PRE/POST ASSESSMENT
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Pre/post assessment of gas laws and kinetic molecular theory
1. An air mattress can hold weight because the air inside exerts pressure onto the walls of the
mattress. Air is made of gas molecules, so how do the gas molecules cause pressure? Please
explain.
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Answer key: Gas molecules are constantly moving. When they collide with the interior walls of
the mattress, they exert impact forces. The sum of all the individual impact forces on the interior
surface of the mattress is the internal pressure that allows the air mattress to hold weight.
Source: Developed by the authors and validated through classroom testing.
Adaptation: Not applicable.
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PRE/POST ASSESSMENT
2. A steel tank is filled with hydrogen gas at a temperature of 20°C and a pressure of 3 atm
(atmospheres). In the following diagram, the circle represents the tank, and the dots represent the
distribution of hydrogen molecules.
If the tank is cooled to 5°C, hydrogen still remains a gas. Which of the following diagrams
illustrates the distribution of hydrogen molecules at 5°C?
a)
b)
c)
d)
Please explain your choice for question 7-Part I:
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Answer key: A. Gas molecules are randomly distributed in the space they occupy as long as they
remain in the gaseous state.
Source: (Sanger & Phelps, 2007)
Adaptation: We reworded the question stem to accommodate for the average reading ability of
high school students. The original question asked about the distribution of hydrogen gas
molecules when the temperature is lowered to -20°C. As Sanger and Phelps indicated, many
students mistakenly believed that hydrogen gas would turn liquid at that temperature, and their
misunderstanding led them to choose the wrong alternatives. To eliminate this irrelevant factor,
we changed the condition on temperature to “cooled to 5°C” and also explicitly indicated that
hydrogen still remains a gas at 5°C.
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PRE/POST ASSESSMENT
3. A scientist filled a steel container with air and sealed it so that no air would leak out. She
measured the pressure inside the container in the morning when it was cold and then in the
afternoon when it was hot. She found that the pressure increased a lot. Please explain what
happened to the gas molecules causing the pressure to increase. Feel free to draw to help you
explain.
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Morning
Afternoon
Answer key: When temperature increases in the afternoon, the gas molecules inside the steel
container move faster on average and have greater kinetic energy on average. Because of their
increased speeds and kinetic energy, they collide with the walls of the container more frequently
with greater impact forces. The increased collision frequency and the greater impact force of
each collision both cause the gas pressure to increase.
Source: Developed by the authors and validated through classroom testing.
Adaptation: Not applicable.
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PRE/POST ASSESSMENT
4. A flask is closed by a stopper connected to a glass tube. The glass tube is sealed by a drop of
mercury as shown in the following figure. If we move the whole apparatus from a room with a
temperature of 26 °C to an outdoor yard with a temperature of 5 °C, and the indoor air pressure
and the outdoor air pressure is the same, what will happen to the mercury drop?
a) The mercury will not move.
b) The mercury will move to the right.
c) The mercury will move to the left.
d) It is impossible to predict.
Explain your prediction:
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Answer key: C. When the apparatus is moved to the colder outdoor yard, the gas molecules in
the flask will move slower, thus they collide with the flask less frequently with less force,
resulting in a lower internal pressure. As the atmospheric pressure stays the same, there will be a
net force pushing the mercury drop toward the flask.
Source: (Lin & Cheng, 2000)
Adaptation: We added two alternatives “the mercury will not move” and “it is impossible to
predict” as they are plausible and attractive distractors.
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PRE/POST ASSESSMENT
5. After being cooled to 5°C, the pressure inside the steel tank (described in question 2)
described in question 7A will be:
a) 0 atm
b) In-between 0 atm and 3 atm
c) 3 atm
d) Greater than 3 atm
Please explain your choice:
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Answer key: B. As the temperature decreases, the gas molecules inside the steel tank move
slower with lower kinetic energy. Because their slower speeds and lower kinetic energy, they
would collide with the walls of the container less frequently with weaker impact forces, resulting
in a decrease in gas pressure. However, gas pressure will not go down to 0 atm because they are
always moving as long as the temperature is above absolute zero.
Source: (Sanger & Phelps, 2007)
Adaptation: We added this question as a supplement to question 2 to see how students relate gas
pressure and the distribution of gas molecules.
