Chemistry The Science in Context

Thomas R. Gilbert

Chapter 10

Intermolecular Forces: The Uniqueness of Water - all with Video Answers

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Chapter Questions

Problem 1

Look at the pairs of ions in the structures of KF and KI represented in Figure P10.1. Which substance has the stronger cation-anion attractive forces and the higher melting point? Explain your answer.

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Problem 2

In Figure $\mathrm{P} 10.2,$ identify the physical state (solid, liquid, or gas) of xenon and classify the attractive forces between the xenon atoms.

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Problem 3

Figure $\mathrm{P} 10.3$ depicts molecules of $\mathrm{XH}_{3}$ and $\mathrm{YH}_{3}$ (not shown to scale) and the boiling points of $\mathrm{XH}_{3}$ and $\mathrm{YH}_{3}$ at 1 atm pressure. One substance is phosphine $\left(\mathrm{PH}_{3}\right)$ and the other substance is ammonia (NH$_{3}$). Which molecule is phosphine? Explain your answer.

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Problem 4

Figure P10.4 shows representations of the molecules pentane, $\mathrm{C}_{5} \mathrm{H}_{12},$ and decane, $\mathrm{C}_{10} \mathrm{H}_{22} .$ Which substance has the lower freezing point? Explain your answer.

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Problem 5

The graphs in Figure $\mathrm{P} 10.5$ have the same scales and describe the change in $\ln \left(P_{\text {vap }}\right)$ of two pure liquids as a function of temperature. Which liquid has the stronger intermolecular attractive forces? Explain your answer.

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Problem 6

Which of the drawings in Figure P10.6, both of which are at constant temperature, most likely illustrates the pure liquid with the lower normal boiling point? Explain your choice.

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Problem 7

Examine the phase diagram of substance $Z$ in Figure P10.7. (a) Does the freezing point of the substance increase or decrease with increasing pressure? (b) Do you predict the solid phase of substance $Z$ would float on the liquid phase? Why?

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Problem 8

Use representations $[\mathrm{A}]$ through $[\mathrm{I}]$ in Figure $\mathrm{P} 10.8$ to answer questions a-f.
a. What are the predominant intermolecular forces in each of the substances in the electrostatic potential maps?
b. Which substance requires the lowest temperature to condense?
c. Which representations are isomers of one another?
d. Consider ethylene glycol and iodine. Which will dissolve in ethanol? Which will dissolve in carbon tetrachloride? Name the
intermolecular forces responsible.
e. Which process(es) show(s) an increase in partial pressure?
f. Which process would occur due to an increase in temperature?
(FIGURE CAN'T COPY)

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Problem 9

Which type of intermolecular force exists in all substances?

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Problem 10

At room temperature, bromine $\left(\mathrm{Br}_{2}\right)$ is a corrosive red liquid, whereas iodine $\left(\mathrm{I}_{2}\right)$ is a volatile violet solid. The differences point to different strengths of intermolecular forces between these halogens, with those for $\mathrm{I}_{2}$ being stronger. What kind of intermolecular force is responsible for these differences?

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Problem 11

Why do gases behave nonideally at high pressures and low temperatures?

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Problem 12

Why are normal boiling points generally lower for branched hydrocarbons than for straight-chain hydrocarbons of the same molecular mass?

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Problem 13

In each of the following pairs of molecules, which one experiences the stronger dispersion forces? (a) $\mathrm{CCl}_{4}$ or $\mathrm{CF}_{4};$ (b) $\mathrm{CH}_{4}$ or $\mathrm{C}_{3} \mathrm{H}_{8}$

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Problem 14

What kinds of intermolecular forces must be overcome as solid CO$_{2}$ sublimes?

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Problem 15

Consider the two molecules shown in Figure $\mathrm{P} 10.15$ One of these compounds is a liquid at room temperature; the other is a solid. Which is which? Explain why.

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Problem 16

Consider the two molecules shown in Figure P10.16. One has a boiling point of $57^{\circ} \mathrm{C} ;$ the other, $84^{\circ} \mathrm{C} .$ Predict which has the higher boiling point, and explain your choice.

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Problem 17

How are the water molecules preferentially oriented around the anion in an aqueous solution of sodium chloride?

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Problem 18

How are the water molecules preferentially oriented around the cation in an aqueous solution of potassium bromide?

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Problem 19

Why are dipole-dipole interactions generally weaker than ion-dipole interactions?

