NAME:
Midterm Exam 4
Chemistry 211
April 20, 2005
PAGE 1 0f 6
- Show all work or NO CREDIT will be awarded.
- Work problems to proper number of significant figures.
- Be sure to include units in your answers.
- Circle your final answers.
- Put your name on ALL pages.
- Multiple choice questions are worth 3 pts. each.
1.
What set of hybrid orbitals would sulfur, in SCl2 , employ?
(a) sp
2.
(c) bent
(d) see-saw
(e) T-shaped
(b) BF3
(c) SiCl4
(d) PCl3
(e) BeCl2
(b) 109º
(c) 180º
(d) 120º
(e) 45º
(b) C2 H4
(c) C3 H6
(d) C4 H8
(e) C5 H10
An unknown gas was found to diffuse 2.2 m in the same time that methane gas (CH4 , MW
= 16.05 g/mol) diffuses 3.0 m. The unknown gas was found to contain 80% C and 20%
H. What is the molecular weight of the gas?
(a) 15
7.
(b) triangular
A gaseous carbon-hydrogen compound is found to have the empirical formula CH2 . A 4.6
-gram sample is found to occupy 4 L at l atm pressure and 27º C. The molecular formula
of the gas is therefore:
(R = 0.082 L • atm )
mol • k
(a) CH2
6.
(e) sp3 d2
In the molecule XeF 4 the F-Xe-F bond angle would be expected to be
(a) 90º
5.
(d) sp3 d
Which of the following would be a polar molecule?
(a) PCl5
4.
(c) sp3
The best description of the molecular geometry of ClF3 would be
(a) pyramidal
3.
(b) sp2
(b) 16
(c) 30
(d) 45
(e) 120
If equal weights of O2 and N2 are placed in identical containers at the same temperature,
which of the following statements is true?
(a) Both flasks contain the same number of molecules
(b) The pressure in the flask that contains N2 will be greater than the pressure in the
flask that contains O2 .
(c) There will be more molecules in the flask that contains O2 than the flask that contains
N2 .
(d) This question can’t be answered unless we know the weights of O2 and N2 in the
flasks.
(e) None of the above are correct.
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8. Which of the following graphs is not a straight line for an ideal gas?
a) V versus T (n and P constant)
b) T versus P (n and V constant)
c) P versus 1/V (n and T constant)
d) n versus 1/T (P and V constant)
e) n versus 1/P (V and T constant)
9. Which of the following is an assumption of the kinetic theory of gases that we know is
not a true statement of the behavior of a real gas at 25 oC and 1 atm?
a) Gases consist of a large number of tiny particles in constant random motion.
b) Gas particles move in a straight line until they collide with another gas particle or
the walls of the container.
c) The distance between gas particles is large compared with their diameters, and
therefore most of the volume of a gas is empty space.
d) The average kinetic energy of the particles in a gas is proportional to the
temperature of the gas and that factor alone.
e) There is no force of attraction between the particles in a gas.
10. A unit of pressure is the
a) Newton
c) Pascal
e) Manometer
b) Joule
d) Tortelini
11. Which of the following molecules or ions does NOT have a tetrahedral shape
according to VSEPR theory?
a) SiF4
c) SeF4
e) BF4-
b) NF4+
d) CF4
12 A balloon has a volume of 0.30 L at one atmosphere pressure (760 torr). When it is
placed in a vacuum chamber with pressure = 1/5 atmosphere (152 torr), its volume will
be:
a) 0.060 L
b) 0.30 L
c) 1.5 L
d) 3.0 L
e) 5.0 L
13. The gas pressure in an aerosol can is 1.5 atm at 25°C. What would the pressure be if
the can were heated to 450°C?
a) 3.6 atm
b) 27 atm
c) 0.62 atm
d) 0.28 atm
e) none of the above
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14. In which of the following gases do molecules have the highest average kinetic energy
at 25º C?
a) H2
d) Cl2
b) O2
e) All have the same.
c) N2
15. Deviations from the ideal gas law are greatest at
a)
b)
c)
d)
e)
low temperature and low pressure
low temperature and high pressure
high temperature and high pressure
high temperature and low pressure
No deviations occur--all gases are ideal and the world is better
because of it--particularly the free world.
