Which of the following ions is the conjugate base of oxalic acid.. Since calcium carbonate is a salt of a strong base, calcium hydroxide, and a weak acid, carbonic acid, it will be basic
Trang 1GENERAL CHEMISTRY TOPICAL:
Acids and Bases
Test 1
Time: 23 Minutes*
Number of Questions: 18
* The timing restrictions for the science topical tests are optional If you are using this test for the sole purpose of content reinforcement, you may want to disregard the time limit
Trang 2DIRECTIONS: Most of the questions in the following
test are organized into groups, with a descriptive passage preceding each group of questions Study the passage, then select the single best answer to each question in the group Some of the questions are not based on a descriptive passage; you must also select the best answer to these questions If you are unsure of the best answer, eliminate the choices that you know are incorrect, then select an answer from the choices that remain Indicate your selection by blackening the corresponding circle on your answer sheet A periodic table is provided below for your use with the questions
PERIODIC TABLE OF THE ELEMENTS
1
H
1.0
2
He
4.0 3
Li
6.9
4
Be
9.0
5
B
10.8
6
C
12.0
7
N
14.0
8
O
16.0
9
F
19.0
10
Ne
20.2 11
Na
23.0
12
Mg
24.3
13
Al
27.0
14
Si
28.1
15
P
31.0
16
S
32.1
17
Cl
35.5
18
Ar
39.9 19
K
39.1
20
Ca
40.1
21
Sc
45.0
22
Ti
47.9
23
V
50.9
24
Cr
52.0
25
Mn
54.9
26
Fe
55.8
27
Co
58.9
28
Ni
58.7
29
Cu
63.5
30
Zn
65.4
31
Ga
69.7
32
Ge
72.6
33
As
74.9
34
Se
79.0
35
Br
79.9
36
Kr
83.8 37
Rb
85.5
38
Sr
87.6
39
Y
88.9
40
Zr
91.2
41
Nb
92.9
42
Mo
95.9
43
Tc
(98)
44
Ru
101.1
45
Rh
102.9
46
Pd
106.4
47
Ag
107.9
48
Cd
112.4
49
In
114.8
50
Sn
118.7
51
Sb
121.8
52
Te
127.6
53
I
126.9
54
Xe
131.3 55
Cs
132.9
56
Ba
137.3
57
La *
138.9
72
Hf
178.5
73
Ta
180.9
74
W
183.9
75
Re
186.2
76
Os
190.2
77
Ir
192.2
78
Pt
195.1
79
Au
197.0
80
Hg
200.6
81
Tl
204.4
82
Pb
207.2
83
Bi
209.0
84
Po
(209)
85
At
(210)
86
Rn
(222) 87
Fr
(223)
88
Ra
226.0
89
Ac †
227.0
104
Unq
(261)
105
Unp
(262)
106
Unh
(263)
107
Uns
(262)
108
Uno
(265)
109
Une
(267)
*
58
Ce
140.1
59
Pr
140.9
60
Nd
144.2
61
Pm
(145)
62
Sm
150.4
63
Eu
152.0
64
Gd
157.3
65
Tb
158.9
66
Dy
162.5
67
Ho
164.9
68
Er
167.3
69
Tm
168.9
70
Yb
173.0
71
Lu
175.0
†
90
Th
232.0
91
Pa
(231)
92
U
238.0
93
Np
(237)
94
Pu
(244)
95
Am
(243)
96
Cm
(247)
97
Bk
(247)
98
Cf
(251)
99
Es
(252)
100
Fm
(257)
101
Md
(258)
102
No
(259)
103
Lr
(260)
GO ON TO THE NEXT PAGE.
