12
Chapter 2 Water
Multiple Choice Questions
1. Weak interactions in aqueous systems
Which of these statements about hydrogen bonds is not true?
A) Hydrogen bonds account for the anomalously high boiling point of water.
B) In liquid water, the average water molecule forms hydrogen bonds with three to four other water
molecules.
C) Individual hydrogen bonds are much weaker than covalent bonds.
D) Individual hydrogen bonds in liquid water exist for many seconds and sometimes for minutes.
E) The strength of a hydrogen bond depends on the linearity of the three atoms involved in the
bond.
2. Weak interactions in aqueous systems
A true statement about hydrophobic interactions is that they:
A) are the driving force in the formation of micelles of amphipathic compounds in water.
B) do not contribute to the structure of water-soluble proteins.
C) have bonding energies of approximately 20–40 Kjoule per mole.
D) involve the ability of water to denature proteins.
E) primarily involve the effect of polar solutes on the entropy of aqueous systems.
3. Weak interactions in aqueous systems
Hydrophobic interactions make important energetic contributions to:
A) binding of a hormone to its receptor protein.
B) enzyme-substrate interactions.
C) membrane structure.
D) three-dimensional folding of a polypeptide chain.
E) All of the above are true.
4. Weak interactions in aqueous systems
Dissolved solutes alter some physical (colligative) properties of the solvent water because they
change the:
A) concentration of the water.
B) hydrogen bonding of the water.
C) ionic bonding of the water.
D) pH of the water.
E) temperature of the water.
Chapter 2 Water
13
5. Weak interactions in aqueous systems
Osmosis is movement of a:
A) charged solute molecule (ion) across a membrane.
B) gas molecule across a membrane.
C) nonpolar solute molecule across a membrane.
D) polar solute molecule across a membrane.
E) water molecule across a membrane.
6. Ionization of water, weak acids, and weak bases
A hydronium ion:
A) has the structure H3O+.
B) is a hydrated hydrogen ion.
C) is a hydrated proton.
D) is the usual form of one of the dissociation products of water in solution.
E) All of the above are true.
7. Ionization of water, weak acids, and weak bases
The pH of a solution of 1 M HCl is:
A) 0.
B) 0.1.
C) 1.
D) 10.
E) –1.
8. Ionization of water, weak acids, and weak bases
The pH of a solution of 0.1 M NaOH is:
A) 0.1.
B) 1.0.
C) 12.8.
D) 13.
E) 14.
9. Ionization of water, weak acids, and weak bases
Which of the following is true about the properties of aqueous solutions?
A) A pH change from 5.0 to 6.0 reflects an increase in the hydroxide ion concentration ([OH–]) of
20%.
B) A pH change from 8.0 to 6.0 reflects a decrease in the proton concentration ([H+]) by a factor of
100.
C) Charged molecules are generally insoluble in water.
D) Hydrogen bonds form readily in aqueous solutions.
E) The pH can be calculated by adding 7 to the value of the pOH.
Chapter 2 Water
14
10. Ionization of water, weak acids, and weak bases
The pH of a sample of blood is 7.4, while gastric juice is pH 1.4. The blood sample has:
A) 0.189 times the [H+] as the gastric juice.
B) 5.29 times lower [H+] than the gastric juice.
C) 6 times lower [H+] than the gastric juice.
D) 6000 times lower [H+] than the gastric juice.
E) one million times lower [H+] than the gastric juice.
11. Ionization of water, weak acids, and weak bases
The aqueous solution with the lowest pH is:
A) 0.01 M HCl.
B) 0.1 M acetic acid (pKa = 4.86).
C) 0.1 M formic acid (pKa = 3.75).
D) 0.1 M HCl.
E) 10–12 M NaOH.
12. Ionization of water, weak acids, and weak bases
The aqueous solution with the highest pH is:
A) 1 M HCl.
B) 1 M NH3 (pKa = 9.25).
C) 0.5 M NaHCO3 (pKa = 3.77).
D) 0.1 M NaOH.
E) 0.001 M NaOH.
13. Ionization of water, weak acids, and weak bases
Phosphoric acid is tribasic, with pKa’s of 2.14, 6.86, and 12.4. The ionic form that predominates at pH
3.2 is:
A) H3PO4.
B) H2PO4–.
C) HPO42–.
D) PO43–.
E) none of the above.
14. Buffering against pH changes in biological systems
Which of the following statements about buffers is true?
A) A buffer composed of a weak acid of pKa = 5 is stronger at pH 4 than at pH 6.
B) At pH values lower than the pKa, the salt concentration is higher than that of the acid.
