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PAGE 5-1
ANSWERS TO
END-OF-CHAPTER QUESTIONS
CHAPTER 5: THE WATER WE DRINK
Emphasizing Essentials
1. a. The text states that 50–65% of adult body weight is water. How many pounds is this for a
150-lb adult? Report your answer as a range of values.
b. Given that a gallon of water weighs about 8 lb, how many gallons of water will this be for
a 150-lb adult? Report your answer as a range of values.
Answer:
a. 75–98 lb
b. 9–12 gal
2. a. What is an aquifer?
b. Why is it important to prevent unwanted substances from reaching a clean aquifer?
Answer:
a. An aquifer is a large natural underground supply of water trapped in sand and gravel and is
typically 50 to 500 feet below the surface.
b. If an aquifer becomes polluted, it may be next to impossible to remove the contaminant(s),
depending on what they are. This could threaten the source of clean water for decades to
come.
3. If the water in a 500-L drum were representative of the world’s total supply, how many liters
would be suitable for drinking? Hint: See Figure 5.5.
Answer:
The drinking water supply is limited by the amount of available fresh water. Figure 5.5
shows 97.4% is salt water, which means only 2.6% (13 L of the 500-L drum) is fresh and
possibly suitable for drinking. About 65% of this fresh water, however, is frozen in
Antarctica and Greenland.
4. Based on your experience, what is the solubility of each of these substances in water? Use
terms such as very soluble, partially soluble, or not soluble. Cite supporting evidence.
a. orange juice concentrate
b. liquid laundry detergent
c. household ammonia
d. chicken broth
e. chicken fat
Answer:
a. Partially soluble. Orange juice concentrate contains some solids (pulp) that do not dissolve
in water.
PAGE 5-2
b. Very soluble. The detergent dissolves in water forming a solution that spreads evenly
throughout the clothes being cleaned.
c. Very soluble. Household ammonia is a solution of the gas ammonia in water. It will
dissolve in water in any proportion.
d. Partially soluble if the chicken broth contains fat or suspended solids. Neither of these
dissolve in water. If the broth is clear and fat free, it is very soluble.
e. Not soluble. Chicken fat does not dissolve in water.
5. a. Bottled water consumption was reported to be 21 gal per person in the United States in
2002. The last census reported 2.9 108 people in the United States. Given this, estimate the
total bottled water consumption.
b. If the per capita consumption of bottled water increased 20% in the last 10 years, what was
the per capita consumption 10 years ago?
Answer:
a. 6.1 x 109 gallons
b. 17 gallons
6. a. A certain bottled water lists a calcium concentration of 55 mg/L. What is its calcium
concentration expressed in parts per million?
b. How does this concentration compare with that for Evian listed in Table 5.2?
Answer:
a. 55 mg of Ca2+
per liter of bottled water is the same as 55 ppm.
55 mg Ca 2+
1 L H2O
1 L
103 mL
1 mL H2O
1 g H 2O
1g
103 mg
= 55 mg Ca 2+
106 mg H 2O or 55 ppm Ca2+
b. Evian, with 78 mg Ca2+
/L, has a slightly higher Ca2+
concentration.
7. One particular vitamin tablet contains 162 mg of calcium and supplies 16% of the
recommended daily amount of calcium required by a person on a typical 2000-Calorie diet.
How many 500-mL bottles of Evian bottled water would you have to drink each day to
obtain the same mass of calcium? Hint: See Your Turn 5.7 and Table 5.2.
Answer:
A 500-mL bottle of Evian provides 4% of the recommended daily requirement of calcium, so
you would have to drink four bottles of Evian each day to obtain the amount of calcium
provided by the vitamin tablet (16% of the recommended daily amount).
8. The acceptable limit for nitrate, often found in well water in agricultural areas, is 10 ppm. If a
water sample is found to contain 350 g/L, does it meet the acceptable limit? Show a
calculation to support your answer.
PAGE 5-3
Answer:
The nitrate concentration in this sample is well below the 10 ppm limit.
350 g nitrate
1 L H2O
1 mg
103 g
1g
103 mg
1 L
103 mL
1 mL H2O
1g H2O
0.35 g nitrate
106 g H2Oor 0.35 ppm
9. One reagent bottle on the shelf in a laboratory is labeled 12 M H2SO4 and another is labeled
12 M HCl.
a. How does the number of moles of H2SO4 in 100 mL of 12 M H2SO4 solution compare
with the number of moles of HCl in 100 mL of 12 M HCl solution?
b. How does the number of grams of H2SO4 in 100 mL of 12 M H2SO4 solution compare
with the number of grams of HCl in 100 mL of 12 M HCl solution?
Answer:
a. The number of moles of H2SO4 in 100 mL of 12 M H2SO4 would be the same as the
number of moles of HCl in 100 mL of 12 M HCl.
