CHM 136 General Chemistry II
Exam 1, Spring 2009 – Dr. Steel
Name
SOLUTIONS
Indicate your answer to each question in the space provided next to each question –
please write clearly. Each question on this portion of the exam is worth 2 points.
B
1. If the volume of a confined gas is doubled while its pressure remains constant, what
change will be observed?
a) The temperature of the gas will decrease to 1/2 its original value.
b) The temperature of the gas will double.
c) The density of the gas will increase to twice its original value.
d) The number of the molecules will quadruple.
e) The number of the molecules will double.
E
2. If the pressure of a confined gas is reduced to one-third its original value while its
temperature constant, what change will be observed?
a) The amount of the gas will decrease to 1/3 its original value.
b) The volume of the gas will decrease to 1/3 its original value.
c) The density of the gas will increase to 3 times its original value.
d) The average speed of the gas particles will increase to 3 times its original value.
e) The density of the gas will decrease to 1/3 its original value.
D
3. Avogadro’s hypothesis states that equal volumes of gases under the same conditions
of temperature and pressure have equal ________.
a) densities
b) particle velocities
c) molar masses
d) numbers of particles
e) masses
D
4. If the amount of a confined gas is quadrupled while its volume and temperature are
held constant, what change will be observed?
a) The average speed of the gas will be decreased to 1/4 its original value.
b) The pressure of the gas will decrease to 1/4 its original value.
c) The average speed of the gas will increase to 4 times its original value.
d) The pressure of the gas will increase to 4 times its original value.
e) The density of the gas will decrease to 1/4 its original value.
E
5. Which of the following gases has the greatest density at 25 °C and 5.0 atm?
a) CH4
B
c) N2
c) N2
b) Ne
a)
b)
c)
d)
e)
mass
density
pressure
average speed
molar mass
___.
A
B
B
C
C
9. Which is a correct statement of the law of conservation of energy?
a)
b)
c)
d)
e)
D
d) SO2
8. On the figure at right, which distribution
describes the sample at the highest
temperature?
A
C
e) CO2
7. Boyle learned that at constant temperature and number of molecules, the volume of a
gas is inversely proportional to its _____
C
d) F2
6. Which of the following gases has the greatest average velocity at 325 K:
a) Ar
C
b) O2
Energy is neither lost nor gained by an object.
Energy is stored as matter.
The energy lost by a system is gained by the surroundings.
The energy gained by a system is also gained by the surroundings.
Energy can only be transferred as heat and work.
10. Which of the following processes must always result in a net increase in internal
energy?
a)
b)
c)
d)
e)
A system gains energy as heat and does work on the surroundings.
A system loses energy as heat and has work done on it by the surroundings.
A system loses energy as heat and does work on the surroundings.
A system gains energy as heat and has work done on it by the surroundings.
A system gains does work on it by the surroundings at a constant temperature.
A
11. Which of the following exchanges of energy has a positive sign for heat (relative to
the system)?
a)
b)
c)
d)
e)
A
12. Which of the following is the result of an exothermic process?
a)
b)
c)
d)
e)
D
snow melts on a mountaintop (snow is the system)
water vapor condenses on a cold surface (water vapor is the system)
a rock tumbles from a mountain (the rock is the system; neglect friction forces)
both (a) and (b) are correct
both (b) and (c) are correct
water freezes
ice melts
dry ice sublimes
gasoline evaporates from a spill
All of the above are endothermic processes.
13. Which one of the following statements is INCORRECT?
a) In an exothermic process heat is transferred from the system to the surroundings.
b) The greater the heat capacity of an object, the more thermal energy it can store.
c) The SI unit of specific heat capacity is joules per gram per kelvin.
d) Specific heat capacity values are positive for liquids and negative for gases.
e) When heat is transferred from the system to the surroundings, q is negative.
D
14. The enthalpy change is the heat absorbed or evolved in a reaction that occurs at
.
constant
a) temperature
b) volume
c) mass
d) pressure
e) energy
B
15. If the same amount of heat is added to 5.00 g samples of each of the metals below,
which metal will experience the largest temperature change?
