Unlock document.
This document is partially blurred.
Unlock all pages and 1 million more documents.
Get Access
solution | Raoult's law MSC: Conceptual
60. A liquid-liquid solution is called an ideal solution if:
I.
It obeys PV = nRT.
II.
It obeys Raoult's law.
III.
Solute-solute, solvent-solvent, and solute-solvent interactions are very similar.
IV.
Solute-solute, solvent-solvent, and solute-solvent interactions are quite
different.
A)
I, II, III
B)
I, II, IV
C)
II, III
D)
II, IV
E)
I, III, IV
61. Liquid A and liquid B form a solution that behaves ideally according to Raoult's law. The
vapor pressures of the pure substances A and B are 233 torr and 135 torr, respectively.
Determine the vapor pressure over the solution if 1.21 moles of liquid A is added to 5.30
moles of liquid B.
A)
153 torr
B)
188 torr
C)
215 torr
D)
760 torr
E)
43.3 torr
62. Two liquids form a solution and release a quantity of heat. How does the pressure above the
solution compare to that predicted by Raoult's law?
A)
It will be greater.
B)
It will be less.
C)
It will be the same.
D)
It will show positive deviation.
E)
None of these.
63. Consider a solution containing liquids A and B where the mole fraction of B is 0.60.
Assuming ideality, calculate the mole fraction of A in the vapor at equilibrium with this
solution at 25°C. (The vapor pressures of pure liquid A and B at 25°C are 117.7 torr and
400.0 torr, respectively.)
A)
0.16
B)
196
C)
0.31
D)
3.40 10–3
E)
4.90 10–3
64. Consider a solution containing liquids A and B where the mole fraction of B is 0.60.
Assuming ideality, calculate the mole fraction of B in the vapor at equilibrium with this
solution at 25°C. (The vapor pressures of pure liquid A and B at 25°C are 129.4 torr and
400.0 torr, respectively.)
A)
0.18
B)
0.22
C)
0.33
D)
0.78
E)
0.82
65. A solution with a positive enthalpy of solution (Hsoln) is expected to show positive
deviations from Raoult's law.
66. Determine the mass of a nonvolatile, nonionizing compound that must be added to 3.83 kg
of water to lower the freezing point to 98.70°C. The molar mass of the compound is 50.0
g/mol and the Kf for water is 1.86°C kg/mol.
A)
203 g
B)
134 g
C)
463 g
D)
9.12 g
E)
18.7 g
67. The freezing point (Tf) for t-butanol is 25.50°C and Kf is 9.1°C/m. Usually t-butanol absorbs
water on exposure to the air. If the freezing point of a 12.9-g sample of t-butanol is
measured as 24.59°C, how many grams of water are present in the sample?
A)
0.10 g
B)
0.023 g
C)
10. g
D)
2.3 g
E)
23 g
68. The molar mass of a solid as determined by freezing point depression is 10% higher than the
true molar mass. Which of the following experimental errors could not account for this
discrepancy?
A)
Not all the solid was dissolved.
B)
More than the recorded amount of solvent was pipetted into the solution.
C)
The solid dissociated slightly into two particles when it dissolved.
D)
Some solid was left on the weighing paper.
E)
Before the solution was prepared, the container was rinsed with solvent and not
dried.
69. A solution consisting of 0.250 mol of methylbenzene, C6H5CH3, in 244 g of nitrobenzene,
C6H5NO2, freezes at –1.2°C. Pure nitrobenzene freezes at 6.0°C. What is the freezing-point
depression constant of nitrobenzene?
A)
4.8°C/m
B)
3.5°C/m
C)
29°C/m
D)
7.0°C/m
E)
14°C/m
70. A 6.06-gram sample of a compound is dissolved in 250. grams of benzene. The freezing
point of this solution is 1.02°C below that of pure benzene. What is the molar mass of this
compound? (Note: Kf for benzene = 5.12°C/m.) Ignore significant figures for this problem.
A)
30.4 g/mol
B)
122 g/mol
C)
243 g/mol
D)
4.83 g/mol
E)
60.8 g/mol
71. Thyroxine, an important hormone that controls the rate of metabolism in the body, can be
isolated from the thyroid gland. If 0.455 g of thyroxine is dissolved in 10.0 g of benzene, the
freezing point of the solution could be measured as 5.144°C. Pure benzene freezes at
5.444°C and has a value for the molal freezing point depression constant of Kf of 5.12°C/m.
What is the approximate molar mass of thyroxine?
