2008 AP Chemistry Practice Exam Commentary
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
The answer is “A.” To oxidize means to lose an electron. It becomes harder to remove an
electron from something that has a filled energy level. Noble Gasses are completely filled, so
they will be the hardest to take an electron off. (72% correct)
The answer is “E.” Very large atoms, all of those over atomic number 83 will be radioactive.
Actinides are the second set of “F-block” elements on the periodic table. (70% correct)
The answer is “C.” Anions are those elements that gain electrons, those that are close, but not
filled. Halogens each have 7 valence electrons, have high electronegativity and will likely gain
electrons. (63% correct)
The answer is “C.” This would be the equivalence point when moles of acid equal moles of base.
(65% correct)
The answer is “E.” Once we go well beyond equivalence the majority of the solution is the
strong base that was being added. As we add more, it becomes closer to what we were adding.
(69% correct)
The answer is “B.” When the moles of weak acid, equal the moles of conjugate base present,
this would be “halfway” to equivalence. If we started with one mole of weak acid and conjugate
base is produced when weak acid is used…then when we’ve used half, they are equal. (38%
correct)
The answer is “A.” In order to be trigonal pyramidal, you must have 3 bonds with one lone pair
on the central atom. This is the case in NH3. For reference the shapes are as follows, trigonal
pyramidal, trigonal planar, linear, bent and linear. (68% correct)
The answer is “E.” Strong electrolytes are soluble ionic salts, strong acids or strong bases. HBr is
one of the six strong acids. (50% correct)
The answer is “E.” Slow effusion is related to slow velocity. Velocity is inversely related to
molar mass. HBr has the greatest molar mass, so it has the lowest velocity and slowest effusion.
(62% correct)
The answer is “C.” By definition and the third law of thermodynamics the only substance that
can have zero entropy is a crystal solid at absolute zero. (41% correct)
The answer is “A.” This is unlikely to be discussed on the AP exam. By relating the rate of
reaction to temperature, we can the energy that is required to initiate the reaction, which will
be constant regardless of the temperature of the reaction. (33% correct)
The answer is “C.” In order to be spontaneous at all temperatures, both the entropy (ΔS) and
enthalpy (ΔH) terms must be thermodynamically favorable. – ΔH and + ΔS are
thermodynamically favorable. (57% correct)
The answer is “A.” Evaporation requires energy, so ΔH is positive. Entropy is increasing, going
from liquid to gas ΔS is positive. (41% correct)
The answer is “C.” Combustion reaction release energy, so it has a negative ΔH. A liquid is being
converted into gasses, so ΔS is positive. As an aside, this too would be spontaneous regardless
of temperature. (34% correct)
The answer is “B.” A precipitation reaction involves ions from aqueous solutions combining to
produce an insoluble ionic solid. (80% correct)
16. The answer is “E.” A coordination complex is another name for a complex ion. The [Ag(NH3)]+ is
an example of this. As you can see, it has two ligands (NH3) surrounding our cation, Ag+, which
makes sense, as the rule is the number of ligands is twice the central ion charge. (58% correct)
17. The answer is “A.” Most all synthesis (I think all, but I don’t want to misspeak) are red-ox.
Synthesis is when two or more compounds/elements combine to make a single larger
compound. (54% correct)
1
18. The answer is “C.” At STP, 1 mole of gas occupies 22.4 L. So 1 L is 22.4 mole. It is a 2:3 mole
2
ratio for KClO3 : O2. So if you have 3 mole KClO3, you can produce 1 mole of O2. (62% correct)
19. The answer is “C.” Bromine has 35 e-, but as Br- it will have 36 e-. If we look at each of the
other ions, they have 18, 18, 36, 54 and 18 respectively. (78% correct)
20. The answer is “E.” CH4 is a covalent compound with polar bonds. However, the polar bonds are
cancel each other out, due to the positive charge on the outer hydrogens, the only
intermolecular force is dispersion, which all substance with electrons exhibit. (46% correct)
21. The answer is “B.” An excited electron is one that does not have everything in its lowest
configuration. (71% correct)
22. The answer is “C.” All but “A” feature polar bonds. B, D and E each feature dipoles (B and D) or
hydrogen bonding (E). In CO2 it is a linear molecule that features negative oxygens (due to
increased electronegativity). The positive is contained on the inside, making it a non-polar
molecule. (39% correct)
23. The answer is “B.” When in doubt “D-block” elements will have a 2+ ion. Silver is likely the only
exception you will need to know (only 1+). (82% correct)
24. The answer is “B.” The fact that it is Cesium, we can look on the periodic table and find its
atomic number 55, which is its protons. Given that it is neutral, protons will equal electrons, so
55. 134 represents the mass number, which is protons + neutrons. 55 + x = 134. x = 79. (82%
correct)
25. The answer is “A.” This is an ideal gas law problem. 𝑝𝑉 = 𝑛𝑅𝑇, by rearranging for P, we get
𝑝=
𝑛𝑅𝑇
,
𝑉
given that 𝑅 = 0.0821
𝐿∙𝑎𝑡𝑚
,
𝑚𝑜𝑙∙𝐾
we need to convert our 35°C to 308 K. (87% correct)
26. The answer is “D.” By halving a first order reaction (X) the rate would be cut in half. By doubling
a second order reaction (Y) the rate would quadruple. Together, halving and quadrupling would
result in a doubling of the overall rate. (47% correct)
