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Teacher Answer Key
January 23, 2012
'see explanation below'
1. Base your answer on the information below.
A student prepared two mixtures, each in a labeled beaker. Enough water at 20.oC was used to
make 100 milliliters of each mixture.
Describe a procedure to physically remove the water from mixture 1. [1]
Allowing the water to evaporate will leave the solid NaCl behind.
[1 point]
'see explanation below'
2. Base your answer on the information below.
A student prepared two mixtures, each in a labeled beaker. Enough water at 20.oC was used to
make 100 milliliters of each mixture.
Classify each mixture using the term "homogeneous" or the term "heterogeneous." [1]
All solutions are homogeneous (uniform) mixtures. Mixture 1 is homogeneous. The iron filings in
water clearly result in a nonuniform mixture. Mixture 2 is heterogeneous.
[1 point]
'see explanation below'
3. Base your answer on the information below.
Natural gas is a mixture that includes butane, ethane, methane, and propane. Differences in boiling
points can be used to separate the components of natural gas. The boiling points at standard
pressure for these components are listed in the accompanying table.
Identify a process used to separate the components of natural gas. [1]
The components of natural gas can be separated because of the differences in their boiling points.
The process is known as fractional distillation.
[1 point]
'see explanation below'
4. Base your answer on the information below.
A student performed a laboratory activity to observe the reaction between aluminum foil and an
aqueous copper(II) chloride solution. The reaction is represented by the accompanying balanced
equation.
2Al(s) + 3CuCl2(aq) → 3Cu(s) + 2AlCl3(aq) + energy
The procedures and corresponding observations for the activities are given in the accompanying
table.
State one observation that indicates Cu2+ ions became Cu atoms. [1]
The blue-green color is characteristic of the Cu2+ ion. As this ion is reduced to Cu, the blue-green
color disappears and a red-brown solid appears. Either observation is acceptable for credit.
[1 point]
'see explanation below'
5. Base your answer on the information below.
Polonium-210 occurs naturally, but is scarce. Polonium-210 is primarily used in devices designed
to eliminate static electricity in machinery. It is also used in brushes to remove dust from camera
lenses.
Polonium-210 can be created in the laboratory by bombarding bismuth-209 with neutrons to create
bismuth-210. The bismuth-210 undergoes beta decay to produce polonium-210. Polonium-210 has
a half-life of 138 days and undergoes alpha decay.
State one beneficial use of Po-210. [1]
In the first paragraph, two beneficial uses of polonium-210 are given:


Eliminate static electricity in machinery
Remove dust from camera lenses
Only one of these uses need be given in order to receive credit.
[1 point]
'see explanation below'
6. Base your answer on the information below.
Polonium-210 occurs naturally, but is scarce. Polonium-210 is primarily used in devices designed
to eliminate static electricity in machinery. It is also used in brushes to remove dust from camera
lenses.
Polonium-210 can be created in the laboratory by bombarding bismuth-209 with neutrons to create
bismuth-210. The bismuth-210 undergoes beta decay to produce polonium-210. Polonium-210 has
a halflife of 138 days and undergoes alpha decay.
Complete the nuclear equation on the answer sheet or on a separate piece of paper for the decay
of Po-210, by writing a notation for the missing product. [1]
In order to complete a nuclear equation, the atomic numbers and the mass numbers on both sides
of the arrow must be equal. Use Reference Table O. The completed nuclear equation for the alpha
decay of polonium-210 is shown:
(see image)
[1 point]
'see explanation below'
7. Base your answer on the information below.
Polonium-210 occurs naturally, but is scarce. Polonium-210 is primarily used in devices designed
to eliminate static electricity in machinery. It is also used in brushes to remove dust from camera
lenses.
Polonium-210 can be created in the laboratory by bombarding bismuth-209 with neutrons to create
bismuth-210. The bismuth-210 undergoes beta decay to produce polonium-210. Polonium-210 has
a halflife of 138 days and undergoes alpha decay.
Determine the total mass of an original 28.0-milligram sample of Po-210 that remains unchanged
after 414 days. [1]
Use the information given in the question and the radioactive decay formula given on Reference
Table T:
(see image)
[1 point]
'see explanation below'
8. Base your answer on the information below.
In 1864, the Solvay process was developed to make soda ash. One step in the process is
represented by the balanced equation shown.