12 mol H2SO4
1 L solution
1 L
103 mL 100 mL solution =1.2 mol H2SO4
12 mol HCl
1 L solution
1 L
103 mL 100 mL solution =1.2 mol HCl
b. The number of grams is not the same because the molar mass of H2SO4 is greater than that
of HCl.
1.2 mol H2SO4 98.1 g H 2SO 4
1 mol H2SO4
= 120 g H 2SO4
1.2 mol HCl 36.5 g HCl
1 mol HCl = 44 g HCl
10. A student weighs out 5.85 g of NaCl to make a 0.10 M solution. What size volumetric flask
does she need? Hint: See Figure 5.6.
Answer:
The student is weighing out 5.85 g, and the molar mass of NaCl is 58.5 g/mol, so she is
effectively weighing out 0.1 mol. Therefore she will need a 1.0 L volumetric flask.
11. Both methane, CH4, and water are compounds in which hydrogen atoms are bonded with a
nonmetallic element. Yet, methane is a gas at room temperature and pressure and water is a
liquid. Offer a molecular explanation for the difference in properties.
Answer:
The electronegativities of C, H, and O are 2.5, 2.1 and 3.5 respectively. The C-to-H bond in
CH4 is nonpolar (the difference in electronegativity between C and H is small) and the
molecule is nonpolar. The forces between nonpolar molecules are relatively weak, allowing
the molecules to escape from the liquid to the gas phase at room temperature and pressure. In
contrast, the O-to-H bonds in H2O are polar (the difference in electronegativity between C
PAGE 5-4
and O is larger), and so is the molecule. Water molecules strongly attract each other.
Accordingly, more energy is required to evaporate water than to evaporate methane.
12. Explain why the term universal solvent is applied to water.
Answer:
Water dissolves many substances, including a variety of acids, bases, salts and polar
compounds. It is also a common solvent, and in this sense universal as well. However, water
is not really a universal solvent as it does not dissolve everything. Gasoline and limestone,
for example, are not soluble in water.
13. Here are four sets of atoms. Consult Table 5.3 to answer these questions.
N and C N and H
S and O S and F
a. What is the electronegativity difference between the atoms?
b. Assume that a single covalent bond forms between each pair of atoms. Which atom
attracts the electron pair in the bond more strongly?
c. Arrange the bonds in order of increasing polarity.
Answer:
a. N and C; 3.0 – 2.5 = 0.5
S and O; 3.5 – 2.5 = 1.0
N and H; 3.0 – 2.1 = 0.9
S and F; 4.0 – 2.5 = 1.5
b. N will attract the bonded electron pair more strongly than C.
O will attract the bonded electron pair more strongly than S.
N will attract the bonded electron pair more strongly than H.
F will attract the bonded electron pair more strongly than S.
c. N–C < N–H < S–O < S–F
14. NaCl is an ionic compound, but SiCl4 is a covalent compound.
a. Use Table 5.3 to determine the electronegativity difference between chlorine and sodium,
and between chlorine and silicon.
b. What correlations can be drawn about the difference in electronegativity between bonded
atoms and their tendency to form ionic or covalent bonds?
c. How can you explain on the molecular level the conclusion reached in part b?
Answer:
a. Cl and Na; electronegativity difference is 3.0 – 0.9 = 2.1
Cl and Si; electronegativity difference is 3.0 – 1.8 = 1.2
b. Larger differences in electronegativity are associated with ionic bonds; smaller differences
with covalent bonds.
c. When electronegativity differences are relatively large, one or more electrons are
transferred, forming ions. When electronegativity differences are smaller, neither atom is
able to release its outer electrons to the other, so the outer electrons are shared, resulting in
formation of covalent bonds, in this case a polar covalent bond.
PAGE 5-5
15. Consider a molecule of ammonia, NH3.
a. Draw its Lewis structure.
b. Does the NH3 molecule contain polar bonds?
c. Is the NH3 molecule polar? Hint: Consider its geometry.
d. Is NH3 soluble in water? Explain.
Answer:
a. The Lewis structure for ammonia is.
b. Each N-to-H bond is polar. The difference in electronegativity between N and H is
3.0 – 2.1 = 0.9.
c. The molecule is polar. The molecular shape is triangular pyramidal.
d. Ammonia and water are polar molecules and “like dissolves like.” Ammonia is highly
soluble in water.
16. This diagram represents two water molecules in a liquid state. What kind of bonding force
does the arrow indicate? Is this an intermolecular or intramolecular force?
Answer:
The arrow points to a hydrogen bond, an example of an intermolecular force, which is a force
between water molecules and not within each water molecule.
17. The density of liquid water at 0 °C is 0.9987 g/cm3; the density of ice at this same
temperature is 0.917 g/cm3.
a. Calculate the volume occupied at 0 °C by 100.0 g of liquid water and by 100.0 g of ice.
b. Calculate the percentage increase in volume when 100.0 g of water freezes at 0 °C.