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Problem 20

Two liquids-one polar, one nonpolar-have the same molar mass. Which one is likely to have the higher boiling point? Explain your answer.

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Problem 21

Why are hydrogen bonds considered a special class of dipole-dipole interactions?

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Problem 22

Can all polar hydrogen-containing molecules form hydrogen bonds? Why or why not?

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Problem 23

The permanent dipole moment of $\mathrm{CH}_{2} \mathrm{F}_{2}(1.93 \mathrm{D})$ is larger than that of $\mathrm{CH}_{2} \mathrm{Cl}_{2}(1.60 \mathrm{D}),$ yet the boiling point of $\mathrm{CH}_{2} \mathrm{Cl}_{2}\left(40^{\circ} \mathrm{C}\right)$ is much higher than that of $\mathrm{CH}_{2} \mathrm{F}_{2}$ $\left(-52^{\circ} \mathrm{C}\right) .$ Why?

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Problem 24

How is it that the permanent dipole moment of HCl $(1.08 \mathrm{D})$ is larger than the permanent dipole moment of HBr $(0.82 \mathrm{D}),$ yet HBr boils at a higher temperature?

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Problem 25

Hydrogen peroxide $\left(\mathrm{H}_{2} \mathrm{O}_{2}\right)$ and water $\left(\mathrm{H}_{2} \mathrm{O}\right)$ are both liquids at room temperature, but their standard heats of vaporization $\Delta H_{\mathrm{vap}}^{o}$ are different: $52 \mathrm{kJ} / \mathrm{mol}$ and $41 \mathrm{kJ} / \mathrm{mol},$ respectively. Which substance has the stronger intermolecular forces? Can you suggest why?

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Problem 26

From the thermochemical data for $\mathrm{H}_{2} \mathrm{O}_{2}$ and $\mathrm{H}_{2} \mathrm{O}$ given in Problem $10.25,$ which liquid has the higher boiling point?

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Problem 27

Explain why the melting point of methyl fluoride, $\mathrm{CH}_{3} \mathrm{F}$ $\left(-142^{\circ} \mathrm{C}\right),$ is higher than the melting point of methane, $\mathrm{CH}_{4}\left(-182^{\circ} \mathrm{C}\right)$.

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Problem 28

Explain why the boiling point of $\mathrm{Br}_{2}\left(59^{\circ} \mathrm{C}\right)$ is lower than that of iodine monochloride, ICl $\left(97^{\circ} \mathrm{C}\right)$, even though they have nearly the same molar mass.

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Problem 29

Why doesn't fluoromethane (CH $_{3} \mathrm{F}$ ) exhibit hydrogen bonding, whereas hydrogen fluoride, HF, does?

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Problem 30

The boiling point of phosphine, $\mathrm{PH}_{3}\left(-88^{\circ} \mathrm{C}\right),$ is lower than that of ammonia, $\mathrm{NH}_{3}\left(-33^{\circ} \mathrm{C}\right)$, even though $\mathrm{PH}_{3}$ has twice the molar mass of $\mathrm{NH}_{3} .$ Why?

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Problem 31

In which of the following compounds do the molecules experience the strongest dipole-dipole attractions? (a) $\mathrm{CF}_{4}$ (b) $\mathrm{CF}_{2} \mathrm{Cl}_{2} ;$ (c) $\mathrm{CCl}_{4}$

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Problem 32

Which of the following compounds, $\mathrm{CO}_{2}, \mathrm{NO}_{2}, \mathrm{SO}_{2},$ or $\mathrm{H}_{2} \mathrm{S},$ is expected to have the weakest interactions between its molecules?

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Problem 33

Which of the following molecules can form hydrogen bonds among themselves in pure samples of bulk material?
(a) methanol $\left(\mathrm{CH}_{3} \mathrm{OH}\right) ;$ (b) ethane $\left(\mathrm{CH}_{3} \mathrm{CH}_{3}\right) ;(\mathrm{c})$ dimethyl ether $\left(\mathrm{CH}_{3} \mathrm{OCH}_{3}\right) ;$ (d) acctic acid $\left(\mathrm{CH}_{3} \mathrm{COOH}\right)$

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Problem 34

Which of the following molecules can form hydrogen bonds with molecules of water? (a) methanol (CH $_{3} \mathrm{OH}$ ); (b) ethane $\left(\mathrm{CH}_{3} \mathrm{CH}_{3}\right) ;$ (c) dimethyl ether $\left(\mathrm{CH}_{3} \mathrm{OCH}_{3}\right)$ (d) acetic acid (CH $_{3} \mathrm{COOH}$ )

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Problem 35

What is the difference between the terms miscible and insoluble?

