Download Chapters 13 and 14

Chapters 13 and 14
1. 1980 #3
A solution containing 3.23 grams of an unknown compound dissolved in 100.0 grams of water
freezes at -0.97 °C. The solution does not conduct electricity.
a. Calculate the molecular weight of the compound. (The molal freezing point depression
constant for H2O is 1.86 °C kg mole¯1.)
b. Elemental analysis of this unknown compound yields the following percentages by
weight: H = 9.74%; C = 38.70%; O = 51.56%. Determine the molecular formula of the
compound.
c. Complete combustion of a 1.05-gram sample of the compound with the stoichiometric
amount of oxygen gas produces a mixture of H2O(g) and CO2(g). What is the pressure of
this gas when it is contained in a 3.00-liter flask at 127 °C?
2. 1982 #8
Describe a laboratory procedure needed to carry out each of the following.
a. Separate a mixture of powdered solid CaCl2 and CaCO3.
b. Determine the concentration of solute in an aqueous sodium chloride solution and give
the concentration units that your method provides.
c. Separate a mixture of two volatile liquids.
3. 1984 #4 a d
Give a scientific explanation for the following observations. Use equations of diagrams if they
are relevant.
a. It takes longer to cook an egg until it is hard-boiled in Denver (altitude 1 mile above sea
level) than it does in New York City (near sea level).
d. The addition of antifreeze to water in a radiator decreases the likelihood that the liquid in
the radiator will either freeze or boil.
4. 1984 #8
Discuss some differences in physical and chemical properties of metals and nonmetals. What
characteristic of the electronic configurations of atoms distinguishes metals from nonmetals?
On the basis of this characteristic, explain why there are many more metals than nonmetals.
5. 1985 #3
The formula and the molecular weight of an unknown hydrocarbon compound are to be
determined by elemental analysis and the freezing-point depression method.
a. The hydrocarbon is found to contain 93.46 percent carbon and 6.54 percent hydrogen.
Calculate the empirical formula of the unknown hydrocarbon.
b. A solution is prepared by dissolving 2.53 grams of p-dichlorobenzene (molecular weight
147.0) in 25.86 grams of naphthalene (molecular weight 128.2). Calculate the molality of
the p-dichlorobenzene solution.
c. The freezing point of pure naphthalene is determined to be 80.2 °C. The solution
prepared in (b) is found to have an initial freezing point of 75.7 °C. Calculate the molal
freezing-point depression constant of naphthalene.
d. A solution of 2.43 grams of the unknown hydrocarbon dissolved in 26.7 grams of
naphthalene is found to freeze initially at 76.2 °C. Calculate the apparent molecular
weight of the unknown hydrocarbon on the basis of the freezing-point depression
experiment above.
e. What is the molecular formula of the unknown hydrocarbon?
6. 1985 #9
Substance
Melting Point, °C
H2
-129
C3H8
-190
HF
-92
CsI
621
LiF
870
SiC
>2,000
a. Discuss how the trend in the melting points of the substances tabulated above can be
explained in terms of the types of attractive forces and/or bonds in these substances.
b. For any pairs of substances that have the same kind(s) of attractive forces and/or bonds,
discuss the factors that cause variation in the strengths of the forces and/or bonds.
7. 1986 #5
The first ionization energy of sodium is +496 kilojoules per mole, yet the standard heat of
formation of sodium chloride from its elements in their standard state is -411 kilojoules per
mole.
a. Name the factors that determine the magnitude of the standard heat of formation of solid
sodium chloride. Indicate whether each factor makes the reaction for the formation of
sodium chloride from its elements more or less exothermic.
b. Name the factors that determine whether the reaction that occurs when such a salt
dissolves in water is exothermic or endothermic and discuss the effect of each factor on
the solubility.
8. 1986 #8
Give a scientific explanation for each of the following observations. Use equations or
diagrams if they seem relevant.
a. Graphite is used to make electrodes, while diamond, another allotrope of carbon, is a
very poor conductor of electricity.
b. Putting rock salt on an icy driveway melts the ice even when the air temperature is -10
°C.
c. A hot-air balloon must be much larger than a helium-filled balloon in order to lift the same
weight.
d. Carbon dioxide, rather than a stream of water, should be used to extinguish an oil fire.
