AP CHEMISTRY MIDTERM REVIEW
FREE RESPONSE QUESTIONS
1.The molecular formula of a hydrocarbon is to be determined by analyzing its combustion products and
investigating its colligative properties.
(a) The hydrocarbon burns completely, producing 7.2 grams of water and 7.2 liters of CO2 at standard
conditions. What is the empirical formula of the hydrocarbon?
(b) Calculate the mass in grams of O2 required for the complete combustion of the sample of the
hydrocarbon described in (a).
(c) The hydrocarbon dissolves readily in CHCl3. The freezing point of a solution prepared by mixing 100.
grams of CHCl3 and 0.600 gram of the hydrocarbon is -64.0ºC. The molal freezing-point depression
constant of CHCl3 is 4.68ºC/molal and its normal freezing point is -63.5ºC. Calculate the molecular
weight of the hydrocarbon.
(d) What is the molecular formula of the hydrocarbon?
2.
Answer the following questions about BeC2O4(s) and its hydrate.
(a) Calculate the mass percent of carbon in the hydrated form of the solid that has the formula
BeC2O4•3H2O.
(b) When heated to 220.ºC, BeC2O4•3H2O(s) dehydrates completely as represented below.
BeC2O4•3H2O(s)  BeC2O4(s) + 3 H2O(g)
If 3.21 g of BeC2O4•3H2O(s) is heated to 220.ºC calculate
(i)
the mass of BeC2O4(s) formed, and
(ii)
the volume of the H2O(g) released, measured at 220.ºC and 735 mm Hg.
(c) A 0.345 g sample of anhydrous BeC2O4, which contains an inert impurity, was dissolved in sufficient
water to produce 100. mL of solution. A 20.0 mL portion of the solution was titrated with KMnO4(aq). The
balanced equation for the reaction that occurred is as follows.
16 H+(aq) + 2 MnO4-(aq) + 5 C2O42-(aq)  2 Mn2+(aq) + 10 CO2(g) + 8 H2O(l).
The volume of 0.0150 M KMnO4(aq) required to reach the equivalence point was 17.80 mL.
(i)
Identify the reducing agent in the titration reaction.
(ii) For the titration at the equivalence point, calculate the number of moles of each of the following that
reacted.
• MnO4-(aq)
• C2O42-(aq)
(ii)
Calculate the total number of moles of C2O42-(aq) that were present in the 100. mL of prepared
solution.
(iv) Calculate the mass percent of BeC2O4(s) in the impure 0.345 g sample.
3.
Three volatile compounds X, Y, and Z each contain element Q. The percent by weight of element Q in each
compound was determined. Some of the data obtained are given below.
Percent by weightMolecular
Compound
of Element Q Weight
X
64.8%
?
Y
73.0%
104.
Z
59.3%
64.0
(a) The density of gaseous compound X at 27C and 750. mm Hg was determined to be 3.53 grams per liter.
Calculate the molecular weight of compound X.
(b) Determine the mass of element Q contained in 1.00 mole of each of the three compounds.
(c) Calculate the most probable value of the atomic weight of element Q.
(d) Compound Z contains carbon, hydrogen, and element Q. When 1.00 gram of compound Z is oxidized and
all of the carbon and hydrogen are converted to oxides, 1.37 grams of CO 2 and 0.281 gram of water are
produced. Determine the most probable molecular formula of compound Z.
4.
Ne
2000
F
N
1500
O
first ionization energy (kJ/mol)
1000
Be
C
B
500
Li
0
The diagram shows the first ionization energies for the elements from Li to Ne. Briefly (in one to three
sentences) explain each of the following in terms of atomic structure.
(a) In general, there is an increase in the first ionization energy from Li to Ne.
(b) The first ionization energy of B is lower than that of Be.
(c) The first ionization energy of O is lower than that of N.
(d) Predict how the first ionization energy of Na compares to those of Li and of Ne. Explain.
5.
Account for each of the following in terms of principles of atom structure, including the number, properties, and
arrangements of subatomic particles.