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Problem 36

What properties of water molecules enable them to hydrate and separate cations and anions in aqueous solutions?

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Problem 37

One of the compounds in Figure $\mathrm{P} 10.16$ is insoluble in water; the other has a water solubility of $0.87 \mathrm{g} / 100 \mathrm{mL}$ at $20^{\circ} \mathrm{C} .$ Identify which is which, and explain your reasoning.

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Problem 38

Explain why the solubility of the series of alcohols with the formula $\mathrm{C}_{n} \mathrm{H}_{2 n+2} \mathrm{OH}$ decreases with increasing $n$.

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Problem 39

In what context do the terms bydropbobic and bydropbilic relate to the solubilities of substances in water?

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Problem 40

How does the presence of increasingly longer hydrocarbon chains in the structure affect the solubility of a series of structurally related molecules in water?

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Problem 41

In each of the following pairs of compounds, which compound is likely to be more soluble in water?
a. $\mathrm{CCl}_{4}$ or $\mathrm{CHCl}_{3}$
b. $\mathrm{CH}_{3} \mathrm{OH}$ or $\mathrm{C}_{6} \mathrm{H}_{11} \mathrm{OH}$
c. NaF or MgO
d. $\mathrm{CaF}_{2}$ or $\mathrm{BaF}_{2}$

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Problem 42

In each of the following pairs of compounds, which compound is likely to be more soluble in $\mathrm{CCl}_{4} ?$
a. $\mathrm{Br}_{2}$ or $\mathrm{NaBr}$
b. $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}$ or $\mathrm{CH}_{3} \mathrm{OCH}_{3}$
c. $\mathrm{CS}_{2}$ or $\mathrm{KOH}$
d. $I_{2}$ or $C a F_{2}$

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Problem 43

Which of these pairs of substances is likely to be miscible?
a. $\mathrm{Br}_{2}$ and benzene $\left(\mathrm{C}_{6} \mathrm{H}_{6}\right)$
b. $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OCH}_{2} \mathrm{CH}_{3}$ (diethyl ether) and $\mathrm{CH}_{3} \mathrm{COOH}$ (acetic acid)
c. $\mathrm{C}_{6} \mathrm{H}_{12}$ (cyclohexane) and hexane $\left(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\right)$
d. $\mathrm{CS}_{2}$ (carbon disulfide) and $\mathrm{CCl}_{4}$ (carbon tetrachloride)

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Problem 44

Which of these pairs of substances is likely to be miscible?
a. $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}$ (ethanol) and $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OCH}_{2} \mathrm{CH}_{3}$ (diethyl ether)
b. $\mathrm{CH}_{3} \mathrm{OH}$ (methanol) and methyl amine $\left(\mathrm{CH}_{3} \mathrm{NH}_{2}\right)$
c. CH $_{3} \mathrm{CN}$ (acetonitrile) and acetone (CH $_{3} \mathrm{COCH}_{3}$ )
d. $\mathrm{CF}_{3} \mathrm{CHF}_{2}$ (a Freon replacement) and $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}$ (pentane)

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Problem 45

Which of the following compounds is likely to be the most soluble in water? (a) $\mathrm{NaCl} ;$ (b) $\mathrm{KI} ;$ (c) $\mathrm{Ca}(\mathrm{OH})_{2} ;$ (d) $\mathrm{CaO}$.

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Problem 46

From the data in Figure $P 10.46,$ which has a greater effect on the solubility of oxygen in water: (a) decreasing the temperature from $20^{\circ} \mathrm{C}$ to $10^{\circ} \mathrm{C}$ or (b) raising the pressure from 1.00 atm to 1.25 atm?

