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Chapter 11
Chemistry in Water: Salts, Acids, and Bases
Review Questions
1. Salts are solid compounds consisting of a cation (except H+) and an anion (except
OH-).
2. Ionic compounds dissolve when both anions and cations dissociate from the ionic
lattice and associate with a number of molecules of water that surround them. In the
process of dissolution, the attractive force between the ions of opposite charge is
replaced by the attraction of the ions with several molecules of water.
3. A solute is a substance that dissolves in a liquid. A solvent is defined as the
dissolving liquid.
4. A solute dissolves in a solvent to produce a solution.
5. Solutions that contain a relatively high concentration of solute are called concentrated
solutions. Conversely, solutions that contain a relatively low concentration of solute
are called dilute.
6. Solutions that contain the maximum amount of a dissolved solid at a given
temperature are called saturated solutions.
7. A solid that forms because its solubility is exceeded in a solution is called a
precipitate.
8. Parts per million (ppm) concentrations represent the mass of a dissolved substance in
one million grams of the solution. The numerical concentration of any solute on the
ppm scale is identical to that on the milligrams per liter scale.
9. Mineral water is water obtained from natural sources that has a higher concentration
of dissolved salts than regular water.
10. The mineral component of tooth enamel is apatite, Ca5(PO4)3OH.
11. In fluorapatite, a fluoride ion replaces the hydroxide ion to produce Ca5(PO4)3F. This
conversion makes the enamel in teeth more resistant to the carboxylic acids
produced by the bacteria in the mouth.
12. An acid is a substance that increases the concentration of H+ ions in solution. Acids
have several distinguishing characteristics including their sour taste, their ability to
turn litmus paper red, and their ability to react with many metals to produce
hydrogen gas, or calcium carbonate to produce carbon dioxide gas.
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13. A base is defined as a substance that increases the concentration of the hydroxide ion
in solution. Characteristics of a base include its bitter taste, its slimy feel, and its
ability to turn litmus paper blue.
14. When a strong acid such as HCl ionizes, the hydrogen atom loses its half-share of
the electron pair that represented the covalent bond in the HCl molecule to the
chlorine atom. The two ions are then separated and surrounded by several molecules
of water. The chemical reaction for this process is: HCl  H+ + Cl15. When a weak acid ionizes, the hydrogen atom loses its half share of the electron pair
that represented the covalent bond in the acid in a process identical to that for a
strong acid. In a weak acid, however, the hydrogen ion that is formed readily
recombines with the anion thereby reforming a molecule of the unionized acid. The
equation for this partial ionization is given below.
HF