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PRE/POST ASSESSMENT
6. When you pump your bicycle tires, the air pump feels harder and harder to push as you push in.
Please explain what happen to the gas molecules inside the pump and how they make it harder to
push.
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Answer key: There are two mechanisms that make it harder to push as you pump a bicycle tire.
First, as more gas molecules enter the tire, the frequency of collision between gas molecules and
interior wall of the tire increases, resulting in higher gas pressure. Second, as you push down the
pump, the volume of the gas decreases, which also makes gas molecules collide with the tire
more frequently, resulting in higher pressure.
Source: Developed by the authors and validated through classroom testing.
Adaptation: Not applicable.
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PRE/POST ASSESSMENT
7. The evening before a birthday party, you fill several balloons with helium gas. The birthday
party occurs on a hot summer day. The next day (the same hot temperature and the same
atmospheric pressure) the latex balloons are noticeably smaller. Which of the following explains
this observation?
a)
b)
c)
d)
The molecules moved closer to each other.
The helium molecules lost their strength.
Some of the helium molecules escaped through pores in the latex.
The helium molecules became smaller than before.
Please explain your choice:
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Answer key: C. According the gas laws, the volume of a gas is determined by its temperature,
pressure, and quantity. As the temperature and the pressure of the gas stays the same, it must be a
decrease in the quantity of the gas that caused the volume of the gas to decrease. At the
molecular level, gas molecules are constantly moving and colliding with the interior of the latex
balloons. As latex is a porous material, it is possible for gas molecules to penetrate through it and
escape to the atmosphere.
Source: Matter Concept Inventory-v5 (American Modeling Teachers Association, n.d.)
Adaptation: We added a prompt for students to explain their choices.
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PRE/POST ASSESSMENT
8. Two containers of the same size are at the same temperature. One contains 1 mole of hydrogen
gas, and the other contains 1 mole of nitrogen gas. The mass of nitrogen molecules is greater
than that of hydrogen molecules. Which of the following statements is true?
a)
b)
c)
d)
The pressure of the hydrogen gas is higher than that of the nitrogen gas.
The pressure of the hydrogen gas is lower than that of the nitrogen gas.
The hydrogen gas and the nitrogen gas have the same pressure.
It is difficult to tell.
Please explain your choice:
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Answer key: C. According to the gas laws, the pressure of a gas is determined by its temperature,
volume, and quantity. Molecular mass of a gas has no impact on its pressure. At the molecular
level, with the same temperature, heavier gas molecules move slower than lighter gas molecules,
but they have greater momentum due to their greater mass. As a result, heavier gas molecules
collide with the container walls less frequently than lighter gas molecules, but their collisions
were more forceful than the lighter ones. These two aspects cancel out each other, resulting in the
same pressure on the container walls.
Source: Developed by the authors and validated through classroom testing.
Adaptation: Not applicable.
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PRE/POST ASSESSMENT
9. A mixture of 0.5 moles of hydrogen gas and 0.5 moles of chlorine gas in a container at 300 K
has a total gas pressure of 98.8 kPa. If the hydrogen gas is taken out and only the chlorine gas
remains in the container, what would the pressure be? Explain your answer.
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1. Answer key: In a mixture of gases, each gas has a partial pressure which is the
hypothetical pressure of that gas if it alone occupied the volume of the mixture at the same
temperature. The total pressure of an ideal gas mixture is the sum of the partial pressures of each
individual gas in the mixture. The partial pressure of a component gas is proportional to its
quantity. Thus, the hypothetical pressure of the chlorine gas is half of the total pressure of the
mixture. At the molecular level, the reason why molecular mass has no impact on gas pressure is
similar to that for question 8.
Source: Developed by the authors and validated through classroom testing.
Adaptation: Not applicable.
PRE/POST ASSESSMENT
References
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American Modeling Teachers Association. (n.d.). Matter Concept Inventory, version 5.
Retrieved from http://modelinginstruction.org/researchers/evaluation-instruments/
Lin, H., & Cheng, H. (2000). The assessment of students and teachers’ understanding of gas laws.
Journal of Chemical Education, 77(2), 235–238.
Sanger, M. J., & Phelps, A. J. (2007). What are students thinking when they pick their answer? A
content analysis of students’ explanations of gas properties. Journal of Chemical Education,
84(5), 870–874.