Chemistry 400
Chapter 5 Homework Problems
1. In the olden days, the pennies were pure copper but copper got so expensive that a pure copper penny was worth more than 1 cent.
Modern pennies are composed of zinc coated with copper. After making several scratches in a penny, a student determines the mass of
a penny to be 2.482 g. The student puts the penny into hydrochloric acid, where the zinc (but not the copper) reacts with hydrochloric
acid via a single replacement reaction. The hydrogen is collected at 25°C over water. The collected gas occupies 0.899 L at a total
pressure of 791 mm Hg. Calculate the percent zinc (by mass) in the penny. (Assume that all of the zinc in the penny dissolves.) Show
all of your work to receive credit for this problem.
2. In the "olden" days (pre-1982), pennies were pure copper, but copper got so expensive that a pure copper penny was worth more
than 1 cent. Modern pennies are composed of zinc coated with copper. After making several scratches in a penny student determines
the mass of a penny to be 2.513 g. The student puts the penny into concentrated nitric acid, where the copper (but not the zinc) reacts
with nitric acid via an oxidation-reduction reaction to produce NO2, a poisonous brown gas and H2O. The NO2 is collected at 25°C
over water. The collected gas occupies 54.0 mL at a total pressure of 791 mm Hg. Show all of your work to receive credit for this
problem.
A. Determine the oxidation number of each element in each chemical species.
Cu(s) + 4 HNO3(aq) ——> Cu(NO3)2(aq) + 2 NO2(g) + 2 H2O(l)
B. Calculate the percent copper (by mass) in the penny. (Assume that all of the copper in the penny dissolves.)
3. In a common classroom demonstration, a balloon is filled with air and submerged in liquid nitrogen. The balloon contracts as the
gases in the balloon cool. Suppose the balloon initially contains 2.95 L of air at a temperature of 25.0°C and a pressure of 0.998 atm.
Calculate the expected volume of the balloon upon cooling to –196°C (the boiling point of liquid nitrogen). When the demonstration is
carried out, the actual volume of the balloon decreases to 0.61 L. How does the observed volume of the balloon compare to your
calculated value? Why?
4. Hydrogen gas (a potential future fuel) can be formed by the reaction of methane with water according to the equation:
CH4(g) + H2O(g) → CO(g) + 3 H2(g)
In a particular reaction, 25.5 L of methane gas (measured at a pressure of 732 torr and a temperature of 25°C) mixes with 22.8 L of
water vapor (measured at a pressure of 702 torr and a temperature of 125°C). The reaction produces 26.2 L of hydrogen gas at STP.
What is the percent yield of the reaction?
5. You have a 965 mL sample of methane at 92ºC and a pressure of 1.29×103 torr.
A. If you lower the temperature to –43ºC without changing the volume, what pressure will the methane exert?
B. Using the results from part A, if you then decrease the volume to 623 mL while keeping the temperature at –43ºC, what
pressure will the methane exert?
C. Using the results from part B, if you now increase the temperature to 83ºC while keeping the pressure constant, what will
the volume of the methane be?
D. If you add 1.2 moles of oxygen to the methane in part C while keeping temature and volume constant, what will be the
partial pressure of methane, the partial pressure of oxygen, and the total pressure?
E. What is the mass of the oxygen?
6. You have a 455 mL sample of oxygen at 18ºC and a pressure of 739 torr.
A. If you lower the temperature to –61ºC without changing the volume, what pressure will the oxygen exert?
B. Using the results from part A, if you then increase the volume to 623 mL while keeping the temperature at –61ºC, what
pressure will the oxygen exert?
C. Using the results from part B, if you then increase the temperature to 83ºC while keeping the pressure constant, what will
the volume of the oxygen be?
D. If you add 2.6 moles of nitrogen to the oxygen in part C while keeping temperature and volume constant, what will be the
partial pressure of oxygen, the partial pressure of nitrogen, and the total pressure?
E. What is the mass of the oxygen?
7. A compound contains 88.8% carbon and 11.2% hydrogen. At 150ºC and 565 torr, this compound is a gas with a density of 2.32 g/L.
Determine the molecular formula of this compound.
