Chemistry Review Using OGT
questions
2005—2009 retired exam questions
Concept: Conservation of matter
and energy.
Neither energy or matter can be
created or destroyed, but it may be
converted from form to form.
During a chemical process, all atoms
and all energy that entered the
process via the reactants must be
accounted for in the ending products.
In chemical equations,
Reactants products
Balancing chemical equations shows that no
new atoms or energy are created and that
none are lost.
When methane (CH4) is burned in the presence of oxygen
(O2), the two chemicals react together in a process called
combustion.
Which of these compounds could be a possible product of
this combustion reaction?
1.
2.
3.
4.
NH3
SO2
H2O
CS2
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Reactants:
CH4 + O2  ?
What makes each choice correct or incorrect?
1. NH3
No N atoms in either reactant
2. SO2
No S atoms in either reactant
3. H2O
possible—both H & O atoms in reactants
4. CS2
No S atoms in either reactant
During combustion reactions, C & H containing
compounds react with O2 to produce heat (released
from bonds broken in the CH4), CO2, & H2O.
CH4 + O2  CO2 + H2O + heat energy
Concept: density= mass/volume
density is a physical property
density shows how much mass
occupies a certain amount of space
Materials will float on top of liquids or
gases having higher density, but will
sink if the liquids or gases have lower
density.
Density can change with temperature.
Gertrude cut two bars of different types of soap into four pieces
each. She put one piece from each bar into each of four
beakers, labeled Beaker W, Beaker X, Beaker Y and Beaker Z.
Each beaker contained a different unknown liquid.
According to the results shown above, which beaker contained
the liquid that was densest?
1.
2.
3.
4.
Beaker W
Beaker X
Beaker Y
Beaker Z
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Explanation—materials will float on top of liquids or
gases having higher density, but will sink if the liquids
or gases have lower density.
Density = mass/volume
Both soaps float in
Beaker W, so the liquid
In beaker W is more dense
Than both soaps.
Order of increasing density:
Liquid in Y least dense,
Liquid in X 2nd least dense,
Liquid in Z 2nd most dense,
Liquid in W most dense
At 25°C, water has a density of 1.0 g/mL and vegetable oil
has a density of 0.90 g/mL.
How would a substance with a density of 0.95 g/mL
behave when placed in both oil and water?
1. Sink in both oil and water
2. Sink in oil and float in
water
3. Float in oil and sink in
water
4. Float in both oil and water
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Substance will float on top of objects more dense than
themselves.
The object having D= 0.95 g/mL is more dense than oil
with D=0.90 g/ml, so it will sink in oil. The object’s less
dense than water (D=1.0 g/ml), so it will float on top of
water.
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Two identical flasks containing different liquids are placed
on identical balances.
Based only on what you can observe from the picture,
what property differs between the two liquids?
1.
2.
3.
4.
density
volume
alkalinity
conductivity
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Explanation
Students are expected to remember that:
Density=mass/volume, to know that balances and
units of grams show mass, and to know that
graduated cylinders and units of ml show volume.
alkalinity refers to basic (>7) pH.
conductivity is a measure of the ability of a substance
to allow electricity to flow through it.
Suppose scientists discovered four new elements (W, X, Y,
Z) while studying rock and soil samples brought back from
a Mars mission. Which Lewis dot structure represents an
element that should be placed in column VIIA (17) of the
periodic table?
1.
2.
3.
4.
Element W
Element X
Element Y
Element Z
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Explanation: Elements in families 1, 2, & 3—18 possess
the same numbers of valence electrons as other
members of their families. The dots in a Lewis dot
structure show only an atom’s valence electrons.
Members of family 17 (VIIA) have 7 valence electrons.
Concept: the atomic # of an element shows the number of
protons in its atoms. The atomic weight of an element shows its
total # neutrons & protons, each having a mass of 1 atomic mass
unit (1 amu).
In neutral atoms, the number protons and electrons are equal.
In ions, metal atoms have lost 1 electron for each + charge or
nonmetal atoms have gained 1 electron for each – charge.
Fill in this table.
F
9p+
S
16p+
Mg 12p +
O
8p +
10 average no
16 average no
12 average no
8 average no
10 e16 e11 e10 e-
charge 1charge 0
charge 1+
charge 2-
A neutral atom of silicon has
1.
2.
3.
4.