9. 1987 #7a-b
In 1884 the Swedish chemist Svante Arrhenius proposed that salts dissociate into two or
more separate, independent, ionic fragments when they dissolve in water.
a. Give one piece of experimental evidence that more than 1 mole of particles is formed
when 1 mole of a salt dissolves in water.
b. Give one piece of experimental evidence that the particles formed when a salt dissolves
in water are charged.
10. 1987 #8b
When crystals of barium hydroxide, Ba(OH)2 . 8H2O, are mixed with crystals of ammonium
thiocyanate, NH4SCN, at room temperature in an open beaker, the mixtures liquefies, the
temperature drops dramatically, and the odor of ammonia is detected. The reaction that
occurs is the following:
Ba(OH)2 . 8H2O(s)+2 NH2SCN(s)  Ba2++2SCN¯ +2 NH3(g)+10H2O(l)
If the beaker in which the reaction is taking place is put on a block of wet wood, the water on
the wood immediately freezes and the beaker adheres to the wood. Yet the water inside the
beaker, formed as the reaction proceeds, does not freeze even though the temperature of the
reaction mixture drops to -15 °C. Explain these observations.
11. 1988 #5a-c
Using principles of chemical bonding and/or intermolecular forces, explain each of the
following.
a. Xenon has a higher boiling point than neon has.
b. Solid copper is an excellent conductor of electricity, but solid copper chloride is not.
c. SiO2 melts at a very high temperature, while CO2 is a gas at room temperature, event
though Si and C are in the same chemical family.
12. 1988 #8
The normal boiling and freezing points of argon are 87.3 K and 84.0 K, respectively. The
triple point is at 82.7 K and 0.68 atmosphere.
a. Use the data above to draw a phase diagram for argon. Label the axes and label the
regions in which the solid, liquid, and gas phases are stable. On the phase diagram,
show the position of the normal boiling point.
b. Describe any changes that can be observed in a sample of solid argon when the
temperature is increases from 40 K to 160 K at a constant pressure of 0.50 atmosphere.
c. Describe any changes that can be observed in a sample of liquid argon the pressure is
reduced from 10 atmospheres to 1 atmosphere at a constant temperature of 100 K,
which is well below the critical temperature.
d. Does the liquid phase of argon have a density greater than, equal to, or less than the
density of the solid phase? Explain your answer, using information given in the
introduction to this question.
13. 1989 #6
The melting points of the alkali metals decrease from Li to Cs. In contrast, the melting of the
halogens increase from F2 to I2.
a. Using bonding principles, account for the decrease in the melting point of the alkali
metals.
b. Using bonding principles, account for the increase in the melting points of the halogens.
c. What is the expected trend in the melting points of the compounds LiF, NaCl, KBr, and
CsI? Explain this trend using bonding principles.
14. 1991 #2
The molecular formula of a hydrocarbon is to be determined by analyzing its combustion
products and investigating its colligative properties.
a. The hydrocarbon burns completely, producing 7.2 grams of water and 7.2 liters of CO 2 at
standard conditions.
b. Calculate the mass in grams of O2 required for the complete combustion of the sample of
the hydrocarbon described in (a).
c. The hydrocarbon dissolves readily in CHCl3. The freezing point of a solution prepared by
mixing 100. grams of CHCl3 and 0.600 gram of the hydrocarbon is -64.0 °C. The molal
freezing-point depression constant of CHCl3 is 4.68 °C / molal and its normal freezing
point is -63.5 °C. Calculate the molecular weight of the hydrocarbon.
d. What is the molecular formula of the hydrocarbon?
15. 1991 #8
Experimental data provide the basis for interpreting differences in properties of substances.
Table 1
Compound
Melting Point
Electrical conductivity of
(°C)
molten state
BeCl2
405
0.086
MgCl2
714
>20
SiCl4
-70
0
MgF2
1261
>20
Substance
F2
Br2
N2
Table 2
Bond Length (angstroms)
1.42
2.28
1.09
Account for the differences in properties given in Tables 1 and 2 above in terms of the
differences in structure and bonding in each of the following pairs.
a. MgCl2 and SiCl4
b. MgCl2 and MgF2
c. F2 and Br2
d. F2 and N2
16. 1992 #8
Explain each of the following in terms of atomic and molecular structures and/or
intermolecular forces.
a. Solid K conducts an electric current, whereas solid KNO3 does not.
b. SbCl3 has a measurable dipole moment, whereas SbCl5 does not.
c. The normal boiling point of CCl4 is 77 °C, whereas that of CBr4 is 190 °C.
d. NaI(s) is very soluble in water whereas I2(s) has a solubility of only 0.03 gram per 100
grams of water.