Trang 3Passage I (Questions 1–6)
Geological change occurs on wide-ranging time
scales Dramatic geologic changes can occur during
extremely brief periods, as with earthquakes, or can take
eons to have a significant effect, such as the slow building
of mountain ranges While much of this change is due to
physical effects, many geological processes, such as cave
formation, have chemical components Caves are formed
when limestone rock strata come into contact with
aqueous carbonic acid This process is described by the
following reactions:
CO2(g) + pressure H 2 O CO2(aq)
Reaction 1
CO2(aq) + H2O(l) H2CO3(aq)
Reaction 2
H2CO3(aq) H+(aq) + HCO3(aq)
Reaction 3
CaCO3(s) + H+(aq) Ca2+(aq) + HCO3(aq)
Reaction 4
Carbon dioxide from the atmosphere dissolves in
water according to Reaction 1 At atmospheric pressure
and 25°C, the concentration of dissolved carbon dioxide is
quite low, however, due to the higher pressures possible
in the cracks and fissures of the limestone, the
concentration is much higher The dissolved carbon
dioxide then reacts with water, forming carbonic acid
(Reaction 2) The carbonic acid dissociates (Reaction 3),
and the acidic hydrogens react with limestone according to
Reaction 4 At some point, the water drains from the
caves, leaving a network of cracks still filled with
dissolved limestone As the dissolved limestone seeps out
of these cracks and is exposed to the lower pressure of the
cave interior, Reactions 1 through 4 reverse, resulting in
the precipitation of calcium carbonate If the precipitation
occurs before the drop of dissolved limestone leaves the
cave ceiling, a stalactite forms
If the precipitate builds-up on the cave floor, a
stalagmite forms
1 Which of the following compounds is the major
component of stalagmites?
A Ca(HCO3)2
B CaCO3
C H2CO3
D Ca2CO3
2 If CaCO3 were water soluble, what would be the
approximate pH of a 1.0 M solution?
A 3
B 5
C 7
D 8
3 Many substances that are considered acids contain an
H+ ion Similarly, many bases contain OH– ions BCl3, containing neither H+ nor OH–, would be classified as:
A a Brønsted base.
B a Brønsted acid.
C a Lewis base.
D a Lewis acid.
4 If the pKa1 value for H2CO3 is 6.4, what is the pH of
a 10–3 M solution of this acid?
A 3.0
B 4.7
C 6.3
D 9.3
GO ON TO THE NEXT PAGE.
Trang 45 Consider the following acid-base reaction:
HCO3 + HCl H2CO3 + Cl–
A HCl is an acid and HCO3 is its conjugate base
B Cl– is an acid and HCl is its conjugate base
C HCO3 is a base and Cl– is its conjugate acid
D HCO3 is a base and H2CO3 is its conjugate
acid
6 Which of the following is a strong acid?
A Nitric acid
B Phosphoric acid
C Carbonic acid
D Acetic acid
GO ON TO THE NEXT PAGE.
Trang 5Passage II (Questions 7–13)
Acids are defined, according to the Arrhenius
definition, as materials that give up a proton when added
to water or reacted with another substance Since the
proton is always closely associated with at least one water
molecule, it is sometimes written as H3O+ It was once
thought that all acids contained oxygen and, in fact, the
Greek word oxygen means acid-forming This belief was
refuted with the elemental analysis of hydrochloric acid
Some acids are almost 100% dissociated: these compounds
are called strong acids Acids that have a low percentage
ionization are known as weak acids
There are literally thousands of compounds that are
acids, most of them weak organic acids The relative
strength of an acid is indicated by its Ka value The table
below shows the acid dissociation constants for several
common acids
Table 1 Name Formula Acidity constant (Ka)
Hydrochloric acid
Acetic acid
Hydrocyanic acid
HCl
HC2H3O2 HCN
2 × 10–5
6 × 10–10 Oxalic acid H2C2O2 Ka1 = 6 × 10–2
Ka2 = 6 × 10–5
Nitrous acid
Nitric acid
HNO2 HNO3
4 × 10–4
Carbonic acid H2CO3 Ka1 = 4 × 10–7
Ka2 = 6 × 10–11
7 Which of the following ions is the conjugate base of
oxalic acid?
A HC2O4
B C2O42–
C HC2O4+
D H3C2O4+
8 Which of the following is the strongest acid?
A Acetic acid
B Oxalic acid
C Hydrocyanic acid
D Nitrous acid
9 What would be the pH of a 0.05 M solution of acetic
acid?
A 0.05
B 2.0
C 3.0
D 6.0
1 0 Which of the following correctly represents the
titration curve for oxalic acid when it has been completely titrated with NaOH?
