
1. Using the table above.
(a) What is the Hvap of liquid mercury?
(b) What is the normal boiling point (b.p. at 1 atm pressure) of liquid mercury?
(c) What is the Hfo of mercury vapor?
(d) What is the Hfo of liquid mercury?
(e) What is the Hsub of solid mercury?
(f) What is the Hfus of solid mercury?
2. Even at high temperatures, the formation of nitric oxide is not favored.
N 2(g )  O2(g )  2NO(g )

K c  4.1x104 at 2000C
What is the equilibrium concentration of NO(g) when a mixture of 0.20 mol N2(g) and
0.15 mol O2(g) is allowed to come to equilibrium in a 1.0 L container?
3. Ammonium hydrogen sulfide decomposes according to the following reaction for
which Kp=0.11 at 250 C.
NH4 HS(s)  H 2 S(g )  NH3(g )
If 55.0 g of NH4HS(s) is placed in a sealed 5.0 L container what is the pressure of NH3 at
equilibrium? Note: ammonium hydrogen sulfide is a solid (i.e., how does it appear in the

equilibrium constant?)
4. Consider the following data for Carbon Disulfide, CS2. Assume that Hfo and So
do not vary significantly with temperature (we always make this assumption).
CS2(g)
CS2(l)
Hfo (kJ/mol)
117
87.9
So (J/Kmol)
237.79
151.0
(aa). Calculate Ho and So for this reaction CS2(l)  CS2(g).
Ho=
So=
(a) Calculate the Go for the vaporization process CS2(l)  CS2(g) at 25 C.
Go(25)=
(b) Write a very simple expression in terms of the natural units for the
equilibrium constant of this vaporization process
Kp=
(c) Calculate the numerical value of this equilibrium constant for this
process at 25 C?
Kp(25)=
(d) What is the vapor pressure of carbon disulfide at 25C? (use your results
from b and c)
(e) Using your results from (aa), Calculate the Go for the vaporization
process at 55 C?
Go(55)=
(f) Calculate the numerical value of the equilibrium constant for this process at 55
C?
Kp(55)=
(h) What is the normal boiling point of Carbon disulfide? (i.e., the
temperature at which it is at equilibrium under standard conditions)
T=
BONUS:
:
(i) At what temperature is the equilibrium vapor pressure .693 atm.
T=
5. Consider the following reaction
N2O4(g)  2 NO2(g)
Three experiments are done for this reaction and the results are given below
in Table A. Experiments 1 and 2 are known to involve systems that are at
equilibrium.
The Jury is still out on experiment 3.
Table A
Exp.
1 at
equilibrium
2 at
equilibrium
3 ???????
Table B
Exp
K/Q
1
K=
Temp C
25
P(N2O4) atm
0.137
P(NO2) atm
0.209
50
0.225
0.351
25
0.216
0.404
Go
G
Go=
G=
K=
Go=
G=
Q=
Go=
G=
2
3
(a) Use the data in Table A to fill out the data in Table B.
(b) Is the system in Experiment B at equilibrium and if not will it proceed
in the forward or reverse direction?
(c) How much work can be exchanged between the surroundings and the
systems in experiments 1, 2, and 3 (also, if work flows, which direction does
it flow?)
Equations:
 K 2   H o
 
ln 
K
R
 1
 k   Ea
ln  2  
R
 k1 
 Ea
k  Ae
1 1
  
 T2 T1 
1 1
  
 T2 T1 
RT
1
1

 kt
 At A 0
ln A t  ln A0  kt
G  G o  RT ln Q
K e

G o
RT