(a) The second ionization energy of sodium is about three times greater than the second ionization energy
of magnesium.
(b) The difference between the atomic radii of Na and K is relatively large compared to the difference
between the atomic radii of Rb and Cs.
(c) A sample of nickel chloride is attracted into a magnetic field, whereas a sample of solid zinc chloride is
not.
(d) Phosphorus forms the fluorides PF3 and PF5, whereas nitrogen forms only NF3.
6.
Use principles of atomic structure and/or chemical bonding to answer each of the following.
(a) The radius of the Ca atom is 0.197 nanometer; the radius of the Ca2+ ion is 0.099 nanometer. Account
for this difference.
(b) The lattice energy of CaO(s) is -3,460 kilojoules per mole; the lattice energy for K2O(s) is -2,240
kilojoules per mole. Account for this difference.
(c) Explain the difference between Ca and K in regard to
(i) their first ionization energies,
(ii) their second ionization energies.
K
Ca
Ionization Energy
(kJ/mol)
First
Second
419
3,050
590
1,140
(d) The first ionization energy of Mg is 738 kilojoules per mole and that of Al is 578 kilojoules per mole.
Account for this difference.
7.
Explain each of the following observations using principles of atomic structure and/or bonding.
(a) Potassium has a lower first-ionization energy than lithium.
(b) The ionic radius of N3- is larger than that of O2-.
(c) A calcium atom is larger than a zinc atom.
(d) Boron has a lower first-ionization energy than beryllium.
8.
First
Second
Third
Ionization Energy
Ionization Energy
Ionization Energy
(kJ mol-1)
(kJ mol-1)
(kJ mol-1)
Element 1
1,251
2,300
3,820
Element 2
496
4,560
6,910
Element 3
738
1,450
7,730
Element 4
1,000
2,250
3,360
The table above shows the first three ionization energies for atoms of four elements from the third period of the
periodic table. The elements are numbered randomly. Use the information in the table to answer the following
questions.
(a) Which element is most metallic in character? Explain your reasoning.
(b) Identify element 3. Explain your reasoning.
(c) Write the complete electron configuration for an atom of element 3.
(d) What is the expected oxidation state for the most common ion of element 2?
(e) What is the chemical symbol for element 2?
(f) A neutral atom of which of the four elements has the smallest radius?
9.
Answer the following questions that relate to the analysis of chemical compounds.
(a) A compound containing the elements C, H, N, and O is analyzed. When a 1.2359 g sample is burned in
excess oxygen, 2.241 g of CO2(g) is formed. The combustion analysis also showed that the sample
contained 0.0648 g of H.
(i) Determine the mass, in grams, of C in the 1.2359 g sample of the compound.
(iii)
When the compound is analyzed for N content only, the mass percent of N is found to be 28.84
percent. Determine the mass, in grams, of N in the original 1.2359 g sample of the compound.
(iv)
Determine the mass, in grams, of O in the original 1.2359 g sample of the compound.
(v)
Determine the empirical formula of the compound.
(b) A different compound, which has the empirical formula CH2Br, has a vapor density of 6.00 g L-1 at 375 K
and 0.983 atm. Using these data, determine the following.
(i) The molar mass of the compound
(ii) The molecular formula of the compound
10..
A student collected a sample of hydrogen gas by the displacement of water as shown by the diagram above.
The relevant data are given in the following table.
GAS SAMPLE DATA
Volume of sample
90.0 mL
Temperature
25ºC
Atmospheric Pressure
745 mm Hg
Equilibrium Vapor Pressure
of H2O (25ºC)
23.8 mm Hg
(a) Calculate the number of moles of hydrogen gas collected.
(b) Calculate the number of molecules of water vapor in the sample of gas.
(c) Calculate the ratio of the average speed of the hydrogen molecules to the average speed of the water
vapor molecules in the sample.
(d) Which of the two gases, H2 or H2O, deviates more from ideal behavior? Explain your answer.
11.