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Problem 47

Which of these substances is the least soluble in water?
a. $\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{2} \mathrm{CH}_{2} \mathrm{OH}$
b. $\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{4} \mathrm{CH}_{2} \mathrm{OH}$
c. $\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{6} \mathrm{CH}_{2} \mathrm{OH}$
d. $\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{8} \mathrm{CH}_{2} \mathrm{OH}$

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Problem 48

Which of these substances is the most soluble in water?
a. $\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{2} \mathrm{CH}_{2} \mathrm{NH}_{2}$
b. $\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{4} \mathrm{CH}_{2} \mathrm{Cl}$
c. $\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{6} \mathrm{CH}_{2} \mathrm{Br}$
d. $\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{8} \mathrm{CH}_{2} \mathrm{I}$

Jennifer Hudspeth

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Problem 49

Which of the compounds in Figure $\mathrm{P} 10.49$ are alcohols and which ones are ethers? Place them in order of increasing boiling point.

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Problem 50

Which of the compounds in Figure P10.50 are alcohols and which ones are ethers? Place them in order of increasing vapor pressure at $25^{\circ} \mathrm{C}$.

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Problem 51

Why does the solubility of most gases in most liquids increase with decreasing temperature?

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Problem 52

Which term, $k_{\mathrm{H}}$ or $P$, in Henry's law is affected by temperature?

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Problem 53

Air is primarily a mixture of nitrogen and oxygen. Is the Henry's law constant for the solubility of air in water the sum of $k_{\mathrm{H}}$ for $\mathrm{N}_{2}$ and $k_{\mathrm{H}}$ for $\mathrm{O}_{2} ?$ Explain why or why not.

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Problem 54

Why is the Henry's law constant for $\mathrm{CO}_{2}$ so much larger than those for $\mathrm{N}_{2}$ and $\mathrm{O}_{2}$ at the same temperature?

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Problem 55

Which sulfur oxide would you predict to be more soluble in nonpolar solvents, $\mathrm{SO}_{2}$ or $\mathrm{SO}_{3} ?$

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Problem 56

Which oxide would you predict to be more soluble in polar solvents, $\mathrm{CO}_{2}$ or $\mathrm{NO}_{2} ?$

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Problem 57

Arterial blood contains about $0.25 \mathrm{g}$ of oxygen per liter at $37^{\circ} \mathrm{C}$ and standard atmospheric pressure. What is the Henry's law constant $[\mathrm{mol} /(\mathrm{L} \cdot \mathrm{atm})]$ for $\mathrm{O}_{2}$ dissolution in blood? The mole fraction of $\mathrm{O}_{2}$ in air is $0.209 .$

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Problem 58

The solubility of $\mathrm{O}_{2}$ in water is $6.5 \mathrm{mg} / \mathrm{L}$ at an atmospheric pressure of 1 atm and temperature of $40^{\circ} \mathrm{C}$. Calculate the Henry's law constant of $\mathrm{O}_{2}$ at $40^{\circ} \mathrm{C}$. The mole fraction of $\mathrm{O}_{2}$ in air is 0.209.

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Problem 59

Use the Henry's law constant for $\mathrm{O}_{2}$ dissolved in arterial blood from Problem 10.57 to calculate the solubility of $\mathrm{O}_{2}$ in the blood of (a) a climber on Mt. Everest $\left(P_{\text {atm }}=0.35 \text { atm }\right)$ and (b) a scuba diver at 100 feet $(P \approx 3 \text { atm })$.

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Problem 60

The solubility of air in water is approximately $7.9 \times 10^{-4} \mathrm{M}$ at $20^{\circ} \mathrm{C}$ and 1.0 atm. Calculate the Henry's law constant for air. Is the $k_{\mathrm{H}}$ value of air approximately equal to the sum of the $k_{\mathrm{H}}$ values for $\mathrm{N}_{2}$ and $\mathrm{O}_{2}$ because these two gases make up $99 \%$ of the gases in air?

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Problem 61

Manufacturers of carbonated beverages dissolve $\mathrm{CO}_{2}(g)$ in water under pressure to produce sodas. If a manufacturer uses a pressure of 4.16 atm at $25^{\circ} \mathrm{C}$ to carbonate the water, how many grams of
$\mathrm{CO}_{2}$ are in the average can of soda, the volume of which is 355 mL? The Henry's law constant for $\mathrm{CO}_{2}$ at $25^{\circ} \mathrm{C}$ is $0.03360 \mathrm{mol} /(\mathrm{L} \cdot \mathrm{atm})$.

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Problem 62

What would be the effect on the results of the process described in Problem 10.61 (a) if the temperature of the carbonation process was changed from $25^{\circ} \mathrm{C}$ to $10^{\circ} \mathrm{C},$ or (b) if the temperature were held constant but the pressure was doubled?