H+ + F-
16. Strong acids are acids that completely ionize in aqueous solutions. Weak acids are
acids that are only partially ionized in aqueous solutions.
17. Polyprotic acids are acids that contain more than one hydrogen atom that can ionize
in water to produce H+ ions.
18. A chemical equilibrium is a condition in which a reaction proceeds in both directions
at equal rates.
19. a) A solution of hydrochloric acid in water is commonly referred to as muriatic acid
and is used to clean the surface of metals, masonry, and cement.
b) Vinegar is a dilute (5%) solution of acetic acid, a weak acid.
c) Strong bases such as sodium hydroxide are used in drain cleaners.
d) Ammonia, a weak base, dissolved in water is used as a household cleaner.
20. A mole is the name given to 6.02 x 1023 particles, such as molecules, atoms, or ions.
A mole refers to a quantity of objects in the same way that a dozen refers to 12
objects.
21. Molarity is defined as the number of moles of a substance that are present in one liter
of solution. The units of molarity are moles per liter or moles / liter.
22. The pH of a solution tells you the concentration of H+ ions in solution. The lower the
pH, the higher the concentration of H+ ions. Solutions with a pH of 7 are considered
neutral, less than 7 acidic, and greater than 7 basic.
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23. Weak acids are typically found in foods and provide a sour or tart flavor.
24. Both strong acids (HCl in gastric juice) and weak acids (bicarbonate in the blood and
the amino acids in proteins) are found in the human body. The bicarbonate anion and
the amino acids in proteins also serve as weak bases in the body.
25. a) Acids will attack metals, dissolving them in the process. The reaction with iron
metal is given below.
2 H+ + Fe  H2 + Fe+2
b) Acids react with anions to form a covalent bond. The reaction of H+ with the
carbonate anion is given below.
H+ + CO3-2  HCO3c) Acids can react with the bicarbonate anion to produce carbonic acid.
H+ + HCO3-  H2CO3
26. The functions of gastric or stomach acid are to suppress the growth of bacteria and to
facilitate the digestion of food by enzymes in the stomach.
27. Even though the pH of stomach acid is about 1, the stomach is not damaged because
the lining of the stomach is protected by a mucous lining that is constantly being
replaced. Further, the lining provides a source of bicarbonate anions that react with
the strong acid to produce a weak acid, carbonic acid.
28. Some antacids cause you to burp because they contain either the carbonate or the
bicarbonate anion. These antacids react with the acid in your stomach to produce
carbon dioxide gas, CO2.
29. Baking powder consists of a combination of sodium bicarbonate and a weak acid.
When baking powder is mixed with a material containing water, carbon dioxide gas
is produced which expands as the temperature rises. This release and expansion of
carbon dioxide produces baked goods with a greater volume and less density than
the original dough.
30. Major cations and anions in natural water consist of calcium (Ca+2), bicarbonate
(HCO3-), sulfate (SO4-2), magnesium (Mg+2), carbonate (CO3-2), sodium (Na+), and
chloride (Cl-). The ionic components of bottled water vary considerably. Many
bottled waters, however, have the same ionic content as natural water.
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31. Hard water is defined as water with appreciable amounts of calcium (Ca+2) and
magnesium (Mg+2).
32. The reaction of a strong acid with a strong base produces a salt and water. The
general equation is:
HX + MOH  MX + H2O, where “M” is a cation
33. Indicators are materials that take on different colors depending upon whether they
are in an acidic or a basic environment. Examples of indicators include litmus paper
and the juice from grapes or red cabbage.
34. A buffer is a solution that resists changes to pH when a strong acid or base is added
to it.
Understanding Concepts
35. Water does not randomly surround ions in a solution because water is polar in
nature. Water consists of a partially positive region that associates with anions, and a
partially negative region that associates with cations.
36. a) Al2O3 is insoluble in water.
b) KBr is soluble in water.
c) CaSO4 is insoluble in water.
37. When water evaporates from an aqueous salt solution, only the water molecules
enter the gaseous state. The ions are left behind. Eventually, the concentration of the
ionic solute increases past its solubility limit and the salt precipitates out of the
solution.
38. Since the numerical concentration of any solute dissolved in water is the same on the
milligrams per liter scale and the ppm scale, sodium ion concentrations of ppm and
mg/L are identical.
39. Since the numerical concentration of any solute dissolved in water is the same on the
milligrams per liter scale and the ppm scale, a calcium ion concentration of 66 ppm
is identical to a concentration of 66 mg/L. Therefore,
66 mg calcium ion
1L
=
x mg calcium ion
0.200 L
x = 13.2 mg of calcium ion
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40.
112 mg sulfate ion
1L
=
50 mg sulfate ion
xL
x = .446 L or 446 ml
41. The fluoride ion in drinking water is able to replace the hydroxide ion in apatite,
Ca5(PO4)3OH, the mineral component of tooth enamel. Fluorapetite is more resistant