B
C
D
mL NaOH added
A
mL NaOH added
mL NaOH added
mL NaOH added
1 1 What volume of 0.5 M KOH would be necessary to
neutralize 15 mL of 1.0 M nitrous acid?
A 30 mL
B 15 mL
C 7.5 mL
D 1.5 mL
GO ON TO THE NEXT PAGE.
Trang 61 2 What is the approximate pH when 0.1 moles NaOH
has been added to 0.2 moles hydrocyanic acid?
A 10.0
B 9.2
C 7.0
D 3.7
1 3 Which of the following mixtures could be used in a
buffer solution?
A HNO3 and NaNO2
B HC2H3O2 and HCN
C HCl and KCl
D NaHCO3 and K2CO3
GO ON TO THE NEXT PAGE.
Trang 7Questions 14 through 18 are
NOT based on a descriptive
passage
1 4 If a buffer solution that contains 0.15 moles NaNO2
and 1.5 moles HNO2 has a pH of 2.4, what will be
the approximate pH after the addition of 0.7 moles
NaOH?
A 5.4
B 4.4
C 3.4
D 0.7
1 5 Which of the following aqueous solutions would have
the lowest pH?
A 2.0 M NaCl
B 0.5 M HF
C 0.3 M HI
D 1.8 M NaClO
1 6 Which of the following aqueous solutions would have
a pH greater than 7.0?
A 1.0 M HClO
B 0.5 M NH4CL
C 0.25 M HCN
D 0.1 M KCN
1 7 If the Ka of HClO is 33 × 10–8, the Kb value of
ClO– is:
A 3 × 10–7
B 3.3 × 108
C 3.3 × 106
D 14
1 8 The titration curve of alanine, an amino acid, is
shown below The structure of alanine is:
H3C CH COOH
NH 2
pH
7 14
0 I
II
III
mL Titrant
Which of the combinations below correctly represents the structures of alanine at points I, II, and III, respectively?
A
H3C CH COOH
NH 2 I
H3C CH COO –
NH 2 II
H3C CH COOH
NH 3 III
B
H3C CH COO –
NH 2 I
H3C CH COO –
NH 3 II
H3C CH COOH
NH 3 III
C
H3C CH COOH
NH 2 I
H3C CH COO –
NH 3 II
H3C CH COOH
NH 3 III
D
H3C CH COOH
NH 3 I
H3C CH COO –
NH 3 II
H3C CH COO –
NH 2 III
END OF TEST
Trang 8ANSWER KEY:
Trang 9ACIDS AND BASES TEST 1 EXPLANATIONS
Passage I (Questions 1–6)
1 The correct answer for the first problem is B This question is largely a reading comprehension-type
question You are given many hints at the answer throughout the passage At one point, the passage states that, "caves are formed when limestone rock strata comes into contact with aqueous carbonic acid." In further describing the process by four reactions, we can see that only one species is a solid, calcium carbonate The fact that limestone is calcium carbonate is further reinforced by the subsequent description of Reaction 4 as "acidic hydrogens reacting with limestone." Later on, the passage mentions that the reactions reverse and calcium carbonate precipitates A following sentence says that if this
"precipitate builds-up on the cave floor, a stalagmite forms." The stalagmite must be composed of the precipitate, calcium carbonate Thus, the correct answer is B Answer A is incorrect because calcium hydrogen carbonate is soluble and this is in fact the soluble product of the dissolution of limestone Answer C is carbonic acid, which in addition to being incorrect from the statements in the passage, has never been isolated Choice D presents an incorrect formula for calcium carbonate You should know that metallic elements in the calcium column form +2 ions Because the carbonate ion has a –2 charge and the entire molecule is neutral, the charge of each calcium in choice D would be plus 1, an unlikely state Again the correct
answer is B
2 The correct answer for question 2 is D Since calcium carbonate is a salt of a strong base, calcium
hydroxide, and a weak acid, carbonic acid, it will be basic You should know that the hydroxides of all Group I and II metals are strong bases If you didn't remember that carbonic acid is a weak acid, Reaction 3 should remind you Reaction 3 shows that carbonic acid, like any weak acid, is in equilibrium with its ions and does not dissociate completely Many students find that because there are many weak acids and only a very few strong acids, if they memorize the strong acids they can assume all others are weak In predicting whether a salt will be acidic or basic in solution a simple rule holds: the salt will have the characteristics of the stronger of the acid and base from which it was hypothetically produced We can see how this is true form the example of calcium carbonate If calcium carbonate dissolved, the calcium ions would not react with the water but the carbonate ions would pick up hydrogen ions from the water to reestablish the carbonate-hydrogen carbonate-carbonic acid equilibrium When hydrogen ions are removed from water the product is hydroxide ions and the solution would become basic Only one of the answer choices has a basic pH, a pH greater than 7 Choice D is therefore correct