Represented above are five identical balloons, each filled to the same volume at 25ºC and 1.0 atmosphere
pressure with the pure gases indicated.
(a) Which balloon contains the greatest mass of gas? Explain.
(b) Compare the average kinetic energies of the gas molecules in the balloons. Explain.
(c) Which balloon contains the gas that would be expected to deviate most from the behavior of an ideal
gas? Explain.
(d) Twelve hours after being filled, all the balloons have decreased in size. Predict which balloon will be the
smallest. Explain your reasoning.
12.
Use simple structure and bonding models to account for each of the following.
(a) The bond length between the two carbon atoms is shorter in C2H4 than in C2H6.
(b) The H-N-H bond angle is 107.º, in NH3.
(c) The bond lengths in SO3 are all identical and are shorter than a sulfur-oxygen single bond.
(d) The I3- ion is linear.
13.
Butane, chloroethane, acetone, and 1-propanol all have approximately the same molecular weights. Data on
their boiling points and solubilities in water are listed in the table below.
Boiling
Solubility in
Pt.(ºC)
water
Compound
Formula
Butane
CH3CH2CH2CH3
0
insoluble
Chloroethane
CH3CH2Cl
12
insoluble
56
completely
miscible
Acetone
O
||
CH3C C CH3
1-Propanol
CH3CH2CH2OH
97
completely
miscible
On the basis of dipole moments (molecular polarities) and/or hydrogen bonding, explain in a qualitative way
the differences in the
(a) boiling points of butane and chloroethane.
(b) water solubilities of chloroethane and acetone.
(c) water solubilities of butane and 1-propanol.
(d) boiling points of acetone and 1-propanol.
14.
Explain each of the following in terms of atomic and molecular structures and/or intermolecular forces.
(a) Solid K conducts an electric current, whereas solid KNO3 does not.
(b) SbCl3 has measurable dipole moment, whereas SbCl5 does not.
(c) The normal boiling point of CCl4 is 77ºC, whereas that of CBr4 is 190ºC.
(d) NaI(s) is very soluble in water, whereas I2(s) has a solubility of only 0.03 gram per 100 grams of water.
15.
Answer the following questions about the element selenium, Se (atomic number 34).
(a) Samples of natural selenium contain six stable isotopes. In terms of atomic structure, explain what
these isotopes have in common, and how they differ.
(b) Write the complete electron configuration (e.g., 1s2 2s2... etc.) for a selenium atom in the ground state.
Indicate the number of unpaired electrons in the ground-state atom, and explain your reasoning.
(c) In terms of atomic structure, explain why the first ionization energy of selenium is
(i) less than that of bromine (atomic number 35), and
(ii) greater than that of tellurium (atomic number 52).
(d) Selenium reacts with fluorine to form SeF4. Draw the complete Lewis electron-dot structure for SeF4 and
sketch the molecular structure. Indicate whether the molecule is polar or nonpolar, and justify your answer.
16.
Use appropriate chemical principles to account for each of the following observations. In each part, your
response must include specific information about both substances.
(a) At 25˚ C and 1 atm, F2 is a gas whereas I2 is a solid.
(b) The melting point of NaF is 993˚ C, whereas the melting point of CsCl is 645˚.
(c) The shape of ICl4– ion is square planar, whereas the shape of BF4– ion is tetrahedral.
(d) Ammonia, NH3, is very soluble in water, whereas phosphine, PH3, is only moderately soluble in water.
(e) Two Lewis structures can be drawn for the OPF3 molecule, as shown below.
..
: O:
: O:
..
.. ..
..
:F
..: :F
.. P F
.. P F..:
:F
:F
..:
.. :
Structure 1 Structure 2
(i) How many sigma bonds and how many pi bonds are in structure 1?
(ii) Which one of the two structures best represents a molecule of OPF3? Justify your answer in terms of
formal charge.
17.
Give a scientific explanation for the following observations. Use equations or diagrams if they are relevant.
(a) It takes longer to cook an egg until it is hard-boiled in Denver (altitude 1 mile above sea level) than it
does in New York City (near sea level).