A)
7.77 105 g/mol
B)
777 g/mol
C)
7.77 g/mol
D)
11.3 g/mol
E)
42.8 g/mol
72. When a 49.1-g sample of an unknown compound is dissolved in 500.g of benzene, the
freezing point of the resulting solution is 3.77°C. The freezing point of pure benzene is
5.48°C and Kf for benzene is 5.12°C/m. Calculate the molar mass of the unknown
compound.
A)
147 g/mol
B)
28.7 g/mol
C)
251 g/mol
D)
588 g/mol
E)
294 g/mol
73. To calculate the freezing point of an ideal dilute solution of a single, nondissociating solute
of a solvent, the minimum information one must know is:
A)
the molality (of the solute)
B)
the molality (of the solute) and the freezing point depression constant of the
solvent
C)
the same quantities as in B, plus the freezing point of the pure solvent
D)
all of the quantities in C, plus the molecular weight of the solute
E)
all of the quantities in C, plus the weight of the solvent
74. Which of the following will cause the calculated molar mass of a compound determined by
the freezing-point depression method to be greater than the true molar mass?
A)
Water gets into the solvent after the freezing point of the pure solvent is
determined.
B)
Some of the solute molecules break apart.
C)
The mass of solvent is smaller than determined from the weighing.
D)
While adding the solute, some was spilled on the lab bench.
E)
All of the above.
75. Determine the change in boiling point for 397.7 g of carbon disulfide (Kb = 2.34°C kg/mol)
if 35.0 g of a nonvolatile, nonionizing compound is dissolved in it. The molar mass of the
compound is 70.0 g/mol and the boiling point of the pure carbon disulfide is 46.2°C.
A)
0.206 °C
B)
1.86 °C
C)
5.88 °C
D)
24.8 °C
E)
2.94 °C
76. What is the boiling point change for a solution containing 0.736 moles of naphthalene (a
nonvolatile, nonionizing compound) in 250. g of liquid benzene? (Kb = 2.53°C/m for
benzene)
A)
7.45 °C
B)
0.859 °C
C)
3.44 °C
D)
1.86 °C
E)
0.466 °C
77. A solute added to a solvent raises the boiling point of the solution because:
A)
The temperature to cause boiling must be great enough to boil not only the solvent
but also the solute.
B)
The solute particles lower the solvent's vapor pressure, thus requiring a higher
temperature to cause boiling.
C)
The solute particles raise the solvent's vapor pressure, thus requiring a higher
temperature to cause boiling.
D)
The solute increases the volume of the solution, and an increase in volume requires
an increase in the temperature to reach the boiling point (derived from PV = nRT).
E)
Two of the above are correct.
78. Adding salt to water decreases the freezing point of the water since it lowers the vapor
pressure of the ice.
79. When a nonvolatile solute is added to a volatile solvent, the solution vapor pressure
__________, the boiling point __________, the freezing point __________, and the osmotic
pressure across a semipermeable membrane __________.
A)
decreases, increases, decreases, decreases
B)
increases, increases, decreases, increases
C)
increases, decreases, increases, decreases
D)
decreases, decreases, increases, decreases
E)
decreases, increases, decreases, increases
80. All of the following are colligative properties except:
A)
osmotic pressure
B)
boiling point elevation
C)
freezing point depression
D)
density elevation
E)
none of these
81. A cucumber is placed in a concentrated salt solution. What will most likely happen?
A)
Water will flow from the cucumber to the solution.
B)
Water will flow from the solution to the cucumber.
C)
Salt will flow into the cucumber.
D)
Salt will precipitate out.
E)
No change will occur.
82. Polyethylene is a synthetic polymer or plastic with many uses. 1.36 g of a polyethylene
sample was dissolved in enough benzene to make 100. mL of solution, and the osmotic
pressure was found to be 1.86 torr at 25oC. What is the approximate molar mass of the
polyethylene?
A)
179 g/mol
B)
1.14 104 g/mol
C)
7.35 103 g/mol
D)
1.24 105 g/mol
E)
1.36 105 g/mol
83. A 0.2 molar solution of a solute, X, in benzene, displays an osmotic pressure given by the
formula = (0.1)RT. Which of the following is most likely to be the case?
A)
X exists in benzene as X.
B)
X exists in benzene as X2.
C)
X exists in benzene dissociated into two particles.
D)
This solution strongly deviates from ideal behavior.
E)
None of these is plausible.