27. The answer is “D.” In all nuclear equations we must balance charge and mass. “D” does that.
(69% correct)
28. The answer is “C.” Oxygens often make double bonds, Nitrogens often make triple bonds. The
greater the ratio of hydrogens to carbons, the more single bonds. (77% correct)
29. The answer is “B.” This is a problem that we need to do a little math, but it isn’t too terrible.
24.5 grams of solute, with a molar mass of 98 grams/mole. That is 0.25 moles. Molality is
𝑚𝑜𝑙𝑒𝑠 𝑠𝑜𝑙𝑢𝑡𝑒 0.25 𝑚𝑜𝑙𝑒𝑠
. 0.1 𝑘𝑔
𝑘𝑔 𝑠𝑜𝑙𝑣𝑒𝑛𝑡
= 2.5 𝑚𝑜𝑙𝑎𝑙. (46% correct)
30. The answer is “B.” Density is mass over volume. When doing calculations, your answer should
be reported based on the number used with the fewest significant figures. Masses each feature
31.
32.
33.
34.
35.
36.
37.
38.
39.
40.
41.
five significant figures, but the volume features four. (Yes, the two zeros are significant). (54%
correct)
The answer is “C.” Brutal question to solve out without a calculator, so we approximate as best
we can. 30 g of sulfur is slightly less than 1 mole. 70 g of fluorine is slightly less than 4 moles.
(65% correct)
The answer is “C.” Lots of information that does not matter in this problem. Substance is
decaying from some value (Let’s call it 1) to 1/8th its value. We want to find half life, so in the
first half life it drops to a half. After the second it is ¼. After the third it becomes our final value
of 1/8. So it takes 124 seconds to under three half-lives (all equal duration). 124 s divided by 3
is 41.3 seconds. (38% correct)
The answer is “A.” H2 combines with O2 in a 2:1 ratio. This means the O2 is the limiting reactant.
If all 0.15 moles of O2 is used, then 0.30 moles of H2 is used. Leaving 0.10 moles remaining.
Molar mass is 2 g/mole. Meaning 0.20 grams. (56% correct)
The answer is “D.” Balance it. Not too bad. Coefficients are 1, 4, 3. (80% correct)
The answer is “C.” This is a Le Châtelier’s Principle question. By adding H2, more HI will be
produced (shift right). The reaction is endothermic, so by increasing temperature, the forward
reaction (needing heat), will be favored thus producing more HI. There are an equal number of
moles of gas on both sides, so pressure or volume won’t have an impact. (56% correct)
The answer is “A.” A tough question without a calculator, but the numbers are really friendly.