3 The correct answer for problem 3 is choice D BCl3 has a central boron atom with only 6 electrons A Lewis acid by definition is a compound which has fewer electrons than the familiar octet and can, therefore, accept a pair of electrons from another compound Choice D is correct A Lewis base, in contrast, has a pair of unshared electrons and can donate a pair of electrons Therefore Lewis acids or bases are defined in terms of their ability to accept or donates an electron pair The Brønsted concept is quite different Brønsted acids and bases are defined in terms of their ability to donate or receive
a proton A Brønsted acid can donate a proton and usually has an H written first in the formula, which boron trichloride does not contain A Brønsted base is a proton acceptor Though it is less easy to see why boron cannot accept a proton, once explained it is easily understood Because a hydronium ion has no electrons and, as mentioned before, boron has only 6
electrons around it, all of which are involved in bonding with the chlorine atoms, none are available to form a boron-hydrogen bond Boron trichloride cannot be a proton acceptor and is therefore not a Brønsted base Again, choice D is the correct
answer
4 The correct answer for question 4 is B You should already be very familiar with the types of calculations
involved in this problem The pKa of an acid is the negative logarithm of the acidity constant for the acid To undue this
operation and get the Ka of the acid we take the inverse log, or antilog, of –6.4 Since the negative log of the dissociation constant is 6.4, the anti-log is 4 ∞ 10–7 (Note: An easy way to estimate anti-logs is as follows: if the log of "X" is equal
to 1, X is equal to 10; if the log of X is 2, X is equal to 100; if the log of X is equal to 3, X is equal to 1000, and so on Now for the negative values: if the log of X is equal to –1, X is equal to 0.1; if the log of X is equal to, say, –4, then X is equal to 0.0001, and so on.) Like all equilibrium constants, the concentrations of products of the reactants appear in the numerator and the reactants in the denominator All the stoichiometric coefficients are one in this case and so no exponents
are involved in the expression In this case, the Ka1 would be the concentration of the hydrogen carbonate ion times the
concentration of the hydrogen ion divided by the concentration of the undissociated carbonic acid Since the concentrations of the hydrogen ion and hydrogen carbonate ion are unknown but equal, because they come from the same source, we can give the concentration of each as x The concentration of the undissociated carbonic acid is the original concentration, 0.001 moles
per liter, minus the amount that dissociates, x Putting this all together, the Ka, 4 ∞ 10–7, equals x times x, or x2, over 0.001 molar minus x To solve this equation we assume x is small compared to 0.001 and so can be ignored We can say this is true because carbonic acid is a weak acid and only dissociates to a small extent and as x represents the amount that
dissociates it would be expected to be small proportional to 0.001 This approximation reduces the Ka expression to x2 over
Trang 10carbonate Here we only need the hydrogen ion concentration Since pH is the negative log of the hydrogen ion
concentration, we take the negative log of 2 ∞ 10–5 and find the pH is 4.7 The correct answer is B
5 The correct answer for question 5 is D To solve this problem you must understand what conjugate bases
and acids are A conjugate acid has one more H+ than the compound with which it is being compared In other words the conjugate acid loses a proton to become its conjugate base In the same way a conjugate base can gain a proton to become its conjugate acid Statement A is false because the product of Reaction 4 is calcium hydrogen carbonate not calcium
hydrocarbonate Statement B is wrong because calcium carbonate cannot act as an acid Statement C is close but
interchanges the terms Only choice D is correct Bicarbonate is a base and is the conjugate acid of carbonate The correct answer is D
6 The correct answer to question 6 is A Perhaps the best way to solve a problem like this is to memorize
the few strong acids that are common and assume that all other acids are weak Some of the most common strong acids
