The empirical formula is a whole number multiple of the molecular formula.. Choice A says that water has a higher boiling point because it is more polar.. Choice B says that water has a
Trang 1General Chemistry Discretes Test
Time: 30 Minutes
Number of Questions: 30
This test consists of 30 discrete questions—questions
that are NOT based on a descriptive passage These discretes comprise 15 of the 77 questions on the Physical Sciences and Biological Sciences sections of the MCAT
Trang 2GENERAL CHEMISTRY DISCRETES TEST DIRECTIONS: The following questions are not based
on a descriptive passage; you must 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 31 Which of the following statements about molecular
and empirical formulas is (are) FALSE?
I A given compound can have the same
molecular and empirical formula
II The empirical formula is a whole number
multiple of the molecular formula
III H2O2 represents the empirical formula of
hydrogen peroxide
A III only
B I and II only
C II and III only
D I, II, and III
2 All of the following statements are consistent with
Bohr’s model of the atom EXCEPT:
A an electron may assume an infinite number of
velocities
B an electron is most stable in its ground state.
C the electron shell numbers represent the principal
energy levels
D electrons in orbitals closest to the nucleus have
the lowest energy
3 Which of the following statements is NOT true of a
gas at pressures greater than 400 atm and temperatures
close to 0 K?
A The pressure of the gas is always lower than that
predicted by the ideal gas law
B The extent to which the gas deviates from the
ideal gas law depends on the molecules’ size and
polarity
C Intermolecular forces become significant.
D The ideal gas equation is less accurate under these
conditions than at higher temperatures and lower
pressures
4 What volume of water would be needed to dilute
50mL of 3M H2SO4 to 0.75M?
B 100 mL
C 150 mL
D 200 mL
5 Among the elements with partially filled p subshells,
atomic radii decrease significantly from left to right across a period while the atomic radii of transition elements change only slightly from left to right across a period This difference is most likely due to:
A the presence of greater numbers of electrons in
the outer shells of transition elements
B the presence of greater numbers of electrons in
the outer shells of elements with partially filled p
subshells
C the greater nuclear charges of transition elements.
D the presence of d electrons in the second
outermost shell in transition elements
6 H2O has a higher boiling point than HF because:
A H2O is more polar than HF
B H2O can form more hydrogen bonds
C H2O has a higher molecular weight
D H2O has more atoms
7 Which of the following compounds contains the
greatest percentage of oxygen by weight?
A C3H6O5Cl
B C3H6O2
C C5H10O5
D C4H8O3
GO ON TO THE NEXT PAGE.
Trang 48 Which of the following conditions guarantees a
spontaneous reaction?
A Positive ∆H, positive ∆S
B Positive ∆H, negative ∆S
C Negative ∆H, negative ∆S
D Negative ∆H, positive ∆S
9 Which of the following statements concerning
equilibrium constants is true?
A An equilibrium constant that is close to 1 means
that the reaction does not favor either reactants or
products heavily
B Equilibrium constants have units of L/mol.
C The stoichiometric coefficients in a reaction are
not part of the equilibrium expression
D The equilibrium constant for a reaction is the
same for any temperature
1 0 If the pressure of a gas sample is doubled at constant
temperature, the volume will be:
A 4 times the original.
B 2 times the original.
C 1
2 of the original.
D 1
4 of the original.
1 1 The Nernst equation:
A relates resistance and voltage.
B relates voltage and current.
C relates resistance and concentration.
D relates concentration and voltage.
1 2 The melting points of ionic solids are:
A high because of their electrostatic attractions.
B high because of their high densities.
C low because of their intermolecular attractions.
D low because of their high dissociation constants.
1 3 The Ksp of Mg(OH)2 in water is 1.2 × 10–11 mol3/L3 If the Mg2+ concentration in an acid solution is 1.2 × 10–5, what is the pH at which Mg(OH)2 just begins to precipitate?
D 11
1 4 The rate law expression for the following reaction:
N2 + 3 H2→ 2 NH3
A can be represented by rate = [NH3]2 / [N2][H2]3
B can be represented by rate = k[NH3]2
C can be represented by rate = k[NH3]2 / [N2][H2]3
D cannot be determined from the information given.
1 5 K+ and Cl– have the same:
A atomic weight.
B electronic configuration.
C ionization constant.
D number of protons and neutrons.
1 6 Which of the following statements correctly
characterizes a galvanic cell?