Molar mass is 28 g/mole. So if we have 2.8 g then we have 0.1 moles of C2H4. If we have 0.1
moles of C2H4, then we have 6.022 x 1022 molecules. There are two carbon atoms per molecule,
so we multiply by 2. (35% correct)
The answer is “A.” Covalent network solids are covalent compounds bonded in a continuous
network. There are only a few examples (Diamond (C), Graphite (C) and Glass (SiO2) are
examples). The reason for their formation has to do with having 4 valence electrons and the
flexibility to connect in many configurations infinitely. The reason it is not C in this question has
to do with combining with Oxygen. Carbon only makes two compounds with oxygen, Carbon
Monoxide or Carbon Dioxide (neither are solid). (44% correct)
The answer is “D.” You want to cut on the concentration by 5, so you need your solution to be
1/5 of the concentrated material. 1/5 of 500 mL is 100 mL. (73% correct)
The answer is “E.” Fluorides are always tricky in terms of solubility rules. Based off the first
statement, Tin Sulfate and Copper Chloride must be soluble. In the second reaction, Tin Sulfate
and Zinc Fluoride are produced. We already know Tin Sulfate is soluble, so Zinc Fluoride must
not be. Zinc commonly exists as a 2+ ion, so it will combine to form ZnF2(s). (58% correct)
The answer is “A.” Two ways to look at this question. Either intermolecular forces which is
what they want you to do. All demonstrate only dispersion. The more electrons, the more
dispersion. Br2 has the most electrons, so it will have the greater intermolecular forces and the
highest melting point. The other way is that every substance other than Br2 is a gas. Br2 is a
liquid. The fact that it is a liquid means that it has greater intermolecular forces. (33% correct)
The answer is “E.” This is essentially a limiting reactant stoichiometry problem. All of the H2 will
be used, but only 1 of the N2 will be used. 3:1 ratio. Also 2 moles of NH3 will be produced. (52%
correct)
42. The answer is “E.” Tricky question. Remember they pick easy numbers knowing you don’t have
a calculator. We know the pressure is 6 atm and we have equal masses of He and Ne. Doesn’t
matter the masses. Pressure is proportional to moles. Let’s just assume we have 20 g of each
gas (doesn’t matter, but it’ll make my point easier to follow). With 20 g of each gas, molar mass
of He is 4 g/mole and Ne is 20 g/mole. This means we have 5 moles of He and 1 mole of Ne.
This means that 5/6 of the moles are He and 5/6 the pressure is due to the He. (29% correct)
43. The answer is “C.” This is just the definition of boiling. Boiling is when a substances’ vapor
pressure equals atmospheric pressure. It’s what differentiates boiling from evaporation. So the
vapor pressure of water at 100°C is 1 atm or 760 mm Hg. (52% correct)
44. The answer is “B.” Cl2 is being reduced, making it the oxidizing agent. This is a reaction that
happens, so Cl2 is more able to gain electrons than I2, otherwise the opposite reaction would
happen spontaneously. (48% correct)
45. The answer is “D.” During the diagonals it is a single physical state, horizontal is the phase
change (so both phases exists). At t4 it is all liquid, just about to being freezing. (57% correct)
46. The answer to “A.” To approximate this quickly, mole fraction CH3OH is moles of CH3OH divided
by total moles. We have 0.5 moles of CH3OH (16/32) and 5 moles of H2O (90/18) so 0.5/5.5 is
approximately 0.1. (61% correct)
47. The answer is “B.” When sulfuric acid is being diluted, it releases energy. The more water that
is there, the lower the temperature change. This will prevent boiling and quick steam release.
(29% correct)
48. The answer is “B.” A conjugate acid is what is left after a substance has absorbed an H+ ion.
(64% correct)
49. The answer is “E.” This is just a solubility rules question. Acetates, Alkalis and Nitrates are all
soluble. Phosphates not so much. (50% correct)
50. The answer is “E.” On a complex redox problem, try to identify and focus on one easy to
identify half reaction. I would recommend looking at the oxidation of Br-.
10 Br-  5 Br2 + ___e-. If we check charge on each side, it is 10 e- that are needed. (34%
correct)
51. The answer is “C.” Gasses behave most ideally when they are not interacting. High
temperatures, low pressures and low intermolecular forces make gasses behave “as they
should.” (37% correct)
52. The answer is “C.” Mass and moles are not impacted by changes in temperature, however
volume measures are. (40% correct)
53. The answer is “D.” Molarity is moles per L of solution. We know moles (10) we need to find
volume, measure it. (64% correct)
54. The answer is “D.” Vapor pressure is lowered with decreasing mole fraction of solvent. The
mole fraction is now 0.9, meaning the vapor pressure is 90% of its original value or 22.5 mm Hg.
(21% correct)
55. The answer is “E.” Not a tough question, even without a calculator. First we write the
equilibrium expression, then we plug in. 𝐾 =
[𝑍]3
[𝑋]2 [𝑌]
=
43
22 ∙0.5
=
64
2
= 32. (69% correct)
56. The answer is “A.” London dispersion forces are related to the quantity of electrons present. H2
has the fewest electrons, so it has the lowest London dispersion forces. (41% correct)
57. The answer is “A.” Hybridization or hybrid orbitals are related to the number of number of
things bonded and the number of lone electron pairs. In this case there are two hybrid orbitals.