Answer:
a. volume of water:
100.0 g 1 cm 3
0.9987 g 100.1 cm3
volume of ice:
100.0 g 1 cm 3
0.917 g 109 cm3
PAGE 5-6
b.
109 cm3 100.1 cm3
100.1 cm3 100 9% volume increase
18. Consider these liquids.
Liquid Density, g/mL
dishwashing detergent 1.03
maple syrup 1.37
vegetable oil 0.91
a. If you pour equal volumes of these three liquids into a 250-mL graduated cylinder, in what
order will you add the liquids to create three separate layers? Explain your reasoning.
b. If a liquid were poured into the cylinder and it formed a layer that was on the bottom of
the other three layers, what can you tell about one of the properties of this liquid?
c. What would happen if a volume of water equal to the other liquids were poured into the
cylinder in part a and then the contents are mixed vigorously? Explain.
Answer:
a. Three factors need to be considered here: solubility, density, and the care with which you
pour each liquid. Maple syrup will probably slowly dissolve in dishwashing liquid; likewise
vegetable oil may be slightly soluble in the detergent. But with careful pouring, these three
liquids should not easily mix and probably could be added in any order. The most foolproof
way to get the layers would be to add the liquids in the order of most dense to least dense.
b. Because the unknown formed a layer at the bottom, it is only moderately soluble (or less)
in any of the three liquids. Because the unknown formed layer below maple syrup, the
density of the unknown must be greater than 1.37 g/cm3.
c. After vigorous stirring, a cloudy emulsion will likely form. At least some of the maple
syrup and dishwashing detergent should dissolve in the water layer, but eventually the
vegetable oil layer may separate out and rise to the top of the mixture. You may want to try
this experiment and observe the results!
19. Why is there the possibility of a water pipe breaking if the pipe is left full of water during
extended frigid weather?
Answer:
Ice is less dense than liquid water, so when the water in the pipe freezes, it expands and can
break the pipe.
20. What ions typically form from these atoms? Draw Lewis structures for each atom and its
corresponding ion. Use the octet rule to explain why each particular ion forms.
Hint: Consider Tables 5.4 and 5.5.
a. Cl
b. Ba
c. S
d. Li
e. Ne
PAGE 5-7
Answer:
a. A chlorine atom gains one electron to form a chloride ion, with a charge of 1–. The octet
rule is satisfied.
b. A barium atom loses two electrons to form a barium ion, with a charge of 2+. The octet
rule is satisfied.
c. A sulfur atom gains two electrons to form a sulfide ion, with a charge of 2–. The octet rule
is satisfied.
d. A lithium atom loses one electron to form a lithium ion, with a charge of 1+. The octet rule
is satisfied.
e. A neon atom does not form an ion. It has 8 valence electrons.
21. Give the chemical formula and name of the ionic compound formed by the reaction of each
pair of elements.
a. Na and S
b. Al and O
c. Ga and F
d. Rb and I
e. Ba and Se
Answer:
a. Na2S sodium sulfide
b. Al2O3 aluminum oxide
c. GaF3 gallium fluoride
d. RbI rubidium iodide
e. BaSe barium selenide
22. Write the chemical formula for each compound.
a. calcium bicarbonate
b. calcium carbonate
c. magnesium chloride
d. magnesium sulfate
Answer:
a. Ca(HCO3)2
b. CaCO3
c. MgCl2
d. MgSO4
PAGE 5-8
23. Name each compound.
a. KC2H3O2
b. Ca(OCl)2
c. LiOH
d. Na2SO4
Answer:
a. potassium acetate
b. calcium hypochlorite
c. lithium hydroxide
d. sodium sulfate
24. Name each compound.
a. CoO
b. MnCl3
c. ZnS
d. SnBr4
Answer: a. cobalt(II) oxide
b. manganese(III) chloride
c. zinc sulfide
d. tin(IV) bromide
25. Solutions can be tested for conductivity using this type of apparatus.
Predict what will happen when each of these dilute solutions is tested for conductivity. Explain
your predictions briefly.
a. CaCl2(aq)
b. C2H5OH(aq)
c. H2SO4(aq)
Answer:
a. CaCl2 is an electrolyte and dissolves in water to form a solution that conducts electricity.
The circuit will be completed and the bulb will light.
b. Although C2H5OH dissolves in water to form a solution, the solution will not conduct
electricity. The circuit will not be completed and the bulb will not light.
PAGE 5-9
c. H2SO4 is an electrolyte that dissolves in water to form a solution that conducts electricity.
The circuit will be completed and the bulb will light.
26. What ions are present in each of these solutions?
a. Ca(OCl)2(aq)
b. C2H5OH(aq)
Answer:
a. Ca2+
and OCl– ions are present.
b. No ions are present. C2H5OH (ethanol) is not an electrolyte.