17. 1993 #2
Elemental analysis of an unknown pure substance indicates that the percent composition by
mass is as follows:
Carbon - 49.02%
Hydrogen - 2.743%
Chlorine - 48.23%
A solution that is prepared by dissolving 3.150 grams of the substance in 25.00 grams of
benzene, C6H6, has a freezing point of 1.12°C. (The normal freezing point of benzene is
5.50°C and the molal freezing-point depression constant, Kf, for benzene is 5.12 C°/molal.)
a. Determine the empirical formula of the unknown substance.
b. Using the data gathered from the freezing point depression method, calculate the molar
mass of the unknown substance.
c. Calculate the mole fraction of benzene in the solution described above.
d. The vapor pressure of pure benzene at 35°C is 150. millimeters of Hg. Calculate the
vapor pressure of benzene over the solution described above at 35°C.
18. 1994 #5 b and d
Discuss the following phenomena in terms of the chemical and physical properties of the
substances involved and general principles of chemical and physical change.
b. A bell jar is connected to a vacuum pump containing an open beaker of water. As the air
pressure under the bell jar decreases, what behavior of the water in the beaker will be
observed? Explain why this occurs.
d. A water solution of I2 is shaken with an equal volume of a nonpolar solvent such as TTE
(trichlorotrifluoroethane). Describe the appearance of the system after shaking. Account
for the observation.
19. 1994 #8 a-c
For each of the following, use appropriate chemical principles to explain the observation.
a. Sodium chloride may be spread on an icy sidewalk in order to melt the ice; equimolar
amounts of calcium chloride are even more effective.
b. At room temperatue, NH3 is a gas and H2O is a liquid, even though NH3 has a molar
mass of 17 grams and H2O has a molar mass of 18 grams.
c. C (graphite) is used as a lubricant, whereas C (diamond) is used as an abrasive.
20. 1995 #5
The conductivity of several substances was tested. The results of the tests are summarized
in the following data table.
AgNO3
Sucrose
Na
H2SO4
Melting point (°C) 212
185
99
Liquid at room
temperature
Liquid
Good conductor
Nonconductor
Good conductor
Poor conductor
Water solution
Good conductor
Nonconductor
Good conductor
Good conductor
(dissolves,
(conduction
accompanied by
increases as the
the evolution of a acid is added to
gas)
water)
Solid
nonconductor
nonconductor
conductor
Not tested
Using models of chemical bonding and atomic or molecular structure, account for the
differences in conductivity between the two samples in each of the following pairs.
a. Sucrose solution and silver nitrate solution
b. Solid silver nitrate and solid sodium metal
c. Liquid (fused) sucrose and liquid (fused) silver nitrate
d. Liquid (concentrated) sulfuric acid and sulfuric acid solution
21. 1995 #6
Pressure (atm)
W
V
1.0
X
Y
Z
Temperature
The phase diagram for a pure substance is shown above. Use this diagram and your
knowledge about changes of phase to answer the following questions.
a. What does point V represent? What characteristics are specific to the system only at
point V?
b. What does each point on the curve between V and W represent?
c. Describe the changes that the system undergoes as the temperature slowly increases
from X to Y to Z at 1.0 atmosphere.
d. In a solid-liquid mixture of this substance, will the solid float or sink? Explain.
22. 1996 #4
Concentrated sulfuric acid (18.4-molar H2SO4) has a density of 1.84 grams per milliliter. After
dilution with water to 5.20-molar, the solution has a density of 1.38 grams per milliliter and
can be used as an electrolyte in lead storage batteries for automobiles.
a. Calculate the volume of concentrated acid required to prepare 1.00 liter of 5.20-molar
H2SO4.
b. Determine the mass percent of H2SO4 in the original concentrated solution.
c. Calculate the volume of 5.20-molar H2SO4 that can be completely neutralized with 10.5
grams of sodium bicarbonate NaHCO3.
d. What is the molality of the 5.20-molar H2SO4?