C) The pH of a buffered solution remains constant no matter how much acid or base is added to the
solution.
D) The strongest buffers are those composed of strong acids and strong bases.
E) When pH = pKa, the weak acid and salt concentrations in a buffer are equal.
Chapter 2 Water
15
15. Buffering against pH changes in biological systems
A compound has a pKa of 7.4. To 100 mL of a 1.0 M solution of this compound at pH 8.0 is added 30
mL of 1.0 M hydrochloric acid. The resulting solution is pH:
A) 6.5.
B) 6.8.
C) 7.2.
D) 7.4.
E) 7.5.
16. Buffering against pH changes in biological systems
The Henderson-Hasselbalch equation:
A) allows the graphic determination of the molecular weight of a weak acid from its pH alone.
B) does not explain the behavior of di– or tri-basic weak acids.
C) employs the same value for pKa for all weak acids.
D) is equally useful with solutions of acetic acid and of hydrochloric acid.
E) relates the pH of a solution to the pKa and the concentrations of acid and conjugate base.
17. Buffering against pH changes in biological systems
Consider an acetate buffer, initially at the same pH as its pKa (4.76). When sodium hydroxide
(NaOH) is mixed with this buffer, the:
A) pH remains constant.
B) pH rises more than if an equal amount of NaOH is added to an acetate buffer initially at pH 6.76.
C) pH rises more than if an equal amount of NaOH is added to unbuffered water at pH 4.76.
D) ratio of acetic acid to sodium acetate in the buffer falls.
E) sodium acetate formed precipitates because it is less soluble than acetic acid.
18. Buffering against pH changes in biological systems
A compound is known to have a free amino group with a pKa of 8.8, and one other ionizable group
with a pKa between 5 and 7. To 100 mL of a 0.2 M solution of this compound at pH 8.2 was added
40 mL of a solution of 0.2 M hydrochloric acid. The pH changed to 6.2. The pKa of the second
ionizable group is:
A) The pH cannot be determined from this information.
B) 5.4.
C) 5.6.
D) 6.0.
E) 6.2.
Chapter 2 Water
16
19. Buffering against pH changes in biological systems
Three buffers are made by combining a 1 M solution of acetic acid with a 1 M solution of sodium
acetate in the ratios shown below.
1 M acetic acid 1 M sodium acetate
Buffer 1: 10 mL 90 mL
Buffer 2: 50 mL 50 mL
Buffer 3: 90 mL 10 mL
Which of these statements is true of the resulting buffers?
A) pH of buffer 1 < pH of buffer 2 < pH of buffer 3
B) pH of buffer 1 = pH of buffer 2 = pH of buffer 3
C) pH of buffer 1 > pH of buffer 2 > pH of buffer 3
D) The problem cannot be solved without knowing the value of pKa.
E) None of the above
20. Buffering against pH changes in biological systems
A 1.0 M solution of a compound with 2 ionizable groups (pKa’s = 6.2 and 9.5; 100 mL total) has a
pH of 6.8. If a biochemist adds 60 mL of 1.0 M HCl to this solution, the solution will change to pH:
A) 5.60.
B) 8.90.
C) 9.13.
D) 9.32.
E) The pH cannot be determined from this information.
21. Buffering against pH changes in biological systems
You want to maintain pH = 7.0 for an enzyme-catalyzed reaction that will produce hydrogen ions
along with the desired product. At equal concentrations, which weak acid, if any, will serve as the
better buffer for the reaction: Acid A, with pKa = 6.5 or Acid B, with pKa = 7.5?
A) Acid A
B) Water is as good as either of the acids available.
C) Acid B
D) Both are equally effective.
22. Water as a reactant
In which reaction below does water not participate as a reactant (rather than as a product)?
A) Conversion of an acid anhydride to two acids
B) Conversion of an ester to an acid and an alcohol
C) Conversion of ATP to ADP
D) Photosynthesis
E) Production of gaseous carbon dioxide from bicarbonate
Chapter 2 Water
17
23. The fitness of the aqueous environment for living organisms
Which of the following properties of water does not contribute to the fitness of the aqueous
environment for living organisms?
A) Cohesion of liquid water due to hydrogen bonding
B) High heat of vaporization
C) High specific heat
D) The density of water is greater than the density of ice
E) The very low molecular weight of water
Short Answer Questions
24. Weak interactions in aqueous systems
Pages: 43–51 Difficulty: 2
Name and briefly define five types of noncovalent interactions that occur between biological
molecules.