Metal
Al
Au
Cu
Fe
K
a) Al
Specific Heat Capicity (J/g·K)
0.897
0.129
0.385
0.449
0.753
b) Au
c) Cu
d) Fe
e) K
16. In an attempt to identify an unknown noble gas, you observe that a 0.103-gram
sample of the noble gas occupies 63.8 mL at 22.0 °C and 747 torr. What is the
identity of the noble gas? (10 points)
Convert the pressure to atmospheres:
1 atm
747 torr ×
= 0.983 atm
760 torr
PV = nRT
PV =
Convert the temperature to Kelvin:
22.0 °C + 273.15 = 295.2 K
m
M
RT
mRT
PV
atm⋅L
(0.103 g) (0.0821 mol
⋅K )(295.2 K)
M=
(0.983 atm)(0.0638 L)
g
M = 39.8 mol
M=
This is approximately the molar mass of argon gas, Ar (39.95 g/mol).
o
17. Calculate ΔH rxn
for the combustion of gaseous ethanol,
C2H5OH(g) + 3 O2(g) → 2 CO2(g) + 3 H2O(g)
using standard molar enthalpies of formation. (10 points)
molecule
C2H5OH(g)
CO2(g)
H2O(g)
ΔHfº (kJ/mol)
-235.3
-393.5
-241.8
o
o
o
Δ H rxn
= ∑ Δ H products
−∑ Δ H reactants
kJ
kJ
= [2 mol(-393.5) + 3 mol(-241.8) mol
] − [1 mol(-235.3) + 3 mol(0) mol
]
= −1512.4 + 235.3 kJ
= -1277.1 kJ
18. The thermochemical equation for the combustion of hexane is shown below.
2 C6H14(g) + 19 O2(g) → 12 CO2(g) + 14 H2O(g)
ΔH° = -8326 kJ
What is the heat change for the combustion of 2.50 g C6H14? (10 points)
2.50 g C 6 H14 ×
1 mol C 6 H14
- 8326 kJ
×
= -121 kJ
86.17 g C 6 H14 2 mol C 6 H14
19. A steel chamber is filled with 2010 torr of nitrogen. The volume of the container is
8.65 L. It is placed in a room that is 22.4 °C. Returning to the room the following
day you notice the pressure gauge on the chamber reads 1930 torr. Determine the
new temperature of the room. (10 points)
P1 P2
=
T1 T2
Convert the temperature to Kelvin:
T2 = T1
22.4 °C + 273.15 = 295.6 K
P2
P1
(295.65 K)(1930 torr)
2010 torr
= 283.8 K
=
= 10.6 o C
20. A 26.4-gram sample of silver is chilled to a temperature of 0.00 °C. It is placed into a
calorimeter containing 106.8 grams of water initially at 21.4 °C. Determine the
equilibrium temperature of the system. The specific heat capacities of silver and
water are 0.235 J/g·°C and 4.184 J/g·°C, respectively. (10 points)
q Ag = − q H 2O
m Ag × C Ag × ΔTAg = − m H 2O × C H 2O × ΔTH 2O
(26.4 g)(0.235 g⋅Jo C )(Tf − 0.00 o C) = −(106.8 g)(4.184 g⋅Jo C )(Tf − 21.4 o C)
6.204Tf = −446.9Tf + 9562.6
453.1Tf = 9562.6
Tf = 21.1 o C
21. According to the reaction below, what mass of octane (C8H18) will be required to
produce 1.984 atm of carbon dioxide at 294.8 K in a 8.44-L vessel? (10 points)
2C8H18(l) + 25 O2(g) → 16 CO2(g) + 18 H2O(g)
PV = nRT
PV
n=
RT
(1.984 atm)(8.44 L)
=
atm⋅L
(0.0821 mol
⋅K )(294.8 K)
= 0.692 mol CO 2
0.692 mol CO 2 ×
2 mol C 8 H18 114.22 g C 8 H18
×
= 9.88 g C 8 H18
16 mol CO 2
1 mol C 8 H18
22. A 1.25-L cannister of compressed air has a total pressure of 25.0 atm when it is stored
at 21.7 °C. If the mole fraction of oxygen in air is 0.210, what mass of oxygen is
present in the cannister? (10 points)
·
0.210 25.0
5.25
PV = nRT =
mRT
M
PVM
RT
g
(5.25 atm)(1.25 L) 32.00 mol
=
atm⋅L
(0.08206 mol
⋅K )(294.85 K)
m=
(
= 8.68 g O 2
)