I Oxidation occurs at the anode, which is negative
II Oxidation occurs at the anode, which is positive
III Reduction occurs at the cathode, which is positive
A II only
B III only
C I and III only
D I, II, and III
1 7 In order to make a buffer solution, a weak monoprotic
acid could be added to:
A another acid.
B another base.
C the salt of its conjugate base.
D the salt of its conjugate acid.
GO ON TO THE NEXT PAGE.
Trang 51 8 When chromium metal is used to form K2Cr2O7 the
oxidation state of chromium changes from:
A 0 to 4.
B 3 to 6.
C 2 to 6.
D 0 to 6.
1 9 Which of the following molecules contains both
ionic and covalent bonds?
A C6H14
B MgCl2
C (NH4)2SO4
D H2O
2 0 Which of the following will increase the rate at which
ice melts in a closed container if all other parameters
are kept constant?
A Adding water with a temperature of 0°C
B Lowering the temperature below 0° C
C Lowering the pressure
D Raising the pressure
2 1 BaCl2 dissociates in water to give one Ba2+ ion and
two Cl– ions If concentrated HCl is added to this
solution:
A [Ba2+] increases
B [Ba2+] remains constant
C [OH–] increases
D the number of moles of undissociated BaCl2
increases
2 2 Avogadro’s number is NOT equal to:
A the number of atoms in 11.2 L of O2 at STP
B the number of atoms in 1 mole of He at STP.
C the number of electrons in 96,500 coulombs.
D the number of
SO ions in 1 L of 1 N sulfuric acid
2 3 What is the range of possible values for the
[OH–] / [H+] ratio in an aqueous acid solution?
A 0 – 1
B 0 – 14
C 1 – 14
D 1 – ∞
2 4 A flask contains three times as many moles of H2 gas
as it does O2 gas If hydrogen and oxygen are the only gases present, what is the total pressure in the flask if the partial pressure due to oxygen is Pl?
A 4P1
B 3P1
C 4
3 P1
D 3
4 P1
2 5 Which of the following molecules is polarized?
A BH3
B NF3
C C2H6
D SF6
2 6 Which of the following is NOT a true statement
about the entropy of a system?
A Entropy is a measure of the randomness in a
system
B The entropy of an amorphous solid is greater
than that of a crystalline solid
C The entropy of a spontaneous reaction cannot
decrease
D At constant temperature and pressure, the entropy
of a system will spontaneously increase
GO ON TO THE NEXT PAGE.
Trang 62 7 From the following reaction and thermodynamic data,
what is the energy released when a C—H bond forms?
(Note: ∆Hf° CO2(g) = –393 kJ/mol, and ∆Hf° H2O(g)
= –242 kJ/mol.)
CH4 + 2 O2(g) → CO2(g) + 2 H2O(g) ∆H°rxn = – 802
kJ/mol
A –75 kJ/mol
B –19 kJ/mol
C 75 kJ/mol
D 167 kJ/mol
2 8 Which of the following generalizations CANNOT be
made about the phase change of a pure substance from
solid to liquid?
A It involves a change in potential energy.
B It involves no change in temperature.
C It involves a change in kinetic energy.
D It involves a change in entropy.
2 9 What would be the stoichiometric coefficient of
hydrochloric acid in the following equation?
Cl2 + H2O → HCl + HClO3
D 10
3 0 Which of the following compounds can act as a
Lewis base?