(b) Burn coal containing a significant amount of sulfur leads to acid rain.
(c) Perspiring is a mechanism for cooling the body.
(d) The addition of antifreeze to water in a radiator decreases the likelihood that the liquid in the radiator
will either freeze or boil.
18.
Draw the Lewis electron-dot structures for CO32-, CO2, and CO, including resonance structures where
appropriate.
(a) Which of the three species has the shortest C-O bond length? Explain the reason for your answer.
(b) Predict the molecular shapes for the three species. Explain how you arrived at your predictions.
19.
Use the principles of atomic structure and/or chemical bonding to explain each of the following. In each part,
your answer must include references to both substances.
(a) The atomic radius of Li is larger than that of Be.
(b) The second ionization energy of K is greater than the second ionization energy of Ca.
(c) The carbon-to-carbon bond energy in C2H4 is greater than it is in C2H6.
(d) The boiling point of Cl2 is lower than the boiling point of Br2.
20.
Answer the following questions about acetylsalicylic acid, the active ingredient in aspirin.
(a) The amount of acetylsalicylic acid in a single aspirin tablet is 325 mg, yet the tablet has a mass of 2.00
g. Calculate the mass percent of acetylsalicylic acid in the tablet.
(b) The elements contained in acetylsalicylic acid are hydrogen, carbon, and oxygen. The combustion of
3.000 g of the pure compound yields 1.200 g of water and 3.72 L of dry carbon dioxide, measured at
750. mm Hg and 25C. Calculate the mass, in g, of each element in the 3.000 g sample.
(c) A student dissolved 1.625 g of pure acetylsalicylic acid in distilled water and titrated the resulting
solution to the equivalence point using 88.43 mL of 0.102 M NaOH(aq). Assuming that acetylsalicylic
acid has only one ionizable hydrogen, calculate the molar mass of the acid.
21.
(a) A solution containing 3.23 grams of an unknown compound dissolved in 100.0 grams of water freezes
at -0.97ºC. The solution does not conduct electricity. Calculate the molecular weight of the compound.
(The molal freezing point depression constant for water is 1.86ºC kg mole-1)
(b) Elemental analysis of this unknown compound yields the following percentages by weight H=9.74%;
C=38.70%; O=51.56%. Determine the molecular formula for the compound.
(c) Complete combustion of a 1.05 gram sample of the compound with the stoichiometric amount of
oxygen gas produces a mixture of H2O(g) and CO2(g). What is the pressure of this gas mixture when it
is contained in a 3.00 liter flask at 127ºC?
22.
Draw Lewis structures for CO2, H2, SO3 and SO32- and predict the shape of each species.
23.
a. Discuss briefly the relationship between the dipole moment of a molecule and the polar character of the
bonds within it. With this as the basis, account for the difference between the dipole moments of CH2F2
and CF4.
b. The boiling points of the following compounds increase in the order in which they are listed below:
CH4 < H2S < NH3
Discuss the theoretical considerations involved and use them to account for this order.
MIDTERM REVIEW PROBLEM SET
You are responsible for completing the 5 problems listed below. You may be asked to present any one of
these problems to your classmates during our review periods, so be prepared to show all work, units, sig
figs, molecule sketches, lewis dot, etc. The entire review packet will be collected, so you are responsible
for listening to other student presentations for the remainder of the 23 problems. Any problems that are
not completed in class will become homework. The entire packet is due on the day of the midterm.
Andy
1
18
12
6
23
Ashley
2
19
13
7
23
Ben
3
20
14
8
22
Charlotte
4
21
15
9
11
Ellie
5
22
16
10
17
Garett
6
23
17
11
2
Jess
7
1
18
12
22
Joe
8
2
19
13
4
Lars
9
3
20
14
5
Liz
10
4
21
15
6
Nav
11
5
22
16
7
Rachel
12
6
23
17
8
Sarah
13
7
1
18
9
Tara
14
8
2
19
10
Taylor
15
9
3
20
11
Tori
16
10
4
21
12