27
8. Consider a 0.480 gram sample of O2(g):
A. Calculate the volume of the oxygen at a pressure of 1.00 atm and a temperature of 0.0°C.
B. What would be the volume of the oxygen if it is collected over water at PO2 = 740 torr and 20ºC?
C. If the oxygen is injected into a 2.0 L cylinder at 20ºC, what will be its pressure?
D. When injected into a 4.0 L cylinder, the oxygen exerted a pressure of 114 torr. What was the temperature of the oxygen?
E. A porous container was filled with the oxygen. At the end of one hour, 250 mL of the oxygen had escaped. What volume
of hydrogen gas (H2) would have escaped from the same container during the same time under the same conditions?
F. The oxygen was mixed with 0.360 moles of H2(g) in a 1.0 L box at 0ºC. What was the pressure of the gas mixture in the
box?
9. A sample of N2O gas has a density of 2.85 g/L at 298 K. What is the pressure of the gas (in mm Hg)?
10. According to the 2000 edition of the Guinness Book Of World Records, ethanethiol is the "smelliest substance" in existence. Its
can be smelled in as little as 0.36 parts per billion (ppb). 1 ppb = 1×10–9 g/mL. What is the pressure of the gas (in mm Hg) at this
concentration? The molecular formula for ethanethiol is C2H6S. Assume rom temperature conditions.
11. A flask at room temperature contains exactly equal amounts (in moles) of nitrogen and xenon.
A. Which of the two gases exerts more pressure?
B. The molecules or atoms of which gas have the greater velocity?
C. The molecules or atoms of which gas have the greater average kinetic energy?
D. If a small hole were opened in the flask, which gas would effuse more quickly?
12. A flask at room temperature contains exactly equal amounts (in grams) of nitrogen and xenon.
A. Which of the two gases exerts more pressure?
B. The molecules or atoms of which gas have the greater velocity?
C. The molecules or atoms of which gas have the greater average kinetic energy?
D. If a small hole were opened in the flask, which gas would effuse more quickly?
13. Calculate the partial pressures (in torr) of the four most abundant gases in dry air (see Table) assuming that the atmospheric
pressure is 752.00 torr.
14. The apparatus below contains methane and oxygen at a temperature of 373 K. The valve in the center is opened and the methane
reacts with the oxygen.
CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(g)
Solve for the final partial pressures of all species (that are not used up!) and the final total pressure.
volume = 1.164 L
PCH4 = 10 atm
volume = 0.827 L
PO2 = 18 atm
28
15. A chemist wishes to study _________________ (get your gas name below) at high pressures and low temperatures.
A. Why are these conditions where a gas would act nonideally?
B. Use the ideal gas law to calculate the number of moles of your gas required to produce a pressure of 200.0 atm in a 2.000
L container at 200.0°C above your gases boiling point.
C. Repeat the calculation in part a, but use the van der Waals equation to obtain a more accurate value. This may need to be
solved iteratively. To solve this equation iteratively, ask your instructor how.
D. What is the percent error in the ideal gas law estimate? (Assume that the van der Waals value is correct.)
If your last name starts
with …
A
B
C
D
E, F
G
H
I, J
K
L
M
N
O
P
Q, R
S
T
U, V
W, X
Y, Z
a
(L2atm/mol2)
17.82
20.16
14.09
17.81
4.448
4.225
1.363
18.24
28.94
14.66
3.64
11.77
1.505
19.7483
6.579
25.77
11.05
7.57
7.769
23.11
Compound
Acetic acid
Acetic anhydride
Acetone
Acetonitrile
Acetylene
Ammonia
Argon
Benzene
Bromobenzene
Butane
Carbon dioxide
Carbon disulfide
Carbon monoxide
Carbon tetrachloride
Chlorine
Chlorobenzene
Chloroethane
Chloromethane
Cyanogen
Cyclohexane
29
b (L/mol)
0.1068
0.1263
0.0994
0.1168
0.05136
0.03707
0.03219
0.1154
0.1539
0.1226
0.04267
0.07685
0.03985
0.1281
0.05622
0.1453
0.08651
0.06483
0.06901
0.1424