27. Based on the generalizations in Table 5.8, which compounds are likely to be water-soluble?
a. KC2H3O2
b. Ca(NO3)2
c. LiOH
d. Na2SO4
Answer:
a. Soluble; all potassium compounds are soluble.
b. Soluble; all nitrate compounds are soluble.
c. Soluble; although most hydroxides are insoluble, LiOH contains a Group 1A element and
is soluble.
d. Soluble; all sodium compounds are soluble.
28. For a 2.5 M solution of Mg(NO3)2, what is the concentration of each ion present?
Answer:
The concentration of Mg2+
is 2.5 M and the concentration of NO3–
is 5.0 M.
29. Explain how you would prepare these solutions from powdered reagents and whatever
glassware you needed:
a. 2.0 L of 1.5 M KOH
b. 1.0 L of .05 M NaBr
c. 0.10 L of 1.2 M Mg(OH)2
d. 300 mL of 3.0 M CaCl2
Answer:
a. To prepare 2.0 L of 1.5 M KOH, measure out 168 g of KOH and place it into a 2 L
volumetric flask. Add water to the mark.
b. To prepare 1.0 L of .05 M NaBr, measure out 5.15 g of NaBr and place it into a 1 L
graduated cylinder. Add water to the mark.
c. To prepare 0.10 L of 1.2 M Mg(OH)2, measure out 7.00 g of Mg(OH)2 and place it into a
100 mL volumetric flask. Add water to the mark.
d. To prepare 300 mL of 3.0 M CaCl2, measure 99.88 g of CaCl2 and place it into a 500 mL
volumetric flask. Add water to the mark.
PAGE 5-10
30. Explain why desalination techniques, despite proven technological effectiveness, are not used
more widely to produce potable drinking water.
Answer:
The two most common desalination techniques are distillation and reverse osmosis. Both of
these require energy to remove salts from seawater or brackish water, and thus inherently are
expensive. If a less expensive option is available (such as hauling fresh water from a
distance), then the less expensive option is used.
Concentrating on Concepts
31. Consider the statement made by the company that makes LeBleu UltraPure Drinking Water:
“Water, the universal solvent, given sufficient time, will dissolve or suspend almost any
material on earth.” Do you agree with this statement? Explain your answer.
Answer:
It is true that water is an excellent solvent. Water dissolves many salts, acids, bases, and polar
molecules. Examples include KCl, HCl, NaOH, and SO2. But it will not dissolve everything,
given sufficient time. Nor will it necessarily suspend everything, especially something in a
big chunk. For example, a piece of gold, marble, or plastic would remain undissolved and
unsuspended in water virtually forever. Similarly many gases would not dissolve in water, no
matter how long you left them in contact with it.
32. Why is the concentration of calcium often given on the label for bottled water?
Answer:
This mineral is listed because consumers are interested in knowing the concentration of
calcium in their drinking water. Calcium helps to build and preserve bone mass.
33. The label on Evian bottled water lists a magnesium concentration of 24 mg/L. The label of a
popular brand of multivitamins lists the magnesium content as 100 mg per tablet. Which do
you think is a better source of magnesium? Explain your reasoning.
Answer:
A person would need to drink more than four liters of Evian to ingest the amount of
magnesium provided by one multivitamin tablet. But since magnesium is readily available in
our diets (green vegetables and whole grains), multivitamins only provide 100 mg of the
300-400 mg required daily. Most people do not need to supplement their magnesium by a
multivitamin and certainly not by bottled water.
34. A new sign is posted at the edge of a favorite fishing hole that says “Caution: Fish from this
lake may contain over 1.5 ppm Hg.” Explain to a fishing buddy what this unit of
concentration means, and why the caution sign should be heeded.
PAGE 5-11
Answer:
A mercury concentration of 1.5 ppm means there are 1.5 parts of mercury for every 106 parts
fish. For fish, this concentration is likely a ratio of mass, so the concentration can be
expressed as 1.5 g Hg for every one million grams (106 g) of fish. The caution sign is
necessary because mercury is toxic and capable of causing severe neurological effects in
humans. The Maximum Contaminant Level for mercury in drinking water is set by the EPA
in the United States at 2 ppm. Because mercury is a cumulative poison for those who eat the
fish, the caution sign has been posted to warn of a mercury level of 1.5 ppm.
35. This periodic table contains four elements identified by numbers.
a. Based on trends within the periodic table, which of the four elements would you expect to
have the highest electronegativity value? Explain.
b. Based on trends within the periodic table, rank the other three elements in order of
decreasing electronegativity values. Explain your ranking.
Answer:
a. The electronegativities of the elements generally increase from left to right across a period
(until Group 8A is reached) and from bottom to top within any group. This means that of the
positions indicated, the element in position 2 is predicted to have the highest
electronegativity.
b. Ranking the other elements is not straightforward. Element 1 is expected to be more
electronegative than element 3, based on their relative positions in the same group. Element 4
will likely be more electronegative than elements 1 and 3 and less electronegative than 2.
