CHEMISTRY 122
Dr. Dolson
EXAM II
100 POINTS
August 4, 2010
NAME
INSTRUCTIONS
1. PRINT YOUR NAME LEGIBLY on the line above.
2. You may use a simple (nongraphing/nonprogrammable) scientific calculator.
calculators or refer to notes or other references.
3. When finished, take your exam to the lecture table and show your picture ID.
You may not share
Some potentially useful equations and relationships
1 atm = 760 mm Hg = 760 torr = 101.3 kPa = 14.7 psi
R = 8.314 J·mole-1·K-1
R = 0.08206 L·atm·mole-1·K-1
van der Waals equation: P = RT/(Vm-b) – a/(Vm)2 , where Vm = V/n = molar volume
gas density =
P1 = X1·Ptotal
nM
PM
=
V
RT
X1 = n1/ntotal
Ptotal = P1 + P2 + P3 + . . .
u=
3RT
Mkg
u1
=
u2
M2
M1
Write the best letter response to each multiple choice question in the spaces provided here.
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PART I. Multiple Choice, 34 Points (3.4 pts/each)
1. Which of the following correctly states Boyle’s and Charles’ Laws?
A. P ∝ V AND V/T = const.
C. V ∝ 1/P AND V ∝ 1/T
B. PV = const. AND V ∝ T
D. V/P = const. AND V/T = const.
2. The mercury level in the open side of a manometer is 55 cm below the level on the side connected to
the closed gas volume. If atmospheric pressure is 740 torr, the pressure of the gas sample is
torr .
A. 795
B. 190
C. 685
D. 1290
3. A gas cylinder contains helium at a pressure of 100 atm when it is outside on the loading dock at an
Antarctic research station where the temperature is -30 ºC. When the cylinder is brought inside to a
room temperature laboratory the helium pressure changes to an approximate value of
.
A. 89 atm
B. 110 atm
C. 123atm
D. 140 atm
4. 27 grams of research grade deuterated methane (CD4, 20 amu) is delivered to the laboratory at room
temperature in a 1 L steel cylinder. What is the approximate gas pressure in the cylinder?
A. 15 atm
B. 27 atm
C. 33 atm
Exam II - 1
D. 52 atm
5. Consider a gas sample confined in a cylinder with a movable piston (such that the volume can be
adjusted by movement of the piston). What is the result if the volume is reduced by a factor of three and
the Kelvin temperature is reduced by a factor of two?
A. The gas pressure increases to six times the initial value.
B. The gas pressure increases to one and one-half times the initial value.
C. The gas pressure is unchanged.
D. The gas pressure decreases to two-thirds of the initial value.
E. The gas pressure decreases to one-sixth of the initial value.
6. In the figure below “a” represents a vessel with a microscopically porous wall, which is entirely
enclosed within a larger evacuated vessel (“b”). At the beginning of this experiment “a” contains a
mixture of helium and neon. After a time, the gases that have effused into volume “b” are analyzed.
According to Graham’s law we should expect that the mole fraction of neon in volume “b” is
that in volume “a”.
A. smaller than
B. larger than
a
C. the same as
b
7. In certain “atomic beam” experiments a stream of metal atoms is produced by heating a sample
inside a small furnace within an evacuated volume. The atoms streaming out of a small hole in the
furnace have a Maxwell-Boltzmann distribution of velocities. What is the average velocity of atoms in
a sodium beam (23 amu) at 550K?
A. 510 m/s
B. 650 m/s
C. 770 m/s
D. 890 m/s
8. A typical gas mixture for the excimer laser used in eye surgery contains a mixture of fluorine,
krypton, and helium in the mole fractions F2(0.005), Kr(0.010) and He(0.985). What is the partial
pressure of fluorine in a cylinder containing a total pressure of 140 atm?
A. 280 Torr
B. 370 Torr
C. 430 Torr
D. 530 Torr
9. The production of sulfur dioxide from burning sulfur-containing coal is a major contributor to which
environmental problem?
A. acid rain
B. the greenhouse effect
C. destruction of the ozone layer
10. Effusion and diffusion are different words for the same process.
A. True
B. False
End of Multiple Choice Questions
Exam II - 2
D. air pollution
Part II. Short or Essay Answers [11 pts]
Clearly state the five postulates (assumptions) of the kinetic-molecular theory of gases (in any order).
These postulates describe an ideal gas.
(1)
(2)
(3)
(4)
(5)
Briefly explain, in one or two complete sentences, how and why a real gas at 1 bar might deviate from
ideal gas behavior at low temperatures. Which parameter ( “a” or “b” , circle one) in the van der
Waals equation accounts for this behavior?
Briefly explain, in one or two complete sentences, how and why a real gas at 300K might deviate from
ideal gas behavior at pressures in excess of 300 bar. Which parameter ( “a” or “b” , circle one) in
the van der Waals equation accounts for this behavior?
End of Part II
Exam II - 3
Part III. Solutions to Problems [60 pts]
SHOW YOUR SETUP and CALCULATIONS CLEARLY FOR CREDIT!
16.
(10 pts) The ocean floor is characterized by a temperature near 3°C and a pressure of 16,000
pounds per square inch. If a methane bubble of 1.0 cm3 volume were to be released from the ocean
floor, how large would it grow at the ocean’s surface at 27°C? Express this volume in liters with
appropriate significant digits.
Show your setup and calculations for full credit
Methane volume =
Liters
Exam II - 4
17.
(15 pts) What is the density (in grams/Liter) of a gas mixture composed of 228 torr of hydrogen
(2.02 amu) gas and 532 torr of carbon dioxide (44.0 amu) gas at 298K?
Show your setup and calculations for full credit
density of gas mixture =
g/L (report with 3 sig. digits)
Exam II - 5
18. (20 pts) Two gas volumes maintained at 298K are connected by a closed valve. One of the
volumes (5.00 L) contains 300. torr of hydrogen chloride gas, and the other volume (3.00 L) contains
600. torr of ammonia. Firstly, assuming that there is no reaction of these gases calculate the total
pressure and the partial pressures of each gas after the valve is opened and the gases have thoroughly
mixed. Secondly, recognize that the gases do react to form a solid ammonium chloride product by the
chemical equation, HCl (g) + NH3 (g) → NH4Cl (s). Identify the excess reactant in this system as
described, and calculate the final pressure of the excess reactant upon completion of the reaction. Write
your answers in the boxes provided. Show your setups and calculations for full credit
(i) Assuming no reaction and complete mixing:
Ptotal =
torr;
PHCl =
(ii) After completion of the reaction:
torr
Pfinal =
Exam II - 6
PNH3 =
torr
torr
19.
(15 pts) Safety air bags for automobiles are inflated by nitrogen gas released when sodium
azaide is decomposed by rapid heating according to the following reaction:
2 NaN3 (s) → 2 Na (s) + 3 N2 (g) .
How many grams of sodium azide must be decomposed in order to generate sufficient nitrogen to fill a
36 liter air bag to a pressure of 1.2 atm at 26°C? Report your answer with the correct number of
significant digits.
Mass of NaN3 =
g
Exam II - 7
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Exam II - 8