16. A mixture of three gases has a total pressure of 1380 mm at 25º C. The mixture is
analyzed and is found to contain 1.27 moles of CO2, 3.04 moles of CO, and 1.50
moles of Ar. What is the partial pressure of Ar?
a) 0.258 atm
d) 1380 mm
b) 301 mm
e) 5345 mm
c) 356 mm
17. What set of hybrid orbitals is used by the central carbon in the following molecule?
H
H2C C CH3
b) sp2
a) sp
c) sp3
d) sp3 d
e) sp3 d2
18. Which of the following gases has a density of 2.104 g/L at 303 K and 1.31 atm?
a) He
d) Kr
b) Ne
e) Xe
c)Ar
19. A real gas differs from an ideal gas in that the molecules of a real gas:
a) Have no appreciable volumes and no molecular attractions for one another.
b) Have appreciable molecular volumes and have molecular attractions for one
another.
c) Have no molecular attractions forone another.
d) Have no kinetic energy.
e) Have low molecular weights.
20. You have two balloons purchased at a recent carnival. One is filled with hydrogen,
one with helium. If you assume that both gases can pass through the pores of the balloon
material:
a) The hydrogen balloon will deflate 2 times faster than the helium balloon.
b) The helium balloon will deflate 2 times faster than the hydrogen balloon.
c) The hydrogen balloon will deflate 1.4 times faster than the helium balloon.
d) The hydrogen balloon will deflate 4 times faster than the helium balloon.
e) The helium balloon will deflate 1.4 times faster than the hydrogen balloon.
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21. Consider the Lewis structure for glycine, the simplest amino acid:
(3 pts.) a) What are the approximate bond angles x, y and z?
x = 109.5, y > 120, z < 120
What are the orbital hybridizations of:
(2 pts.) b) the nitrogen atom?
sp3
(2 pts.) c) the carbon atom bonded to the nitrogen?
sp3
(2 pts.) d) the carbon atom bonded to the two oxygen atoms?
(2 pts.) e) the oxygen atom double bonded to a carbon atom?
sp2
sp2
(2 pts.) f) the oxygen atom bonded to both a hydrogen and a carbon atom?
sp3
(2 pts.) g) What is the total number of σ bonds in the glycine molecule?
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(2 pts.) h) What is the total number of π bonds in the glycine molecule?
1
21. A gas made up of C and O was found to effuse at a rate of 1.21 times that of Cl2 .
a) What is the identity of the unknown gas? (7 pts)
rateCl 2
ratex
=
mx = m Cl2
mx
1
=
m Cl2 1.21
 1 2
 1 2
•
 = 71• 
 = 48
 1.21
 1.21
thus, the only molecule with C and O that weighs close to 48 is CO2
€
4
b) At standard temperature and pressure (STP) what is the density (in g/L) of the
gas (6 pts)
ρ=
€
P • MW
1atm • 44g / mol
=
= 1.98g / L
R•T
(0.0814L • atm / K • mol) • 273K
22. Automobile air bags use the decomposition of sodium azide as their source of gas
for rapid inflation:
2NaN3 (s) → 2Na (s) + 3N 2 (g)
How many grams of NaN3 are required to provide 40.0 L of N 2 at 25o C and 763
torr? (10 pts)
 1atm 
763torr • 
 • 40L
PV
 760torr 
n=
=
= 1.66mol N 2
RT (0.0814L • atm / K • mol) • 298K
 2mol NaN 3   65g NaN 3 
1.66mol N 2 • 
 •
 = 71.7 g NaN 3
 3mol N 2   mol NaN 3 
€
(10 pts.) EXTRA CREDIT. For the following number of pairs of electrons, give all the
possible molecular geometries.
Number of electron pairs
Possible Molecular Geometry
a)
3
1) trigonal planar
2) bent
b)
4
1) tetrahedral
2) pyramidal
3) bent
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USEFUL EQUATIONS:
ln P =
− ΔHvap
RT
µ =
3RT
M
E=
− µq
4 πε or2
+C
En =
λ =
− RH
n2
h
mv
ΔxΔ (mv) > h
Useful Constants:
Atomic mass unit
Avogadro's number
Boltzmann's constant
Electron Charge
Faraday's constant
Gas constant
1 amu
N
k
e
F
R
Planck's constant
Speed of light
h
c
= 1.66054 X 10-27 kg
= 6.0221367 X 1023 /mol
= 1.38066 X 10-23 J/K
= 1.6021773 X 10-19 C
= Ne = 9.6485309 X 104 C/mol
=Nk = 8.314510 J/K-mol
= 0.08205783 L-atm/mol-K
= 6.6260755 X 10-34 J-s
= 2.99792458 X 108 m/s
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