Chapter 11 2 and solute-solvent interactions are quite different

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.