23. 1996 #9 a
Explain each of the following in terms of the electronic structure and/or bonding of the
compounds involved.
a. At ordinary conditions, HF (normal boiling point = 20°C) is a liquid, whereas HCl (normal
boiling point = -114°C) is a gas.
24. 1998 #2
An unknown compound contains only the three elements C, H, and O. A pure sample of the
compound is analyzed and found to be 65.60 percent C and 9.44 percent H by mass.
a. Determine the empirical formula of the compound.
b. A solution of 1.570 grams of the compound in 16.08 grams of camphor is observed to
freeze at a temperature 15.2 °C below the normal freezing point of pure camphor.
Determine the molar mass and apparent molecular formula of the compound. (The molal
freezing-point depression constant, Kf, for camphor is 40.0 kg-K-mol¯1.)
c. When 1.570 grams of the compound is vaporized at 300 °C and 1.00 atmosphere, the
gas occupies a volume of 577 milliliters. What is the molar mass of the compound based
on this result?
d. Briefly describe what occurs in solution that accounts for the difference between the
results obtained in parts (b) and (c).
25. 1998 #9
Answer each of the following using appropriate chemical principles.
a. Why does it take longer to cook an egg in boiling water at high altitude than it does at sea
level?
b. When NH3 gas is bubbled into an aqueous solution of CuCl2, a precipitate forms initially.
On further bubbling, the precipitate disappears. Explain these two observations.
c. Dimethyl ether, H3C-O-CH3, is not very soluble in water. Draw a structural isomer of
dimethyl ether that is much more soluble in water and explain the basis of its increased
water solubility.
d. Identify a chemical species that is
i.
capable of oxidizing Cl¯(aq) under standard conditions
ii.
capable of reducing Cl2(aq) under standard conditions.
In each case, justify your choice.
26. 2000 #5.
The molar mass of an unknown solid, which is nonvolatile and a nonelectrolyte, is to be
determined by the freezing-point depression method. The pure solvent used in the
experiment freezes at 10˚C and has a known molal freezing-point depression constant, Kf,.
Assume that the following materials are also available.
Test tubes
Stirrer
Pipet
Thermometer
Balance
Beaker
Stopwatch
Graph paper
Hot water bath
ice
a. Sketch cooling curves for (i) the pure solvent and for (ii) the solution as each is cooled
from 20˚C to 0.0˚C on two separate temperature versus time graphs.
b. Information from these graphs may be used to determine the molar mass of the unknown
solid.
i.
Describe the measurements that must be made to determine the molar mass of the
unknown solid by this method.
ii.
Show the setup(s) for the calculation(s) that must be performed to determine the
molar mass of the unknown solid from the experimental data.
iii.
Explain how the difference(s) between the two graphs in part (a) can be used to
obtain information needed to calculate the molar mass of the unknown solid.
c. Suppose that during the experiment a significant but unknown amount of solvent
evaporates from the test tube. What effect would this have on the calculated value of the
molar mass of the solid (i.e., too large, too small, or no effect?) Justify your answer.
d. Show the setup for the calculation of the percentage error in a student’s result if the
student obtains a value of 126 g mol-1 for the molar mass of the solid when the actual
value is 120. g mol-1.
27. 2001 #5a
Answer the questions below that relate to the following five 0.10 M aqueous solutions at
25˚C:
Pb(NO3)2, NaCl, KMnO4, C2H5OH, KC2H3O2
a. Which solution has the highest boiling point? Explain
28. 2001 #8
Account for each of the following observations about pairs of substances. In your answers,
use appropriate principles of chemical bonding and/or intermolecular forces. In each part,
your answer must include references to both substances.
a. Even though NH3 and CH4 have similar molecular masses, NH3 has a much higher
normal boiling point (-33˚C) than CH1 (-164˚C).
b. At 25˚C and 1.0 atm, ethane (C2H6) is a gas and hexane (C6H14) is a liquid.
c. Si melts at a much higher temperature (1410˚C) than Cl2 (-101˚C)
d. MgO melts at a much higher temperature (2852 ˚C) than NaF (993˚C)
29. 2002 #6d
Use the principles of atomic structure and/or chemical bonding to explain each of the
following. In each part, your answer must include references to both substances.
d. The boiling point of Cl2 is lower than the boiling point of Br2.
30. 2003 #6.
For each of the following, use appropriate chemical principles to explain the observation.