NaCl + NH3 + CO2 + H2O → NaHCO3 + NH4Cl
Explain, in terms of electronegativity difference, why the bond between hydrogen and oxygen in a
water molecule is more polar than the bond between hydrogen and nitrogen in an ammonia
molecule. [1]
The electronegativity difference between two bonded atoms is a measure of the polarity of the
bond. Use Reference Table S. The electronegativity difference for the O—H bond is 1.4, while the
electronegativity difference for the N—H bond is 0.9. The O—H bond is more polar than the N—
H bond.
[1 point]
'see explanation below'
9. Base your answer on the information below.
In 1864, the Solvay process was developed to make soda ash. One step in the process is
represented by the balanced equation shown.
NaCl + NH3 + CO2 + H2O → NaHCO3 + NH4Cl
In the space on the answer sheet or on a separate piece of paper, draw a Lewis electron-dot
diagram for the reactant containing nitrogen in the equation. [1]
Either of the following electron-dot diagrams for NH3 is acceptable for credit:
[1 point]
'see explanation below'
10. Base your answer on the information below.
In 1864, the Solvay process was developed to make soda ash. One step in the process is
represented by the balanced equation shown.
NaCl + NH3 + CO2 + H2O → NaHCO3 + NH4Cl
Write the chemical formula for one compound in the equation that contains both ionic bonds and
covalent bonds. [1]
Ionic compounds that contain polyatomic ions have both ionic and covalent bonding within the
compound. Either NaHCO3 or NH4Cl is acceptable for credit.
[1 point]
'see explanation below'
11. Base your answer on the information below.
Ozone, O3(g), is produced from oxygen, O2(g), by electrical discharge during thunderstorms. The
unbalanced accompanying equation (see image) represents the reaction that forms ozone.
Identify the type of bonding between the atoms in an oxygen molecule. [1]
In the O2 molecule, the oxygen atoms share two pairs of electrons. The type of bonding within O2
is (nonpolar) covalent.
[1 point]
'see explanation below'
12. Base your answer on the information below.
Ozone, O3(g), is produced from oxygen, O2(g), by electrical discharge during thunderstorms. The
unbalanced accompanying equation (see image) represents the reaction that forms ozone.
Explain, in terms of electron configuration, why an oxygen molecule is more stable than an
oxygen atom. [1]
When the O2 molecule is formed, both oxygen atoms have a complete octet of valence electrons
with a noble gas configuration.
[1 point]
'see explanation below'
13. Base your answer on the information below.
Natural gas is a mixture that includes butane, ethane, methane, and propane. Differences in boiling
points can be used to separate the components of natural gas. The boiling points at standard
pressure for these components are listed in the accompanying table.
List the four components of natural gas in order of increasing strength of intermolecular forces. [1]
Boiling point is an indicator of the strength of intermolecular forces: the higher the boiling point,
the stronger the forces. In order from weakest to strongest intermolecular forces, the gases are
(weakest) methane < ethane < propane < butane (strongest)
[1 point]
'see explanation below'
14. Explain, in terms of protons and neutrons, why U-235 and U-238 are different isotopes of
uranium. [1]
Isotopes of an element have the same atomic number but different mass numbers. This means that
each isotope has the same number of protons but a different number of neutrons. A U-235 atom
has 92 protons and 143 neutrons, and a U-238 atom has 92 protons and 146 neutrons.
[1 point]
'see explanation below'
15. Base your answer on the accompanying information.
The accompanying bright-line spectra for three elements and a mixture of elements are shown.
Explain, in terms of both electrons and energy, how the bright-line spectrum of an element is
produced. [1]
As excited electrons fall back to lower energy levels, energy is emitted in the form of light.
[1 point]
'see explanation below'
16. Base your answer on the accompanying information.
The accompanying bright-line spectra for three elements and a mixture of elements are shown.
Identify all the elements in the mixture. [1]
The bright-line spectrum of a mixture of elements will contain all of the lines of each element in
the mixture. Compare the spectra of each element with the spectrum of the mixture. The mixture
contains strontium and lithium.
[1 point]
'see explanation below'
17. Base your answer on the accompanying information.
The accompanying bright-line spectra for three elements and a mixture of elements are shown.