include: sulfuric acid (H2SO4), nitric acid (HNO3), hydrochloric acid (HCl), hydrobromic acid (HBr), hydroiodic acid (HI), chloric acid (HClO3), and perchloric acid (HClO4) Again the correct answer is A
Passage II (Questions 7–13)
7 For question 7, the correct answer is choice A The conjugate base of a compound is a species which has
one less hydrogen ion and therefore one unit greater negative charge than the original compound In other words, it is
missing one H+ from the formula of the original compound The formula given in choice A meets those conditions compared
to the formula of oxalic acid itself Choice B is the conjugate base of choice A, but is not the conjugate base of oxalic acid, since it differs by two hydrogen ions from the oxalic acid formula Choice D is the conjugate acid for oxalic acid, having one more hydrogen ion than oxalic acid This might indeed exist Choice C probably does not exist, as oxalic acid would need to lose a hydride ion for the charge to be correct Again, the correct answer is A
8 The correct answer for question 8 is B The strongest acid is the one with the highest Ka value This is
because the larger the Ka value is, the greater extent to which the acid dissociates For those acids that are multiprotic, that is
have more than one acidic hydrogen that can be removed, only the first of the Ka's contributes significantly to the total acidity
of the acid For example, because the Ka values for oxalic acids differ by a factor of a thousand, the second proton from oxalic acid is a thousand times less acidic than the first proton The significant contributor to the acidity of oxalic acid is the first
proton that comes off Because the Ka for this first deprotonation is larger than any of the other acid choices, oxalic acid is the strongest acid among the group The correct answer is oxalic acid, choice B
9 The correct answer for question 9 is C The solution may be found by knowing how the acidity constant,
or Ka, is defined The Ka is an equilibrium constant and is defined to be the equilibrium constant for the dissociation process
of an acid Therefore the acidity constant of acetic acid is the equilibrium constant for the equilibrium between the
undissociated acetic acid, CH3COOH, on the left, and the proton, H+, and acetate, CH3COO− on the right The Ka, as an equilibrium constant, therefore equals the product of the concentrations of H+ and acetate divided by the concentration of
undissociated acetic acid Because the H+ and the acetate come from the same source in equal amounts and there is no other significant source of either of them there must be equal concentrations of both of these species We can call the
concentrations of acetate and hydronium ion x since we don't know the value at this point The numerator in the Ka
expression therefore becomes x ∞ x or x2 The concentration of undissociated acetic acid in the denominator is the original
concentration, 0.05 moles per liter, minus the amount that dissociates, x The Ka expression now has x2 in the numerator
and 0.05 minus x in the denominator This expression equals the Ka for acetic acid, which you can see in the table is 2 ∞
10–5 This is a difficult equation to solve algebraically as it is written, so to simplify it and make it easier to solve we make
an approximation The value of x in the denominator would be expected to be small compared to 0.05 We can say this is true because acetic acid is a weak acid and only dissociates to a small extent and as x represents the amount that dissociates it
would be expected to be small proportional to 0.05 This approximation reduces the Ka expression to x2 over 0.05 equals 2
∞ 10–5 Solving for x by multiplying both sides of the equation by 0.05 and taking the square root of both sides we find that
x is equal to 1 ∞ 10–3 This value of x represents the approximate concentration of both H+ and acetate If we needed a more accurate answer we could substitute this value of x into the denominator and resolve the equation for the value of x in the numerator This solution process is called solving by iteration, as a prior answer is substituted in a part of the expression and
a more accurate answer obtained The accuracy of the solution for this particular question is sufficient As pH is defined as the negative log of the hydronium ion concentration, the solution would have a pH of 3 The correct answer is C
1 0 The correct answer for question 10 is C We can eliminate many of the answers quickly Oxalic acid, as