than apetite to the carboxylic acids produced by bacteria in the mouth, thereby
preventing tooth decay. Also, fluoride ions inhibit the conversion of carbohydrates to
the carboxylic acids that attack tooth enamel.
42. Sodium fluoride is soluble in water and ionizes to Na+ and F-.
43. Sodium is a very common cation in natural water. Since natural water represents the
source of most bottled waters, similar sodium concentrations should exist between
the two. It is worth noting that two very popular brands of bottled water, Aquafina
and Dasani, are produced from natural water by a purification process known as
reverse osmosis. These products have negligible amounts of the sodium ion.
44. An acid is a substance that increases the concentration of H+ ions in solution while a
base increases the concentration of the hydroxide ion in solution.
45. A strong acid completely ionizes and is, therefore, composed exclusively of ionic
components. A weak acid contains mostly molecular components with very few ions
present.
46. Polyprotic acids ionize by a multi-step process in which one proton is removed from
the acid at a time. This stepwise process is outlined below.
H2X  H+ + HXHX-  H+ + X-2
47. When a reaction is in equilibrium, rates of the forward and reverse reactions are
equal. In the case of a strong acid, however, the reaction proceeds only in the
forward direction to produce the ionic products.
48. Several of the products that contain strong acids or strong bases are similar in their
use. HCl is a strong acid used for cleaning metals, cement, and brick, while NaOH, a
strong base, is used as a drain cleaner. Similar properties also exist for weak acids,
such as vinegar, and weak bases, such as ammonia, both of which serve as cleaning
agents. Acids and bases, however, have different characteristics. Acids are sour and
tart while bases are bitter. In addition, acidic materials, such as citrus juices, can be
consumed, while many basic materials cannot.
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49. Molarity refers to the number of moles of solute per liter of solution. If the solute is
an acid, the greater the molarity of acid, the greater the molarity of H+ and the lower
the pH.
50. The hydrogen ion concentration of a solution with a pH of 10 is 1 x 10-10 M and the
hydrogen ion concentration of a solution with a pH of 5 is 1 x 10-5 M. Thus, the
solution with a pH of 5 is 105 times more concentrated in hydrogen ions than that
having a pH of 10. The solution with a pH of 5 is more acidic since it has the higher
hydrogen ion concentration.
51. Since the pH of the aqueous solution is 6, which is less than 7, the solution is acidic.
As the pH decreases, the hydrogen ion concentration increases. Thus, a solution with
a pH of 6 contains less hydrogen ions per liter than one with a pH of 4.
52. Using the relationship that the pH of a solution is equal to the negative of the power
of 10 of the hydrogen ion concentration, the following pH values are calculated.
a) 0.01 M = 1 x 10-2 M; pH = 2
b) 0.0000001 = 1 x 10-7 M; pH = 7
c) 1 x 10-9 M; pH = 9
53. Molarity is defined as the number of moles of solute per liter of solution. Thus,
0.010 moles of HCl = x moles of HCl
0.100 L
1L
x = 0.10 moles of HCl
Therefore, the molarity of the solution is 0.10M. Since HCl is a strong acid, the
molarity of the hydrogen ion concentration is equal to that of HCl. Thus, the H+
concentration is 1 x 10-1 M and the pH is 1.
54. It is not technically correct to say that calcium carbonate “dissolves” in acid because
it implies that the acid molecules are simply stabilizing the ions of the salt through
electrostatic interactions. This is not the case. Rather, acid reacts with calcium
carbonate to produce a salt that does “dissolve” in the water. For example, the
reaction of calcium carbonate with hydrochloric acid produces calcium chloride (a
water-soluble salt), carbon dioxide, and water.
CaCO3 + 2 HCl  CaCl2 + CO2 + H2O
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55. Antacids contain compounds that are hydroxides, carbonates, or bicarbonates. These
bases neutralize the acid in your stomach. The reactions for each of these three
neutralizations are given below (M represents a +2 cation).
Hydroxide: M(OH)2 + 2 HCl  MCl2 + 2 H2O
Carbonate: MCO3 + 2 HCl  MCl2 + CO2 + H2O
Bicarbonate: M(HCO3)2 + 2 HCl  MCl2 + 2 CO2 + 2 H2O
56. The name for Al(OH)3 is aluminum hydroxide and the name for Mg(OH)2 is
magnesium hydroxide.
57. Vinegar is a weak acid and baking soda is a weak base. These two compounds react
according to the following equation. (Note: CH3COOH can be written as HC2H3O2.)
HC2H3O2 + NaHCO3  NaC2H3O2 + CO2 + H2O
Yes, a fizz is produced. This fizz results from the CO2 that is released. This reaction
originally produces carbonic acid, H2CO3, which decomposes to CO2 and H2O.
58. Human blood is normally neutral due to the presence of the bicarbonate ion, which
buffers the pH to around 7.4.
59. Vinegar (acetic acid) removes the scale (calcium carbonate) by the following
neutralization reaction.
2HC2H3O2 + CaCO3  Ca(C2H3O2)2 + CO2 + H2O
60. Ammonia is a weak base that donates a pair of electrons to a hydrogen ion from
water to form an ammonium ion and a hydroxide ion. The reaction of ammonia and
water is given below.
NH3 + H2O