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Problem 63

Why does the vapor pressure of a liquid increase as temperature increases?

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Problem 64

Which of the following factors influences the vapor pressure of a pure liquid?
a. the volume of liquid present in a container
b. the temperature of the liquid
c. the surface area of the liquid

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Problem 65

Is vapor pressure an intensive or extensive property of a liquid?

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Problem 66

A chef observes bubbles while heating a pot of water to $60^{\circ} \mathrm{C}$ to poach vegetables. What are the gases in the bubbles and where did they come from?

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Problem 67

Rank the following compounds in order of increasing vapor pressure at $298 \mathrm{K}$. (a) $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH} ;$ (b) $\mathrm{CH}_{3} \mathrm{OCH}_{3}$ (c) $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{3}$

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Problem 68

Rank the compounds in Figure $\mathrm{P} 10.68$ in order of increasing vapor pressure at $298 \mathrm{K}$.

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Problem 69

The smell of fresh cut pine is due in part to the cyclic alkene pinene, whose carbon-skeleton structure is shown in Figure $\mathrm{P} 10.69$.
a. Use the data in the table to calculate the heat of vaporization, $\Delta H_{\mathrm{rap}},$ of pinene.
b. Use the value of $\Delta H_{\mathrm{vap}}$ to calculate the vapor pressure of pinene at room temperature $\left(23^{\circ} \mathrm{C}\right)$

$$\begin{array}{cc}\begin{array}{c}\text { Vapor Pressure } \\\text { (torr) }\end{array} & \begin{array}{c}\text { Temperature } \\\text { (K) }\end{array} \\\hline 760 & 429 \\\hline 515 & 415 \\\hline 340 & 401 \\\hline 218 & 387 \\\hline 135 & 373
\end{array}$$

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Problem 70

Almonds and almond extracts are common ingredients in baked goods. Almonds contain the compound benzaldehyde (shown in Figure $\mathrm{P} 10.70$ ), which accounts for the odor of the nut. Benzaldehyde is also responsible for the aroma of cherries.
a. Use the data in the table to calculate the heat of vaporization, $\Delta H_{\mathrm{vap}},$ of benzaldehyde.
b. Use the value of $\Delta H_{\text {vap }}$ to calculate the vapor pressure of benzaldehyde at room temperature $\left(23^{\circ} \mathrm{C}\right)$

$$\begin{array}{cc}\begin{array}{c}\text { Vapor Pressure } \\\text { (torr) }\end{array} & \begin{array}{c}\text { Temperature } \\\text { (K) }\end{array} \\\hline 50 & 373 \\\hline 111 & 393 \\\hline 230 & 413 \\\hline 442 & 433 \\\hline 805 & 453
\end{array}$$

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Problem 71

Explain the difference between sublimation and evaporation.

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Problem 72

Can ice be melted merely by applying pressure? Explain your answer.

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Problem 73

What phases of a substance are present (a) at its triple point and (b) at its critical point?

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Problem 74

Explain how the solid-liquid line in the phase diagram of water differs in character from the solid-liquid line in the phase diagrams of most other substances, such as $\mathrm{CO}_{2}$.

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Problem 75

Which phase of a substance (gas, liquid, or solid) is most likely to be the stable phase: (a) at low temperatures and high pressures; (b) at high temperatures and low pressures?

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Problem 76

At what temperatures and pressures does a substance behave as a supercritical fluid?

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Problem 77

Freeze-drying is used to preserve food at low temperature with minimal loss of flavor. Freeze-drying works by freezing the food and then lowering the pressure with a vacuum pump to sublime the ice. Must the pressure be lower than the pressure at the triple point of $\mathrm{H}_{2} \mathrm{O}$ ? Why or why not?

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Problem 78

Solid helium cannot be converted directly into the vapor phase. Does the phase diagram of helium have a triple point?

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Problem 79

Consult Figures $10.26,10.28,$ and 10.29.
What is the normal boiling point of bromine?

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Problem 80

Consult Figures $10.26,10.28,$ and 10.29.
If water boils at $50^{\circ} \mathrm{C},$ what is the pressure?

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Problem 81

Consult Figures $10.26,10.28,$ and 10.29.
Which molecules have stronger intermolecular attractive forces: ethylene glycol or ethanol? Explain your choice.