25. Weak interactions in aqueous systems
Pages: 46–48 Difficulty: 3
Explain the fact that ethanol (CH3CH2OH) is more soluble in water than is ethane (CH3CH3).
26. Weak interactions in aqueous systems
Pages: 46–48 Difficulty: 3
Explain the fact that triethylammonium chloride ((CH3CH2)3N•HCl) is more soluble in water than is
triethylamine ((CH3CH2)3N).
27. Weak interactions in aqueous systems
Page: 48 Difficulty: 3
Explain with an appropriate diagram why amphipathic molecules tend to form micelles in water.
What force drives micelle formation?
Chapter 2 Water
18
28. Weak interactions in aqueous systems
Pages: 51–52 Difficulty: 2
(a) Briefly define “isotonic,” “hypotonic,” and “hypertonic” solutions. (b) Describe what happens
when a cell is placed in each of these types of solutions.
29. Ionization of water, weak acids, and weak bases
Page: 57 Difficulty: 1
For each of the pairs below, circle the conjugate base.
RCOOH RCOO–
RNH2 RNH3+
H2PO4– H3PO4
H2CO3 HCO3–
30. Ionization of water, weak acids, and weak bases
Page: 57 Difficulty: 2
Phosphoric acid (H3PO4) has three dissociable protons, with the pKa’s shown below. Which form of
phosphoric acid predominates in a solution at pH 4? Explain your answer.
Acid pKa
H3PO4 2.14
H2PO4– 6.86
HPO42– 12.4
31. Ionization of water, weak acids, and weak bases
Pages: 58–59 Difficulty: 1
Chapter 2 Water
19
Define pKa for a weak acid in the following two ways: (1) in relation to its acid dissociation constant,
Ka, and (2) by reference to a titration curve for the weak acid.
32. Buffering against pH changes in biological systems
Pages: 58–60 Difficulty: 2
Give the general Henderson-Hasselbalch equation and sketch the plot it describes (pH against amount
of NaOH added to a weak acid). On your curve, label the pKa for the weak acid and indicate the
region in which the buffering capacity of the system is greatest.
33. Buffering against pH changes in biological systems
Pages: 59–60 Difficulty: 3
Draw the titration curve for a weak acid, HA, whose pKa is 3.2. Label the axes properly. Indicate with
an arrow where on the curve the ratio of salt (A–) to acid (HA) is 3:1. What is the pH at this point?
34. Buffering against pH changes in biological systems
Pages: 61–62 Difficulty: 2
What is the pH of a solution containing 0.2 M acetic acid (pKa = 4.7) and 0.1 M sodium acetate?
35. Buffering against pH changes in biological systems
Pages: 61–62 Difficulty: 2
You have just made a solution by combining 50 mL of a 0.1 M sodium acetate solution with 150 mL
of 1 M acetic acid (pKa = 4.7). What is the pH of the resulting solution?
Chapter 2 Water
20
36. Buffering against pH changes in biological systems
Pages: 61–62 Difficulty: 2
For a weak acid with a pKa of 6.0, show how you would calculate the ratio of acid to salt at pH 5.
37. Buffering against pH changes in biological systems
Pages: 61–62 Difficulty: 3
Suppose you have just added 100 mL of a solution containing 0.5 mol of acetic acid per liter to 400
mL of 0.5 M NaOH. What is the final pH? (The pKa of acetic acid is 4.7.)
38. Buffering against pH changes in biological systems
Pages: 61–62 Difficulty: 2
A weak acid HA, has a pKa of 5.0. If 1.0 mol of this acid and 0.1 mol of NaOH were dissolved in one
liter of water, what would the final pH be?
39. Buffering against pH changes in biological systems
Page: 65 Difficulty: 3
In proteins, the amino acid histidine (His) plays an important role in many biological reactions. The
pKa for the protonation of His to form HisH+ = 6.0. When pH = 7.0, what is the fraction of total
histidine that will be in the HisH+ form?
Chapter 2 Water
21
40. Buffering against pH changes in biological systems
Page: 65 Difficulty: 2
H+ + HCO3– H2CO3 CO2 + H2O
Severe diarrhea is accompanied by a loss of HCO3–. If untreated, will the condition result in acidosis
or alkalosis? Use the bicarbonate buffer system given in the scheme above and Le Chatelier’s
Principle to explain your answer.
41. Water as a reactant
Page: 65 Difficulty: 1
Give an example of a biological reaction in which water participates as a reactant and a reaction in
which it participates as a product.
42. The fitness of the aqueous environment for living organisms
Pages: 65–66 Difficulty: 1
If ice were denser than water, how would that affect life on earth?