A HClO2
B NH2NH2
C NH4+
D BF3
END OF TEST
Trang 7THE ANSWER KEY IS ON THE NEXT PAGE
Trang 8ANSWER KEY:
Trang 9GENERAL CHEMISTRY DISCRETES TEST EXPLANATIONS
1 The answer to question 1 is choice C To answer this, you must understand the differences between
empirical and molecular formulas The empirical formula of a compound is the simplest whole number ratio of the atoms in
it, whereas the molecular formula represents the actual whole number ratio of the atoms The molecular formula is usually a whole number multiple of the empirical formula, although sometimes the empirical and molecular formulas are the same Given this information, let's go through the statements to determine which of them is false Statement number I is true For instance, water, H2O, has the same empirical and molecular formula Statement II is false because it is the molecular formula which is usually a whole number multiple of the empirical formula, not the other way around Finally, statement III is false H2O2, hydrogen peroxide, has an empirical formula of HO, so the empirical formula is different from the molecular formula given Another is glucose, C6H12O6, which has an empirical formula of CH2O Since statements II and III are both incorrect, choice C is the answer we're looking for
2 In question 2, the right answer is choice A Let's go through each of the choices and see how they apply
to Bohr's model of the atom Choice A, which says that an electron may assume an infinite number of different velocities, is true in classical mechanics but not in Bohr's model Bohr used quantum theory in developing his atomic model and placed specific conditions on the possible values of the electron velocity Since we're looking for the incorrect statement, A is the correct answer Choice B, which says that an electron is most stable in its ground state, is true The ground state of an electron is its lowest possible energy state From thermodynamics, we know that systems tend to be the most stable at low energy; thus, an electron is most stable in the ground state Choice C says the electron shell numbers represent the principal energy levels This is true; the principal quantum number of a given electron is the same as the number of its electron shell
in the Bohr model Choice D says that electrons closest to the nucleus have the lowest energy As an electron absorbs energy, it jumps to a higher orbital and increases its distance from the nucleus The electron drops to a lower orbital releasing energy or it escapes the pull of the nucleus, making the atom a cation Again, choice A is the correct answer
3 For question 3, the correct choice is A One is asked in this question to determine which of the choices
contains a false description of the gas Under conditions of high pressure and low temperature, the gas is not behaving ideally and corrections must be made for the volume of the gas molecules and the intermolecular forces between them Therefore, choice D, which states that the ideal gas equation would not describe the gas accurately is quite correct Choice C is also a characteristic of the gas since the high pressure reduces the volume and thus the distance between the molecules Intermolecular forces, therefore, have a greater effect on each gas molecule Now we have narrowed our choices down to A and B The van der Waals equation is useful in describing and predicting the behavior of non-ideal gases, and one of its features is the introduction of two parameters a and b, which account for intermolecular forces and excluded volumes respectively The values of these parameters depend on the gas in question; more specifically, the magnitude of a is determined by how strongly the gas molecules attract one another, and b is influenced by the size of the molecules The higher the values of a nd b, the more the behavior of the gas is non-ideal (Recall that if a =b=0, one gets back the ideal gas law.) Therefore choice B is also correct By elimination, one can hence conclude that the correct answer is choice A because it contains a false statement In rationalizing the van der Waals equation, one observes that the presence of intermolecular attraction leads to a lower pressure than would be expected from using the ideal gas law, but when coupled with the effect of excluded volume, no generalization can be made as to whether the actual or ideal pressure would be greater (This holds for gas volume as well, unless if condensation has taken place, in which case the volume occupied by the liquid will be much smaller
than that predicted by any gas law.)
4 For question 4, which is a simple dilution problem, the correct answer is C The number of
moles of solute in the solution will be the same after dilution as before, and the number of moles in each case is equal to the molar concentration multiplied by the volume of solution This means that the initial concentration times the initial volume will be equal to the final concentration times the final volume So, the final volume will equal the initial concentration times the initial volume divided by the final concentration If we work out the math, we find that the final volume of the diluted solution will be 0.2 liters, or 200 milliliters Since we started out with 50 milliliters of the solution, we would have
to add 150 milliliters of water to get a final volume of 200 milliliters Therefore, the correct answer is choice C
5 Question 5 is correctly answered by choice D To answer this question, we have to think about the thing
that determines atomic radius: how strongly the outermost electron shell is attracted to the nucleus Electrons are attracted to the nucleus by its positive charge There are two factors that affect the strength of that attraction: more protons for a stronger pull by the nucleus, which reduces the atomic radius, and the shielding of the outermost electron shell by the intervening filled electron shells, which increases the atomic radius So trends in atomic radii result from interaction between these two competing effects The key thing here is that the electrons in the outer shell are not very effective at shielding each other from the nuclear pull, while electron shells further in toward the nucleus do have a strong shielding effect on electrons
in the outer shell
Now, because of the order in which electron orbitals fill, as you go from left to right across the table, each
subsequent transition element generally has one more d electron but doesn't change the number of electrons in the outermost s subshell However, when you look at the elements with the partially filled p subshells, each subsequent element has one more p electron The d electrons are in the next-to-last shell, so they shield the valence s shell pretty effectively Thus, even though the number of protons in the nucleus increases from left to right across the table, the fact that the number of d
Trang 10Choices A and C are wrong because within a given period, the transition elements have fewer electrons and smaller nuclear
charges that the elements with partially filled p orbitals Once again, choice D is the right answer.