(36% correct)
58. The answer is “D.” This is a tricky question for a number of reasons. First, small values are
always tough to handle without a calculator. 360 mg is 0.36 g divide by 180. I would think
about this in a different manner. If you had 1.8 g that would be 0.01 moles. 0.36 g is 1/5 of 1.8
grams. So this would have 0.002 moles. If we divide by L (0.200 L) we end up with 0.01 M. It
doesn’t matter how much of the solution we have its solution is 0.01 M. (33% correct)
59. The answer is “B.” This is an equilibrium question. Pressure is proportional to moles, so we can
use that in an equilibrium expression. The expression will be 𝐾 =
𝑃𝑁2 𝑂4
𝑃𝑁𝑂2 2
and when we plug in
3
our mole/pressure equivalences we get 𝐾 = 62 = 0.08, however we didn’t actually need the
60.
61.
62.
63.
64.
65.
66.
67.
value, just knowing that it was less than 1 was enough. (32% correct)
The answer is “B.” D-Block ions can form complex ions when exposed to excess materials,
commonly small ions (Cl- or OH-) or small molecules with lone electron pairs (H2O or NH3). That
is what is occurring here. (33% correct)
The answer is “D.” A substance that has an oxygen in the main chain and a carbon oxygen
double bond is an ester. Another way to know this esters have scents associated with them.
(32% correct)
The answer is “B.” Though I would argue a little based off of their wording, a VOLUMETRIC pipet
would be the most accurate means of measuring a liquid. A graduated cylinder would be a good
means, but the pipet would be better. (32% correct)
The answer is “B.” Both have 10 e-, but fluorine has 9 protons, which more effectively attract
the electrons than oxygen with its 8 protons. (38% correct)
The answer is “D.” We want a substance that features a cation that has no impact on pH
(associated with a strong base) and an anion that will attract H+ ions, but not fully dissociate
(not associated with a strong acid). (33% correct)
The answer is “C.” By having the water level in the tube at the same height as the liquid
beneath it will keep the pressure of the atmosphere balanced. In its current state, the pressure
is being used to “push” some of the water up the tube, thus leading to an inaccurate volume of
gas. (43% correct)
The answer is “E.” The more times atoms bump together, the greater the probability of a
reaction. Kinetic energy is related to temperature, if particles are moving faster they will collide
more frequently leading to a greater reaction rate. The orientation does impact the rate. In
many reactions, only one part of the molecule is needed to initiate the reaction. If that section
is able to collide more effectively, the reaction will proceed at a greater rate. The role of a
catalyst is very often related to orientation.
The answer is “B.” This is just a downright mean question to through in after 66 questions, but
alas, it is a full on redox balancing. Given that you only have slightly over 1 minute per question
on the MC, I would recommend saving till the end or guessing. If you really have the time, split
68.
69.
70.
71.
72.
73.
74.
75.
into half reactions and proceed as you normally would. (20% correct) Yes, the odds of getting
right by guessing is also 20%....
The answer is “B.” We are diluting a solution of 0.002 M solution by half, leaving us with 0.001
M KOH. This is a strong base, so it dissociates completely, leaving us with 0.001 M OH or 1 x 10-3
M OH-. If we take the –log (1 x 10-3) we get 3 which is the pOH. pH + pOH =14. pH = 11. (24%
correct)
The answer is “A.” When two gasses are present at the same temperature, they have the same
Kinetic Energy. (47% correct)
The answer is “B.” Amino Acids feature an amine group (-NH2) think ammonia, and an organic
acid with a (-COOH) think acetate ion. More trivia than anything else, I’m not expecting a
question like this on the exam. Throw out a guess and hope for the best. (76% correct)
The answer is “C.” Degree of polarity is based on electronegativity differential. Oxygen and
fluorine are very to each on the periodic table, so they have very similar electronegativity
values. (45% correct)
The answer is “E.” Reaction “A” will dissolve Na2CO3 and produce CO2, reaction “B” will produce
H2 gas and the Zn will dissolve, reaction “C” will form a precipitate, reaction “D” will produce a
precipitate, Reaction “E” will visibly do nothing unless an indicator were added. (45% correct)
The answer is “E.” Only temperature will impact the vapor pressure of a particular liquid. (23%
correct)
The answer is “B.” Not the worst problem ever to do without a calculator. Ni has a 2+ charge,
which means that we will need 2 moles of electrons to produce 1 mole of Ni. 2 moles of
electrons is 2 x 96500 coulombs. At 1 amp, that is 1 coulomb per second. So it will take 2 x
96500 seconds. Approximate and get “B.” (24% correct)
The answer is “B.” We can eliminate “A, linear,” “C, tetrahedral,” and “D, linear.” Both O3 and
OF2 are bent. We want something that has the terminal atoms with differing number of bonds
to create the resonance. We will get that in O3, but not in OF2. (41% correct)