However, because element 4 is not in the same period with any other element, this prediction
cannot be made with certainty. Here are the values found in references; they do not appear in
Table 5.4. EN 1 = 0.8; EN 2 = 2.4; EN 3 = 0.7; EN 4 = 1.9. These values confirm the relative
ranking in order of decreasing electronegativity: 2, 4, 1, 3.
36. A diatomic molecule XY that contains a polar bond must be a polar molecule. However, a
triatomic molecule XY2 that contains a polar bond does not necessarily form a polar
molecule. Use some examples of real molecules to help explain this difference.
Answer:
A diatomic molecule with a polar bond must be polar because the molecule itself has to be
linear. An example is HCl. The hydrogen-to-chlorine bond is polar, and so is the molecule. If
the triatomic molecule contains polar bonds, the geometry of the molecule will determine if
the molecule itself is polar or nonpolar. For example, although CO2 has polar carbon-to-
oxygen bonds, the molecule is linear and, as a result, nonpolar. The H2O molecule has polar
hydrogen-to-oxygen bonds, but is bent. This geometry gives rise to a polar molecule.
PAGE 5-12
37. Imagine you are at the molecular level, watching water vapor condense.
a. Sketch four water molecules using a space-filling representation similar to this one.
Sketch them in the gaseous state and then in the liquid state. How does the collection of
molecules change when water vapor condenses to a liquid?
b. What happens at the molecular level when water changes from a liquid to a solid?
Answer:
liquid gas
a. The molecules in a gas
should appear further apart than those in a liquid. The molecules themselves will not change,
only the distances between them. As water vapor condenses, the molecules are now close
enough to each other to hydrogen bond to each other, an intermolecular force. There are no
changes in the intramolecular, or covalent, bonds within each molecule when the gaseous
water condenses to a liquid.
b. As the temperature drops to 4 °C, the water molecules approach each other most closely.
Then, from 4 °C to 0 °C, they are forced somewhat apart as hydrogen bonding between
molecules creates regular lattice patterns. At 0 °C, snow flakes and ice crystals form, both of
which are less dense than the water at 4 °C.
38. Propose an explanation for the fact that NH3, like H2O, has an unexpectedly high specific
heat. Hint: See question 15 for the Lewis structure and H-to-N-to-H bond geometry in NH3.
PAGE 5-13
Answer:
NH3, like water, is a polar molecule. Therefore, despite its low molar mass, considerable
energy must be added to liquid NH3 to overcome the intermolecular forces among NH3
molecules.
39. a. What type of bond holds together the two hydrogen atoms in the hydrogen molecule, H2?
b. Explain why the term hydrogen bonding does not apply to the bond within H2.
Answer:
a. A single covalent bond holds two hydrogen atoms together in H2. It is an example of an
intramolecular force.
b. Hydrogen bonding is a type of intermolecular force, a force between molecules, not within
the molecule.
40. Hydrogen bonding has been offered as a reason why ice cubes and icebergs float in water.
Consider ethanol, C2H5OH.
a. Draw its Lewis structure and use it to decide if pure ethanol will exhibit hydrogen
bonding.
b. A cube of solid ethanol sinks rather than floats in liquid ethanol. Explain this behavior in
view of your answer in part a.
Answer:
a. The Lewis structure for ethanol is C C
H
H
H H
H
O H . This molecule contains a H atom
bonded to an oxygen atom, and this H atom will exhibit hydrogen bonding.
Remember: Hydrogen bonding only occurs with H atoms attached to F, N, or O atoms.
b. The cube sinks because, as is the case for most substances, the density of the solid phase is
greater than the density of the liquid phase. The ethanol molecules in solid ethanol are closer
together than water molecules are in ice. Therefore, solid ethanol has a greater density than
liquid ethanol.
41. The unusually high heat capacity of water is very important in regulating our body
temperature and keeping it within a normal range despite time, age, activity, and
environmental factors. Consider some of the ways that the body produces heat, and some of
the ways that it loses heat. How would these functions differ if water had a much lower heat
capacity?
Answer: Note: This question should read “specific heat” and not “heat capacity.”
The specific heat of a substance is the quantity of heat energy that must be absorbed to
increase the temperature of 1 g of a substance by 1 8C. Heat is produced within our bodies
primarily by respiration; that is, the “burning” of food to produce carbon dioxide and water.
Our bodies also absorb heat when we sit by a fire or are outside in hot weather. If water had a
PAGE 5-14
lower specific heat, our bodies would be more susceptible to a temperature rise both from
respiration and from exposure to warm or hot conditions.
One way that our bodies lose heat is through exposure to cooler conditions. If water had a
lower specific heat, our bodies would be more susceptible to a temperature drop in cooler
conditions. Our bodies also lose heat through excretion of urine and through sweating. Water
has a relatively high specific heat and thus is a very good coolant to carry away excess heat.
The loss of heat by evaporation, however, would not be affected by a change in the specific
heat of water.