12 electrons
13 electrons
14 electrons
15 electrons
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Explanation: The atomic # of silicon is 14, so it
has 14 protons. In neutral atoms, protons are
equal in number to electrons.
Concept: Only noble gases (family 18 or VIIIA) are stable
in elemental form since only these have valence energy
level electron octets.
Other elements undergo chemical reactions to form either
ionic or covalent bonds, thereby achieving valence octets.
Background:
metals have low electronegativity
nonmetals have higher electronegativity
Metals & nonmetals achieve stable valence shell electron
octets by ionic bonding.
Metals donate electrons & become + ions.
Nonmetals accept electrons & become – ions.
Metal Na atoms
nonmetal Cl atoms
11e-,11p+,
17e-17p+
Na ions 10 e-, 11p+
Cl ions 18e-, 17p+
Nonmetals share electrons with other nonmetals, forming
covalent bonds. By sharing valence electrons (1 each for a
single bond , 2 each for a double bond =, 3 each for a triple
bond =) both atoms achieve valence octets (or for H, a duet
of electrons in the 1st shell forms)
H2 H—H
H:H
F2 F—F
CO2 O=C=O
CF4
F
|
F—C—F
|
F
Use the partial periodic table to answer the question.
In terms of electrons, describe the difference between the
formation of a covalent bond in Cl2 & an ionic bond in NaCl.
Respond in the space provided in your Answer Document.
(2 points)
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Sample answer for 2 points:
Na is a metal element and Cl is a nonmetal element, so these
two elements react to form an ionic bond.
Na, a metal atom, donates its one valence electron to form a
1+ ion, and Cl a nonmetal atom, accepts the one valence
electron from Na to form a 1- ion.
Since 2 Cl atoms are both
nonmetal atoms, they share
1 valence electron each
to form a covalently bonded
molecule, Cl2 .
Both NaCl and Cl2 formation
Allows the atoms to form stable
valence energy level octets.
2
The student response describes the difference between the covalent bond in Cl2
and the ionic bond in NaCl in terms of electrons.
OR
The student response may provide two correctly labeled drawings (e.g., Lewis Dot
diagrams, Bohr model, etc.)
OR
The student response may provide one correctly labeled drawing (e.g., Lewis Dot
diagrams, Bohr model, etc.) of the covalent bond in Cl2 or the ionic bond in
NaCl and a description of the bond not illustrated in the drawing.
1
The student response describes the covalent bond in Cl2 OR the ionic bond in
NaCl.
OR
The student response may provide one correctly labeled drawing of either the
covalent bond in Cl2 OR the ionic bond in NaCl in terms of electrons.
OR
The student response may provide two partially correct drawings, but they may
contain errors (e.g., some labels may be missing).
0
The student response indicates inadequate or no understanding of the task. The
response may provide incorrect or irrelevant information.
What score does each answer earn?
1
0
0
The following graph shows the change in temperature of a
sample of H2O, which begins as ice, as thermal energy is
added.
Which region of the graph represents water (H2O) in the
liquid form only?
1.
2.
3.
4.
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2
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Explanation: When a substance reaches its
melting/freezing point or its boiling/condensing
point, it begins to change state. Energy being
absorbed is used for the change in state, not to
increase kinetic energy (temperature) of the
substance, so temperature remains constant until
all of the substance has changed state.
In his investigations of air, Cavendish discovered a bubble of
gas that would not combine with nitrogen. His observations went
unnoticed until Ramsay performed experiments in which he
obtained similar results. Ramsay recalled & repeated
Cavendish’s experiments exactly to verify the results. Then,
using Kirchhoff’s spectroscopy technique, Ramsay was able to
identify the leftover gas as the element he called argon. Upon
further investigation, he found the elements neon, krypton and
xenon.
Based on these results it can be said that:
1. the combined work of Cavendish, Kirchhoff and Ramsay
led to the discovery of the noble gases.
2. Kirchhoff’s work was insignificant in the investigations
leading to the discovery of argon.
3. Ramsay violated ethical practice in science by repeating
Cavendish’s experiments.
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4. Cavendish is directly responsible for the discovery of
argon, but not neon, krypton or xenon.
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Explanation
Students are expected to remember that:
Xe and Ar are noble gases
That science is a process in scientists are expected
to repeat/verify or refute, then build upon, the
findings of other scientists, and that scientific
theories can be discarded or revised if new data
(e.g., data obtained using improved technology)
contradicts the original theory or if it reveals
more information to refine the theory.