84. Osmotic pressure depends on all but which of the following?
A)
atmospheric pressure
B)
the molarity of the solution
C)
temperature
D)
the ratio of moles of solute to solution volume
E)
none of these
85. A solution of water and a nonvolatile, nonionizing compound is placed in a tube with a
semipermeable membrane on one side. The tube is placed in a beaker of pure water. What
initial net effect will occur?
A)
Water will flow from the beaker to the tube.
B)
Water will flow from the tube to the beaker.
C)
The compound will pass through the membrane into the solution.
D)
Nothing will move through the membrane either way.
E)
Equilibrium is immediately established.
86. Determine the osmotic pressure of a solution that contains 0.048 g of a hydrocarbon solute
(molar mass = 340 g/mol) dissolved in benzene to make a 350-mL solution. The
temperature is 20.0°C.
A)
0.50 torr
B)
2.5 torr
C)
3.3 torr
D)
7.4 torr
E)
6.9 torr
87. Consider pure water separated from an aqueous sugar solution by a semipermeable
membrane, which allows water to pass freely but not sugar. After some time has passed, the
concentration of sugar solution:
A)
will have increased
B)
will have decreased
C)
will not have changed
D)
might have increased or decreased depending on other factors
E)
will be the same on both sides of the membrane
88. In osmosis:
A)
Knowing the osmotic pressure can help determine the molar mass of a solute
dissolved in a solvent.
B)
The semipermeable membrane is used to change the freezing and melting points of
a solution.
C)
Solutions cannot have identical osmotic pressures.
D)
Temperature does not affect the osmotic pressure of a solution.
E)
At least two of the above statements are correct.
89. What is reverse osmosis?
A)
the application, to a concentrated solution, of a pressure that is greater than the
osmotic pressure, such that solvent flows from the concentrated solution to the
dilute solution
B)
the application, to a dilute solution, of a pressure that is greater than the osmotic
pressure, such that solvent flows from the concentrated solution to the dilute
solution
C)
the application, to a concentrated solution, of a pressure that is greater than the
osmotic pressure, such that solute flows from the concentrated solution to the
dilute solution
D)
the application, to a dilute solution, of a pressure that is greater than the osmotic
pressure, such that solute flows from the concentrated solution to the dilute
solution
E)
the application, to a concentrated solution, of a pressure that is greater than the
osmotic pressure, such that solvent flows from the dilute solution to the
concentrated solution
90. Solutions that have identical osmotic pressures are called __________ solutions.
A)
hypertonic
B)
isotonic
C)
hypotonic
D)
hemolytic
E)
dyalitic
91. Calculate the molarity of a solution containing KCl and water whose osmotic pressure at
21.6°C is 125 torr. Assume complete dissociation of the salt.
A)
0.00680 M
B)
5.17 M
C)
0.0464 M
D)
0.00340 M
E)
0.0928 M
92. The observed van't Hoff factor for an electrolyte is less than the expected factor because of
__________.
A)
electrolytic repulsion
B)
complete dissociation
C)
coagulation
D)
ion pairing
E)
gelation
93. Which of the following solutions would have the highest osmotic pressure?
A)
0.2 M KBr, potassium bromide
B)
0.2 M MgBr2, magnesium bromide
C)
0.3 M CH3COOH, acetic acid
D)
0.3 M C6H12O6, glucose
E)
0.3 M C12H22O11, sucrose
94. Which of the following solutions has the lowest boiling point?
A)
0.15 M NaCl
B)
0.10 M MgBr2
C)
0.15 M Ba(NO3)2
D)
0.20 M C2H6O2
E)
0.10 M Fe(NO3)3
95. What is the freezing point of an aqueous 1.58 molal NaCl solution? (Kf = 1.86°C/m)
A)
–2.94°C
B)
2.94°C
C)
–5.88°C
D)
5.88°C
E)
0.00°C
96. What is the expected boiling point of a solution prepared by dissolving 7.27 g of sodium
iodide (NaI) in 68.6 g of water (H2O)? For water, Tb = 100.00oC and Kb = 0.512oC m–1.
A)
0.72oC
B)
100.36oC
C)
103.72oC
D)
100.72oC
E)
0.36oC
97. 292.5 g of NaCl completely dissolves (producing Na+ and Cl– ions) in 1.00 kg of water at
25.0°C. The vapor pressure of pure water at this temperature is 23.8 torr. Determine the
vapor pressure of the solution.