6 The correct answer to question 6 is choice B This question asks you to determine why water has a
higher boiling point than hydrogen fluoride Choice A says that water has a higher boiling point because it is more polar Before you decide whether polarity has an effect on the boiling point of a substance, you should be able to eliminate this choice by realizing that water is not more polar that HF Hydrogen fluoride has a greater charge separation between its constituent atoms and thus has a more polar bond Choice B says that water has a higher boiling point because it can form more hydrogen bonds Hydrogen bonding does affect boiling points by increasing the attraction between the molecules of a compound Water is capable of forming as many as four hydrogen bonds per molecule while hydrogen fluoride can only form
2 This intermolecular attraction leads to a complexation of water molecules and contributes to the high boiling point of water Thus choice B is correct Although we have found the right answer, let's look at the rest of the choices Choice C says that water will have a higher boiling point because it has a higher molecular weight Well, this statement is also false
in and of itself because hydrogen fluoride, at 20 grams per mole, has a greater molecular weight than water, which is only 18 grams per mole Finally, choice D says that water has more atoms than hydrogen fluoride Well, this is true, but it has nothing to do with boiling point, so is not the answer we're looking for So, the correct answer is choice B
7 For question 7, the correct answer is C You need to first calculate the weight of oxygen in each
compound and then divide that value by the compound's molecular weight The weight of oxygen in choice A is 80 grams per mole and the molecular weight is 157 grams per mole Dividing this out gives 51% oxygen For B, the weight of oxygen is 32 grams per mole and the molecular weight is 74 grams per mole This gives you 43% oxygen Choice C contains 80 grams of oxygen per mole and its molecular weight is 150 grams per mole Here, you get 53% oxygen Finally, choice D contains 48 grams of oxygen per mole and its molecular weight is 104 grams per mole, which yields 46% oxygen Obviously, choice C contains the most oxygen by weight and is the correct answer
8 The correct answer for question 8 is D This question asks you to predict which combination of ∆H and
T∆S values will always give a spontaneous reaction Recall that spontaneous reactions have negative values of Gibbs free
energy, ∆G, and that ∆G equals ∆H – T∆S, where ∆H is the change in enthalpy, ∆S is the change in entropy, and T is the
absolute temperature From this equation, it's clear that the best way to guarantee a negative ∆G is to have a negative ∆H
value and a positive ∆S value, since T, always expressed in Kelvin, is always positive This corresponds to choice D, so that
is the right answer
9 For question number 9, choice A is correct To answer this question, you need to understand the concept
of equilibrium and know the expression for the equilibrium constant Let's go through all of the choices Choice A says that
if an equilibrium constant is close to 1, then neither reactants nor products are favored heavily This is correct because the equilibrium constant expresses a ratio of product concentrations to reactant concentrations If the reaction is heavily in favor
of products, the value of the equilibium constant will be much greater than 1 Conversely, if the reaction favors reactants, then the equilibrium constant would have a value much less than one Choice B says that equilibrium constants have units of L/mol This is incorrect; the equilibrium constant is actually a dimensionless number Choice C states that stoichiometric coefficients from the reaction have nothing to do with the equilibrium expression The equilibrium expression is derived by multiplying the concentrations of the products raised to their stoichiometric coefficients together and then dividing by the product of the reactant concentrations raised to their stoichiometric coefficients So we know choice C is wrong Finally, choice D says that the equilibrium constant of a reaction is the same for any given temperature This is wrong because an equilibrium constant is a characteristic of a system at a particular temperature Changing temperatures changes the system identity, so it also changes the equilibrium constant Again, the correct answer is choice A
1 0 For question 10, the right answer is choice C The question is an application of Boyle's Law This
states that at constant temperature, the pressure and volume of a gas are inversely proportional to each other Therefore, since the pressure of the gas in the question increased, the volume must decrease Since the pressure and volume are inversely related, when the pressure is doubled, the volume is halved This is choice C
1 1 The answer to question 11 is choice D The Nernst equation expresses the relationship between the
observed potential of a redox system and the concentrations of the reactants and the products involved It is expressed as
follows: the potential of the cell, E, is equal to the standard potential of the cell minus the quantity RT over nF times the natural log of the reaction quotient, Q R is the universal gas constant expressed in joules per mole Kelvin, T is the absolute temperature, n is the number of moles of electrons exchanged in the redox reaction, and F is Faraday's constant Q is just a representation of the extent to which a reaction has proceeded Since potential is expressed in volts and Q is expressed in
concentration units, the equation relates voltage and concentration and the correct answer is D
1 2 Moving on to question 12, the correct answer is A Because an electron is transferred from one atom to
another in an ionic bond rather than being shared as in a covalent bond, the atoms in an ionic solid exist as ions and are held together by very strong electrostatic forces Because these forces hold them in tight crystalline patterns, ionic solids generally have high melting points This makes choice A correct Density, as in choice B, doesn't explain the high melting points of