Include chemical equations as appropriate.
a. In areas affected by acid rain, statues and structures made of lime stone (calcium
carbonate) often show signs of considerable deterioration.
b. When table salt (NaCl) and sugar (C12H22O11) are dissolved in water, it is observed that
i.
Both solutions have a higher boiling point than pure water, and
ii.
The boiling point of 0.10 M NaCl (aq) is higher than that of 0.10 M C12H22O11 (aq).
c. Methane gas does not behave as an ideal gas at low temperatures and high pressure.
d. Water droplets form on the outside of a beaker containing an ice bath.
31. 2004 #7 a-b; d
Use appropriate chemical principles to account for each of the following observations. In
each part, your response must include specific information about both substances.
a. At 25°C and 1 atm, F2 is a gas, whereas I2 is a solid.
b. The melting point of NaF is 993 °C, whereas the melting point of CsCl is 645°C.
d. Ammonia, NH3, is a very soluble in water, whereas phosphine, PH3, is only moderately
soluble in water.
32. 2005 #2
Answer the following question about a pure compound that contains only carbon, hydrogen,
and oxygen.
a. A 0.7549 g sample of the compound burns in O2 (g) to produce 1.9061 g of CO2 (g) and
0.3370 g of H2O (g).
i.
Calculate the individual masses of C, H, and O in the 0.7549 g sample.
ii.
Determine the empirical formula for the compound.
b. A 0.5246 g sample of the compound was dissolved in 10.0012 g of lauric acid, and it was
determined that the freezing point of the lauric acid was lowered by 1.68 ˚C. The value of
Kf of lauric acid is 3.90˚Cm-1. Assume the compound does not dissociate in lauric acid.
i.
Calculate the molality of the compound dissolved in the lauric acid.
ii.
Calculate the molar mass of the compound from the information provided.
c.
Without doing any calculations, explain how to determine the molecular formula of the
compound based on the answers to parts (a)(ii) and (b)(ii).
d. Further tests indicate that a 0.10 M aqueous solution of the compound has a pH of 2.6.
Identify the organic functional group that accounts for this pH.
33. 2005 #7 a-b
Use principles of atomic structure, bonding, and/or intermolecular forces to respond to each
of the following. Your responses must include specific information about all substances
referred to in each question.
a. At a pressure of 1 atm, the boiling point of NH3 (l) is 240 K, whereas the boiling point of
NF3 (l) is 144 K.
i.
Identify the intermolecular force(s) in each substance.
ii.
Account for the difference in the points of the substances.
b. The melting point of KCl (s) is 776 ˚C, whereas the melting point of NaCl (s) is 801 ˚C.
i.
Identify the type of bonding in each substance.
ii.
Account for the difference in the melting points of the substances.
34. 2005 #8 d-e
Use principles of atomic structure, bonding, and intermolecular forces to answer the following
questions. Your responses must include specific information about all substances referred to
in each part.
H
H
H
O
|
|
|
||
H - C - C - C - H
|
|
|
H
H
H
C
H
H
O
d. The complete structural formulas of propane, C3H8, and methanoic acid, HCOOH, are
shown above. In the table below, write the type(s) of intermolecular attractive force(s)
that occur in each substance.
Substance
Boiling Point
Intermolecular Attractive Force(s)
Propane
229
Methanoic acid
374
e. Use principles of intermolecular attractive forces to explain why methanoic acid has a
higher boiling point than propane.
35. 2006 #6 a-c
a. The structures for glucose, C6H12O6, and cyclohexane, C6H12, are shown below.
OH
H
H
OH
H
H
H
H
H
H
OH
H
H
H
HO
OH
H
H
H
OH
H
H
Identify the type(s) of intermolecular attractive forces in
i.
Pure glucose
ii.
Pure cyclohexane
b. Glucose is soluble I water by cyclohexane is not soluble in water. Explain.
c. Consider the two processes represented below.
Process 1: H2O(l)  H2O(g)
ΔH°= +44.0 kJ/mol
Process 2: H2O(l)  H2(g) + 1/2 O2(g) ΔH°= +286 kJ/mol
i.
ii.
For each of the two processes, identify the type(s) of intermolecular or intramolecular
attractive forces that must be overcome for the process to occur.
Indicate whether you agree or disagree with the statement in the box below. Support
your answer with a short explanation.
When water boils, H2O molecules break apart to form
hydrogen molecules and oxygen molecules.