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Problem 82

Consult Figures $10.26,10.28,$ and 10.29.
What is the boiling point of diethyl ether at 300 mmHg of pressure?

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Problem 83

Consult Figures $10.26,10.28,$ and 10.29.
List the steps you would take to convert a 10.0 -g sample of water at $25^{\circ} \mathrm{C}$ and 1 atm of pressure to water at its triple point.

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Problem 84

Consult Figures $10.26,10.28,$ and 10.29.
List the steps you would take to convert a 10.0 -g sample of water at $25^{\circ} \mathrm{C}$ and 2 atm pressure to ice at 1 atm of pressure. At what temperature would the water freeze?

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Problem 85

Consult Figures $10.26,10.28,$ and 10.29.
What phase changes, if any, does liquid water at $100^{\circ} \mathrm{C}$ undergo if the initial pressure of 5.0 atm is reduced to 0.5 atm at constant temperature?

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Problem 86

Consult Figures $10.26,10.28,$ and 10.29.
What phase changes, if any, occur if $\mathrm{CO}_{2}$ initially at $-80^{\circ} \mathrm{C}$ and 8.0 atm is allowed to warm to $-25^{\circ} \mathrm{C}$ at 5.0 atm?

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Problem 87

Consult Figures $10.26,10.28,$ and 10.29.
Below what temperature can solid $\mathrm{CO}_{2}$ (dry ice) be converted into $\mathrm{CO}_{2}$ gas simply by lowering the pressure?

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Problem 88

Consult Figures $10.26,10.28,$ and 10.29.
What is the maximum pressure at which solid $\mathrm{CO}_{2}$ (dry ice) can be converted into $\mathrm{CO}_{2}$ gas without melting?

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Problem 89

Consult Figures $10.26,10.28,$ and 10.29.
Predict the phase of water that exists under the following conditions.
a. 2 atm of pressure and $110^{\circ} \mathrm{C}$
b. 200 atm of pressure and $380^{\circ} \mathrm{C}$
c. $6.0 \times 10^{-3}$ atm of pressure and $0^{\circ} \mathrm{C}$

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Problem 90

Consult Figures $10.26,10.28,$ and 10.29.
Which phase or phases of water exist under the following conditions?
a. 2.0 atm and $50^{\circ} \mathrm{C}$
b. 0.10 atm and $300^{\circ} \mathrm{C}$
c. 1 atm and $0^{\circ} \mathrm{C}$

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Problem 91

Explain why a needle floats on the surface of water but sinks in a container of methanol $\left(\mathrm{CH}_{3} \mathrm{OH}\right)$.

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Problem 92

Explain why different liquids do not reach the same height in capillary tubes of the same diameter.

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Problem 93

Explain why pipes filled with water are in danger of bursting when the temperature drops below $0^{\circ} \mathrm{C}$.

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Problem 94

A hot needle sinks when put on the surface of cold water. Will a cold needle float in hot water? Explain your answer.

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Problem 95

The meniscus of water in a glass tube is concave, but that of mercury (Figure $\mathrm{P} 10.95)$ is convex. Explain why.

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Problem 96

The mercury level in a capillary tube placed in a dish of mercury is below the surface of the mercury in the dish. Explain why.

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Problem 97

Describe the origin of surface tension at the molecular level.

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Problem 98

What is the cause of the high viscosity of molasses?

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Problem 99

Describe how the surface tension and viscosity of a liquid are affected by increasing temperature.

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Problem 100

Explain how strong intermolecular forces are expected to result in a relatively high surface tension and viscosity of a liquid.

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Problem 101

One of two glass capillary tubes of the same diameter is placed in a dish of water and the other in a dish of ethanol $\left(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\right) .$ Which liquid will rise higher in its tube? Explain your answer.

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Problem 102

Would you expect water to rise to the same height in a tube made of a polyethylene plastic as it does in a glass capillary tube of the same diameter? Why or why not? The molecular structure of polyethylene is shown in Figure P10.102.

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Problem 103

The normal boiling points of liquids $A$ and $B$ are $75.0^{\circ} \mathrm{C}$ and $151^{\circ} \mathrm{C},$ respectively. Which of these liquids would you expect to have the higher surface tension and viscosity at $25^{\circ} \mathrm{C} ?$ Explain your answer.