42. Suppose that you are in charge of regulating an industry in your area that manufactures
agricultural pesticides. How will you decide if this plant is obeying necessary environmental
controls? What criteria affect the success of this plant?
Answer:
A complex question! First, you would need to determine the environmental rules and
regulations in your region. Most likely these would apply to releases of chemicals into the
soil, air, and water. Then you would need to monitor what is being released by the industry,
in what amounts, and with what occurrence. Compliance with environmental controls,
economic factors, and community acceptance of the plant all will affect the success of this
plant.
43. Health goals for contaminants in drinking water are expressed as MCLG, or maximum
contaminant level goals. Legal limits are given as MCL, or maximum contaminant levels.
How are MCLG and MCL related for a given contaminant?
Answer:
The MCLG for carcinogens are set at zero, because no exposure may be safe. MCLG and
MCL values are usually the same except for carcinogens, where it may not be possible to set
the legal limit at zero.
44. Provide an explanation why CoCl2 is called cobalt(II) chloride, whereas CaCl2 is called
calcium chloride (no Roman numeral in the latter).
Answer:
The name cobalt(II) includes a Roman numeral because cobalt can form more than one ion
(and the Roman numeral indicates which one). However, calcium only forms the +2 ion, so
no Roman numeral is necessary.
45. Use the calibration graph in Figure 5.23 to determine the Pb2+
concentration (in M) of 5.0 mL
of a PbSO4 solution that has an absorbance reading of 0.4 at a wavelength of 283.8 nm.
Answer:
Using the calibration curve, an absorbance of 0.4 indicates a Pb2+
concentration of about
27 ppb. A solution with 1 ppb of solute has 1 10-6
grams of material per liter of solution.
Using this ratio, we can calculate the following:
PAGE 5-15
27 ppb1106g/L
1 ppb
1 mol Pb
207.7 g Pb1.3
mol
L1.3 M
46. Use the calibration graph in Figure 5.23 to find the absorbance of a solution resulting from
the addition of 5.0 mL of a 20 ppb PbCl2 solution to 10.0 mL of a 16 ppb PbSO4 solution.
Answer:
The 20 ppb solution contains 20µg of lead ions per liter of solution and the 16 ppb solution
contains 16 µg of lead ions per liter of solution. Combining these:
20 g Pb2
L
1L
1000 mL 5 mL 0.1g
16 g Pb2+
L
1L
1000 mL10 mL 0.16 g
Thus, the combined solutions have 0.26 micrograms of lead ion and a total volume of 15 mL.
To convert this to ppb:
0.26 g Pb2+
15 mL
1000 mL
1 L17
g
Lor 17 ppb
Using the calibration curve in Figure 5.23, a lead solution with a concentration of 17 ppb
should have an absorbance of about 2.75.
47. How can you purify your water when you are hiking? Use the Web to explore some of the
possibilities. What are the relative costs and effectiveness of these alternatives? Are any of
the methods similar to those used to purify municipal water supplies? Why or why not?
Answer:
Several options exist for purifying water while hiking. The traditional method is to boil it.
Boiling kills many microorganisms that may make you sick, but requires time and will not
remove chemical contamination. Boiling also requires time, fuel, and may release soot and
CO to the environment.
Another method of water purification is with iodine. It is easy and effective in twenty
minutes, but iodine should not be used long-term., In addition, pregnant women and people
with thyroid conditions should avoid purification with iodine. Two popular iodine treatments
in the U.S. are Potable Aqua (http://www.potableaqua.com/) and Polar Pure
(http://www.polarequipment.com/). While iodine renders water bacteriologically safe, it
doesn't remove chemical contaminants. Many people dislike the taste of iodine-treated water.
A third method of water purification is with a small amount of household bleach (check the
Web for directions). This will kill some but not all microorganisms. Again, many people
dislike the resulting taste
Filtering cleans water mechanically. According to Katadyn, the makers of many travel water
filters (http://www.baproducts.com/filters.htm), a good microfiltration unit can remove:
“harmful cocci, bacteria, protozoa, fungi, cysts, and parasites are totally removed, including
the chemically resistant infectious agents of Giardia, the amoebic and shigella dysenteries,
PAGE 5-16
and those causing typhoid, cholera bilharzia and a long list of other dangerous diseases.”
Filtration has the advantage that it does not require chemicals. While filters are the most
expensive option, a good filter pumps out good water in a few minutes and is reusable
(http://www.artoftravel.com/10water.htm).
48. Water quality in the chemistry building on a campus was continuously monitored because
testing indicated water from drinking fountains in the building had dissolved lead levels
above those established by the Safe Drinking Water Act.
a. What is the likely major source of the lead in the drinking water?
b. Does the chemical research carried out in this chemistry building account for the elevated
lead levels found in the drinking water? Why or why not?
Answer:
a. The likely source of lead is from solder in the pipe joints or from lead pipes themselves.
b. Research activities should not contribute to lead in the drinking water, assuming that any
lead compounds are disposed of using prescribed methods. Although many undergraduate
chemistry experiments used to use lead, most now have been redesigned to avoid it and other
toxic metal ions completely.