State the total number of valence electrons in a cadmium atom in the ground state. [1]
Valence electrons are those in the outermost shell of an atom. Use the Periodic Table of the
Elements. Cadmium (Cd, element 48) has the ground-state electron configuration 2-8-18-18-2.
Therefore, cadmium has two (2) valence electrons.
[1 point]
2
18. Which substance can not be broken down by a chemical change?
1. ammonia
3. propane
2. mercury
4. water
2 Elements cannot be broken down by a
1
chemical change. Of the choices given, only
choice (2), mercury, is an element.
19. Which diagram represents a physical change, only?
Wrong Choices Explained:
(2), (3), (4) These substances are compounds.
They can be broken down by chemical change.
1 A physical change is one in which atoms or
molecules are rearranged only; there is no
combination or decomposition occurs. Of the
choices given, only choice (1) shows the
rearrangement of the two types of atoms.
Wrong Choices Explained:
(2) The six atoms combine to form two
molecules; this is a chemical change.
(3) Two molecules decompose into eight atoms;
this is a chemical change.
(4) Two molecules combine to form a different
molecule; this is a chemical change.
1. fewer valence electrons
2. more valence electrons
1 Valence electrons are the electrons in the
outermost shell of an atom. Refer to the Periodic
Table of the Elements. The metals of Period 3
(Na, Mg, and Al) have 1, 2, and 3 valence
electrons, respectively. The nonmetals of Period
3 (P, S, and Cl) have 5, 6, and 7 valence
electrons, respectively.
1. iodine and silver
2. iodine and xenon
1
20. Compared to the atoms of nonmetals in Period 3, the
atoms of metals in Period 3 have
3. fewer electron shells
4. more electron shells
3
21. Which elements are malleable and good conductors of
electricity?
3. tin and silver
4. tin and xenon
Metallic elements are good conductors of
electricity. Of the choices given, only choice (3),
tin and silver, are both metallic elements.
1. lithium atom
2. potassium atom
3 The first ionization energy measures the
amount of energy needed to remove the most
loosely held electron from a ground-state atom.
Use Reference Table S. Of the choices given,
rubidium (Rb), has the smallest first ionization
and, therefore, requires the least amount of
energy to remove its single valence electron.
1. the same crystal structure and the same
properties
2. the same crystal structure and different
properties
4 Diamond and graphite are allotropes of the
element carbon. As such, they have different
crystal structures and different chemical and
physical properties.
1. each atom in the sample has 29 protons
2. atoms in the sample react with oxygen
1 An element is defined by its atomic number—
the number of protons in the nucleus of each
atom. Refer to the Periodic Table of the
Elements. The atomic number of copper (Cu) is
29.
1. atomic number
2. mass number
1 The Periodic Table is arranged in order of
increasing atomic number because this
arrangement displays the repeating nature of
physical and chemical properties.
1. tantalum
2. rhenium
3
22. Which atom in the ground state requires the least
amount of energy to remove its valence electron?
3. rubidium atom
4. sodium atom
4
23. At STP, both diamond and graphite are solids composed
of carbon atoms. These solids have
3. different crystal structures and the same
properties
4. different crystal structures and different
properties
1
24. A sample of matter must be copper if
3. the sample melts at 1768 K
4. the sample can conduct electricity
1
25. The elements on the Periodic Table are arranged in
order of increasing
3. number of isotopes
4. number of moles
2
26. Which element has the highest melting point?
3. osmium
4. hafnium
1
2 Use Reference Table S. Rhenium (element
27. Given the balanced equation representing a reaction:
number 75), with a melting point of 3453 K, has
a higher melting point than any of the other
Cl2 → Cl + Cl
elements listed in the question.
What occurs during this reaction?
1. A bond is broken as energy is absorbed.
3. A bond is formed as energy is absorbed.
2. A bond is broken as energy is released.
4. A bond is formed as energy is released.
1 Cl2 is a molecule of chlorine in which the two
4
chlorine atoms are joined by a single bond.
When Cl2 is split into two Cl atoms, the single
bond is broken. The breaking of a bond always
requires the absorption of energy.
1. Cl atom
2. F atom
4 The attraction of a bonded atom depends on
the electronegativity of the atom. The smaller
the electronegativity, the weaker the attraction.
Use Reference Table S. Of the choices given,
choice (4), S, has the smallest electronegativity.