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Problem 104

A simple viscometer consists of a thick-walled glass tube with a 0.5 -mm bore. The tube has etched marks at one-quarter and three-quarters of its height. The tube is clamped with its lower end dipped in a container of the liquid to be tested. A pipette filler is used to draw liquid up the bore past the upper mark. The pipette filler is removed from the tube, and the time taken for the liquid meniscus to drain between the upper and lower viscometer marks is measured with a stopwatch. Using this viscometer to measure the drain times for two pure liquids $A$ and $B$ at the same temperature gives 3.45 seconds for liquid $\mathrm{A}$ and 4.64 seconds for liquid B.
a. Which liquid is more viscous?
b. Which liquid has weaker intermolecular forces?
c. Would the measured drain times be longer or shorter at a lower experimental liquid temperature?

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Problem 105

Why do ethers typically boil at lower temperatures than alcohols with the same molecular formula?

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Problem 106

During the winter months in cold climates, water condensing in a vehicle's gas tank reduces engine performance. An auto mechanic recommends adding "dry gas" to the tank during your next fill-up. Dry gas is typically an alcohol that dissolves in gasoline and absorbs water. From the structures shown in Figure $\mathrm{P} 10.106$ which product do you predict would do a better job methanol or 2 -propanol? Why?

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Problem 107

Does the sublimation point of ice increase or decrease with increasing pressure? Explain why.

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Numerade Educator

Problem 108

Does the sublimation point of $\mathrm{CO}_{2}$ increase or decrease with increasing pressure? Explain why.

David Collins

Numerade Educator

Problem 109

Liquid substances are often compared for their physical properties in different applications. Comparison of two liquids $\mathrm{A}$ and $\mathrm{B}$ at constant temperature and atmospheric pressure shows that liquid A has higher viscosity and surface tension, a higher boiling point, and lower vapor pressure than liquid B. Are these data all consistent with stronger intermolecular forces in liquid A than in liquid B?

David Collins

Numerade Educator

Problem 110

Why is methanol $\left(\mathrm{CH}_{3} \mathrm{OH}\right)$ miscible with water, whereas $\mathrm{CH}_{4}$ is almost completely insoluble in water?

David Collins

Numerade Educator

Problem 111

Sketch a phase diagram for element X, which has a triple point at $152 \mathrm{K}$ and a pressure of $0.371 \mathrm{atm},$ a boiling point of $166 \mathrm{K}$ at a pressure of 1.00 atm, and a normal melting point of $161 \mathrm{K}$.

David Collins

Numerade Educator

Problem 112

The melting point of hydrogen is $14.96 \mathrm{K}$ at 1.00 atm of pressure. The temperature at its triple point is $13.81 \mathrm{K}$ Does $\mathrm{H}_{2}$ expand or contract when it freezes?

David Collins

Numerade Educator

Problem 113

Explain why water climbs higher in a capillary tube than in a test tube.

David Collins

Numerade Educator

Problem 114

Explain why ice floats on water.

David Collins

Numerade Educator

Problem 115

Explain why fish in a pond die if water becomes too warm.

David Collins

Numerade Educator

Problem 116

A test done on new pharmaceutical agents early in the development process required the observation of their relative solubilities in octanol and water. A drug had to be sufficiently soluble in water (hydrophilic) to be carried in the bloodstream but also sufficiently hydrophobic (octanol soluble) to move across cell membranes. Pick the molecule from Figure P10.116 that you predict might have comparable solubility in both solvents. Explain your choice.

David Collins

Numerade Educator

Problem 117

Compounds with low boiling points may be sprayed on skin as a topical anesthetic- they chill it as they evaporate, providing short-term relief from injuries. Predict which compound among those in Figure P10.117 has the lowest boiling point.

David Collins

Numerade Educator

Problem 118

Refrigerators have a unit called a compressor that liquefies a gas. The refrigerator is cooled by a continuous cycle of compression of the gas to produce the liquid, followed by evaporation of the liquid to provide the cooling. Ammonia (NH $_{3}$ ) and sulfur dioxide $\left(\mathrm{SO}_{2}\right)$ were the gases used originally, and hexafluoroethane $\left(\mathrm{C}_{2} \mathrm{F}_{6}\right)$ has been used since the 1990 s. What are the intermolecular interactions that characterize these substances?

David Collins

Numerade Educator

Problem 119

Disposable wipes used to clean the skin before receiving an immunization shot contain ethanol. After the nurse wipes your arm, your skin feels cold. Why?

David Collins

Numerade Educator