Exploring Extensions
49. Most people turn on the tap with little thought about where the water comes from. In
Consider This 5.4, you investigated the source of your drinking water. Now take a more
global view. Where does drinking water come from in other areas of the world? Investigate
the source of drinking water in a desert country, in a developed European country, and in an
Asian country. How do these sources differ?
Answer:
Answers will vary depending on the countries investigated. Island desert countries may
depend more heavily on desalination procedures, while European countries may depend on
ground water that has been treated by ozonation or chlorination. Drinking water sources in
Asian countries will vary, but many rural areas will depend heavily on local well water.
50. One of the large aquifers in the United States is under the Pine Barrens of New Jersey.
a. Where are the Pine Barrens in New Jersey?
b. Why are there increasing political pressures to use the water in this aquifer?
Answer:
a. The Cohansey Aquifer, a large aquifer in the United States (17 trillion gallons), lies near
the surface in the Pine Barrens of New Jersey. The Pine Barrens occupy about 1.1 million
acres of forest in south-central New Jersey. The area is neither barren nor populated only by
pine trees! New Jersey is the third-largest producer of cranberries and second in blueberry
production.
b. Because of its proximity to large population centers and because of the pristine quality of
the water, there are many pressures to tap this aquifer for drinking water supplies. Although
PAGE 5-17
development has been kept to a minimum, a few small towns and larger-scale real estate
developments are proposed in the area. The Pine Barrens of New Jersey are protected by law,
and yet there will continue to be pressures to increase the population and take the water from
this area.
51. Aquifers are also important in providing clean drinking water in other parts of the
world. Recently four countries in South America have reached a historic agreement to share
the immense Guaraní aquifer. This is particularly significant because although surface waters
often provoke discord and result in agreements, underground sources are routinely not
considered at least by international law.
a. Where is the Guaraní aquifer and what four countries are part of this international
“underground concordat”?
b. What concerns did each country have about this aquifer that led to the agreement?
Answer:
a. The Guaraní Aquifer System is an immense fresh groundwater reservoir in South America
shared by four countries: Argentina, Brazil, Paraguay, and Uruguay. In the Mercosur Trade
agreement, 2001, the “shared by all four nations” status was agreed upon.
b. “The Guaraní, a sandy sponge beneath a basalt shell, sprawls across 1.2m square
kilometers (460,000 square miles) and stores enough fresh water to supply 5.5 billion people
with 100 liters a day per person for 200 years. It is so big that there is no real competition for
its water; pollution in one part of the aquifer can take thousands of years to taint another. Yet
there are reasons to start worrying now. Some cities in São Paulo state, Brazil's industrial
center, depend on the Guaraní aquifer. One of them, Ribeirão Preto, has pumped so much
water out that the aquifer's level has dropped 60 meters in the area since the 1970s. In
western Paraguay, farmers are chopping down forest in an area that collects much of the
water for replenishing the aquifer. Owners of thermal spas in northern Uruguay fret that their
Argentine competitors will extract too much.” Source: The Economist, September 6, 2003.
52. Is there any such thing as “pure” drinking water? Discuss what is implied by this term, and
how the term’s meaning might change in different parts of the world.
Answer:
“Pure” water is usually interpreted as meaning water that has no dissolved impurities,
something that is very difficult to achieve. If rain has fallen through the atmosphere, it will
have picked up carbon dioxide, which explains why all rain water is slightly acidic. Ground
water can easily pick up dissolved ions, and even ice may contain suspended particulate
matter or gases. Bottled water will usually have dissolved minerals.
53. In the mid-1990s, researchers in Canada and Australia reported that consumption of
drinking water with more than 100 ppb aluminum can lead to neurological damage, such as
memory loss and perhaps to a small increase in the incidence of Alzheimer’s disease. Has
further research substantiated these findings? Find out more about this topic, and write a brief
summary of your findings. Be sure to cite the sources of your information.
PAGE 5-18
Answer:
As of 2007 in the United States, the EPA views aluminum in drinking water as a “nuisance
chemical,” that is, one that does not have health concerns, but does affect the aesthetic
qualities of the water. As such, it does not regulate aluminum but has a non-enforceable
standard of 50-200 µg/liter. The drinking water guidelines in Canada are established by
Health Canada. The standards, however, are set and enforced by the Canadian provinces. See
the Canadian publication “The facts about human health and aluminum in the drinking
water” for some interesting reading (http://www.esemag.com/0197/facts.html).
54. The text states that hydrogen bonds are only about one tenth as strong as the covalent bonds
that connect atoms within molecules. Check out that statement with this information.
Hydrogen bonds vary in strength from about 4 to 40 kJ/mol. Given that the hydrogen bonds
between water molecules are at the high end of this range, how does the strength of a
hydrogen bond between water molecules compare with the strength of a hydrogen-to-oxygen
covalent bond within a water molecule? Hint: Consult Table 4.2 for covalent bond energies.