A)
20.2 torr
B)
21.8 torr
C)
18.7 torr
D)
18.0 torr
E)
23.8 torr
98. The vapor pressure of water at 90°C is 0.692 atm. What is the vapor pressure (in atm) of a
solution made by dissolving 4.13 mole(s) of CsF(s) in 1.00 kg of water? Assume that
Raoult's law applies.
A)
0.644 atm
B)
0.795 atm
C)
0.602 atm
D)
0.692 atm
E)
none of these
99. You have a 10.40-g mixture of table sugar (C12H22O11) and table salt (NaCl). When this
mixture is dissolved in 150. g of water, the freezing point is found to be –2.24°C. Calculate
the percent by mass of sugar in the original mixture.
A)
39.0%
B)
43.8%
C)
53.9%
D)
61.0%
E)
none of these
100. A 5.05-gram sample of a compound is dissolved in enough water to form 100.0 mL of
solution. This solution has an osmotic pressure of 25.0 torr at 25°C. If it is assumed that
each molecule of the solute dissociates into two particles (in this solvent), what is the molar
mass of this solute?
A)
9.88 101 g/mol
B)
6.30 103 g/mol
C)
3.75 103 g/mol
D)
7.51 104 g/mol
E)
none of these
101. Calculate the osmotic pressure (in torr) of 6.00 L of an aqueous 0.958 M solution at 30.°C, if
the solute concerned is totally ionized into three ions (e.g., it could be Na2SO4 or MgCl2).
A)
71.5 torr
B)
5.43 104 torr
C)
3.23 104 torr
D)
1.81 104 torr
E)
6.03 103 torr
102. A 0.20 M solution of MgSO4 has an observed osmotic pressure of 7.6 atm at 25°C.
Determine the observed van't Hoff factor for this experiment.
A)
19
B)
0.31
C)
1.6
D)
1.8
E)
2.0
103. The osmotic pressure of a 0.0100 M solution of NaCl in water at 25°C is found to be
different from 372 torr because:
A)
Osmotic pressures are hard to measure.
B)
Na+ and Cl– ions are strongly hydrated.
C)
Na+ and Cl– ions can form ion pairs.
D)
NaCl does not dissociate in water.
E)
None of these.
104. The most likely reason for colloidal dispersion is __________.
A)
the Tyndall effect
B)
coagulation
C)
precipitation
D)
emulsion formation
E)
electrostatic repulsion
105. What type of colloid is formed when a liquid is dispersed in a gas?
A)
foam
B)
aerosol
C)
emulsion
D)
sol
E)
gel
106. What type of colloid is formed when a gas is dispersed in a liquid?
A)
foam
B)
gel
C)
emulsion
D)
sol
E)
aerosol
107. Shaving cream is an example of which colloid type?
A)
aerosol
B)
foam
C)
emulsion
D)
sol
E)
coagulate
108. Calculate the mole fraction of H2SO4 in 9.61 M H2SO4. The density of the solution is 1.520
g/mL.
109. What is the percent by mass of ethanol (C2H5OH) in a 1.5-m aqueous solution?
110. Diagram and label a vapor pressure diagram for an ideal solution of two volatile liquids.
Indicate the deviation predicted by an endothermic heat of solution.
111. A chemist is given a white solid that is suspected of being pure cocaine. When 1.22 g of the
solid is dissolved in 15.60 g of benzene, the freezing point is lowered by 1.32°C. Calculate
the molar mass of the solid. The molal freezing point constant (Kf) for benzene is 5.12°C/m.
112. A chemist is given a white solid that is suspected of being pure cocaine (molar mass =
303.35 g/mol). When 1.22 g of the solid is dissolved in 15.60 g of benzene, the freezing
point is lowered by 1.32°C. The molar mass is calculated from these data to be 303 g.
Assuming the following uncertainties, can the chemist be sure the substance is not codeine
(molar mass 299.36)? Kf for benzene is 5.12°C/m.
Uncertainties
Mass of solid = ±0.01 g
Mass of benzene = ±0.01 g
T (freezing point lowering) = ±0.04°C
Kf = ±0.01
Support your answer with calculations.
113. What is the molar mass of glucose if 22.5 g gives a freezing point of –0.930°C when
dissolved in 250.0 g of water? If the empirical formula is CH2O, what is the molecular
formula?
114. When 92.0 g of a compound is dissolved in 1000. g of water, the freezing point of the
solution is lowered to –3.72°C. Determine the molar mass of the compound.
115. Calculate both the boiling point and the freezing point if 46.0 g of glycerol, C3H5(OH)3, is
dissolved in 500.0 g of H2O.