1. Molecules of Br2 are polar, and molecules of
I2 are nonpolar.
2. Molecules of I2 are polar, and molecules of
Br2 are nonpolar.
4 The phase of a substance at STP depends on
the strength of the intermolecular forces within
the substance. Solids have stronger
intermolecular forces than liquids.
1. +1
2. +2
28. Which atom has the weakest attraction for the electrons
in a bond with an H atom?
3. O atom
4. S atom
4
29. Which statement explains why Br2 is a liquid at STP
and I2 is a solid at STP?
3. Molecules of Br2 have stronger intermolecular
forces than molecules of I2.
4. Molecules of I2 have stronger intermolecular
forces than molecules of Br2.
3
30. A neutron has a charge of
3. 0
4. -1
3 A neutron is a subatomic particle that has a
charge of 0.
Wrong Choices Explained:
(1) A proton and a positron each have a charge
of +1.
(2) An alpha particle has a charge of +2.
(4) An electron has a charge of −1.
1. alpha particle
2. beta particle
2
31. Which particle has the least mass?
2 A beta particle is an electron and is the least
massive of the particles named in this question.
3
32. In the electron cloud model of the atom, an orbital is
defined as the most probable
3. location of an electron
4. mass of an electron
1. charge of an electron
2. conductivity of an electron
3 An orbital is defined as the region of space in
which an electron is most probably located.
1. combustion
2. decomposition
3. neutron
4. proton
3
33. Which type of reaction releases the greatest amount of
energy per mole of reactant?
3. nuclear fusion
4. oxidation-reduction
3 Nuclear fusion, in which lighter atoms are
joined to form heavier ones, releases the greatest 4
34. Which balanced equation represents natural
amount of energy per mole of reactant. The
transmutation?
energy released is far greater than the energy
released by ordinary (nonnuclear) exothermic
reactions.
4 Natural transmutation involves the
spontaneous decay of a radioactive nucleus; no
bombarding particle is necessary. Of the choices
given, only choice (4), the decay of uranium238, does not include a bombarding particle.
1. stable nuclei emit alpha particles
2. stable nuclei emit beta particles
4 Atoms of I-131 decay spontaneously because
the nuclei of these atoms are unstable. Use
Reference Table N. I-131 is a beta (−) emitter.
1. formation of covalent bonds
2. formation of ionic bonds
4
35. Atoms of I-131 spontaneously decay when the
3. unstable nuclei emit alpha particles
4. unstable nuclei emit beta particles
3
36. Energy is released during the fission of Pu-239 atoms as
a result of the
3. conversion of matter to energy
4. conversion of energy to matter
'see explanation below'
37. A 1.00-mole sample of neon gas occupies a volume of
24.4 liters at 298 K and 101.3 kilopascals. In the space
provided or on a separate piece of paper, calculate the
density of this sample. Your response must include both a
correct numerical setup and the calculated results. [2]
Density is the ratio of mass to volume. Refer to the Periodic Table of the Elements. The mass of
1.00 mole of neon is 20.179 grams. Use the density equation on Reference Table T:
3 The total mass of the products of a nuclear
fission reaction is less than the total mass of the
reactants. The "missing" matter is converted to
energy.
d=m/V
=20.179 g/24.4 L
=0.827 g/L
Note: One point is awarded for a correct numerical setup, and one point is awarded for the correct
answer (or an answer consistent with your setup).
[2 points]
'see explanation below'
38. Base your answer on the information below.
The hydrocarbon 2-methylpropane reacts with iodine as represented by the balanced equation
below. At standard pressure, the boiling point of 2-methylpropane is lower than the boiling point
of 2-iodo-2-methylpropane.
Explain the difference in the boiling points of 2-methylpropane and 2-iodo-2-methylpropane in
terms of both molecular polarity and intermolecular forces. [2]
The difference in the boiling points of 2-methylpropane and 2-iodo-2-methylpropane are due to:


The molecules of 2-iodo-2-methylpropane being more polar than the molecules of 2methylpropane, and
The intermolecular forces between 2-iodo-2-methylpropane molecules being stronger than
the intermolecular forces between 2-methylpropane molecules.
One point is allowed for correctly describing the molecular polarities, and one point is allowed for
correctly describing the intermolecular forces. [2 points]