Answer:
The bond energy of an oxygen-to-hydrogen bond is 467 kJ/mole. Assuming that the bond
energy of the hydrogen bonds between water molecules is 20 kJ/mole, this means that the
bond energy of an oxygen-to-hydrogen bond is about 20 times greater than the bond energy
of a hydrogen bond.
55. The text states that mass and density are often confused. Here is an example of that potential
misunderstanding of terms.
a. What do you think the term heavy metal implies when talking about elements on the
periodic table?
b. Compare the scientific definitions of this term that you may find in different sources, and
discuss whether each definition is related to relative density or to relative mass.
Answer:
a. Answers will vary but often include metals such as lead or mercury.
b. The term “heavy metal” has been used with a variety of meanings. For example, some
references will use the term to mean a metal with density greater than about 5.0 g/cm3,
especially a metal that is poisonous, such as lead or mercury. Other sources apply the term
heavy metal to mean elements in the 6th and 7th periods, ones with atomic masses greater
than approximately 133 g/mol. Still another source defines heavy metals as all metals above
sodium in atomic mass! Without a precise definition, the term is open to misinterpretation.
56. We all have the amino acid glycine in our bodies. This is its structural formula.
a. Is glycine a polar or nonpolar molecule? Use electronegativity differences to help answer
this question.
PAGE 5-19
b. Can glycine exhibit hydrogen bonding? Explain your answer.
c. Is glycine soluble in water? Explain.
Answer:
a. Glycine is a polar molecule and contains several polar bonds.
Bond Electronegativity Difference
O–H 3.5 – 2.1 = 1.4
O–C 3.5 – 2.5 = 1.0
N–H 3.0 – 2.1 = 0.9
N–C 3.0 – 2.5 = 0.5
b. Yes. Hydrogen bonding is possible with O-H and N-H bonds, both of which glycine
contains.
c. Because glycine is polar and has a relatively small molar mass, glycine should be soluble
in water.
57. Hard water is defined as having high concentrations of Mg2+
and Ca2+
ions. The
process of water softening involves removing these ions.
a. How hard is the water in your local area? One way to answer this question is to determine
the number of water-softening companies in your area. Use the resources of the Web, as well
as ads in your local newspapers and yellow pages, to find out if your area is targeted for
marketing water softening devices.
b. Use the Web to research methods for treating hard water, and explain how an ion
exchange process is used for this purpose.
Answer:
a. In the local area of this author, the water is so soft that it can be used in steam irons
without treatment. Few companies offer only water softening services, but there are many
vendors for faucet filters for tap water.
b. Hard water can be treated by chemical methods. A water softener works on the principle
of ion exchange. A special resin captures the hard water ions (Mg2+
and Ca2+
) while releasing
the softer sodium ions (Na+).
58. The calibration curve shown in Figure 5.23 is useful for Pb2
concentrations between 0 and
40 ppm. What are your options if a water sample is expected to contain a much higher
concentration of Pb2
?
Answer:
PAGE 5-20
One possibility is to extend the straight line and use this as an estimate. Another possibility is
to take more data to generate a new calibration curve. A third method is to analyze the lead
ion concentration by a different method, such as titration.
59. Some areas have a higher than normal amount of THMs in the drinking water. Suppose
that you are considering moving to such an area. Write a letter to the local water district
asking relevant questions to be answered before deciding to move.
Answer:
Questions include: (1) what levels of THMs have been observed, (2) what health effects, if
any, have been observed in the local water district, (3) whether the trihalomethanes were
created in the treatment process and, if so, if the process will be changed, and (4) what
options the consumer has to remove the THMs at the faucet.
60. PCBs are very useful chemicals that may end up in the wrong place, causing long-term
damage to birds and mammals. What are the uses of PCBs that made them desirable, and
what are some of the negative effects of these materials?
Answer:
Here is the report from the United States EPA, dated 1979.
“The Environmental Protection Agency today issued final regulations banning the
manufacture of polychlorinated biphenyls (PCBs) and phasing out most PCB uses. PCBs
are toxic and persistent chemicals primarily used as insulating fluids in heavy-duty
electrical equipment in power plants, industries, and large buildings across the country.
The EPA rules will gradually end many industrial uses of PCBs over the next five years,
but will allow their continued use in existing enclosed electrical equipment under
carefully controlled conditions.
PCBs, valued for chemical stability and fire resistance, were manufactured and processed
primarily for use as insulating fluids and coolants in electrical equipment and machinery
from 1929-1977. …
PCBs have caused birth defects and cancer in laboratory animals, and they are a
suspected cause of cancer and adverse skin and liver effects in humans. EPA estimates
that 150 million pounds of PCBs are dispersed throughout the environment, including air
and water supplies; an additional 290 million pounds are located in landfills in this
country.”