NH4+ + OH-
F- + H2O

HF + OH-
61.
62. Water is able to act as both an acid and a base because it undergoes self-ionization to
a very small extent to produce H+ (an acid) and OH- (a base). Thus, water acts as an
acid when it reacts with F- to produce HF or NH3 to produce NH4+.
F- + H2O

HF + OH-
NH3 + H2O

NH4+ + OH-
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Water also acts as a base when it reacts with acetic acid to produce the acetate anion.
HC2H3O2 + H2O 
H3O+ + C2H3O2-
63. Sodium bromide, NaBr, is the salt produced by the reaction of NaOH and HBr.
NaOH + HBr  NaBr + H2O
64. Indicators are useful in monitoring the progress of an acid-base reaction because
they take on different colors depending upon whether they are in an acidic or a basic
solution.
65. Buffer solutions consist of a weak acid and a salt of the weak acid or a weak base
and a salt of the weak base. If hydrogen ion is added to the buffer solution, the large
excess of anion (from the salt of the weak acid) reacts with the hydrogen ion to
produce the molecular form of the weak acid. As a result, the hydrogen ion
concentration, and hence the pH, remains relatively constant. When a base is added
to the buffer, it reacts with the molecular form of the weak acid to produce more
anion and H2O. Again, the hydrogen ion concentration, and hence the pH, remains
relatively constant. The same rational can be used to explain the action of an acid or
base on a buffer system composed of a weak base and a salt of the weak base.
Synthesizing Ideas
66. No, drinking only pure water with no detectable salt content would not be healthy
for two main reasons. First, your body requires ions such as sodium, potassium,
calcium, and magnesium for normal function. Many of the ions we need come from
the water we drink. Second, water devoid of ions is very reactive as it aggressively
hydrates ions. In fact, deionized water has been shown to leach metal ions from
stainless steel piping and metallic water holding tanks.
67. The concentrations of ions in aqueous solutions are generally small and in the
milligram per liter range. Since the numerical value of the concentration of a solute
is identical on the milligram per liter scale and on the parts per million scale, the
ppm unit of measurement is reasonable.
68. If the pH of a solution is 4, then the hydrogen ion concentration is 1 x 10-4 M, or
0.0001 M. Since the numerical value for the concentration of H+ expressed in
molarity is the same as the number of grams of this ion dissolved in one liter, there
are 0.0001 grams of hydrogen ions present in one liter of this solution.
If the pH of a solution is 3, then the hydrogen ion concentration is 1 x 10-3 M, or
0.001 M. There are 0.001 grams of hydrogen ions present in one liter of this
solution.
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69. Strong acids cannot be buffers because in order to have a buffer solution, a
significant portion of the acid must exist in its molecular form. Since strong acids
ionize completely, insignificant amounts of the parent acid exist in solution. Only
weak acids and weak bases can be components of buffer solutions.
70. Ingesting an extraordinary amount of antacids can increase the concentration of
CO3-2 and HCO3- in the blood. These ions react with the H+ in the blood to produce
carbonic acid, H2CO3, thereby lowering the concentration of H+ and increasing the
pH of the blood.
71. The stepwise ionization reactions are:
H2C2O4

H+ + HC2O4-
HC2O4-

H+ + C2O4-2
The net or overall reaction for complete ionization is:
H2C2O4

2 H+ + C2O4-2
72. The stepwise reactions for this neutralization are:
H2SO4 + NaOH  NaHSO4 + H2O
NaHSO4 + NaOH  Na2SO4 + H2O
The overall equation is:
H2SO4 + 2 NaOH  Na2SO4 + 2 H2O
The net ionic reaction is:
H+ + OH-  H2O
73. a) This is not a buffer system since HCl is a strong acid.
b) This is a buffer system since HF is a weak acid and NaF is a salt of the weak acid.
c) This is not a buffer system since HCl is a strong acid.
74. If a concentration of a solute in a solvent is represented in micrograms per liter, then
the corresponding “parts per” scale with equal numerical values would be parts per
billion (1 x 10-6 grams divided by 1 x 103 grams [recall that the density of water is 1
gram / ml] or 1 x 10-9).
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75. A weak acid can react with the hydroxy ion of apatite. It is much more difficult for
the weak acid to abstract the fluoride ion from fluorapatite.
Ca5(PO4)3OH + H+  Ca5(PO4)3+ + H2O
76. Given an NH3 - NH4Cl buffer system, an added strong acid would react with the
NH3 present to form the ammonium ion. This will result in a slight increase in the
ammonium ion concentration and a slight decrease in the ammonia concentration.
The hydroxide concentration, and hence the pH, remains relatively constant.
H+ + NH3  NH4+
If hydroxide were added, it would react with the ammonium ion to produce ammonia
and water.
OH - + NH4+  NH3 + H2O
Again, the hydroxide concentration, and hence the pH, remains relatively constant.
77. When they exist as solids, acids and bases do not react. When they ionize in solution,
however, they readily react to form a salt and water. Thus, the weak acids and bases
in Mad Dawg gum do not react in the solid gum, but when chewed, the compounds
dissolve in the water in your mouth and react to form CO2 and water.
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