College Physics Quiz: Kinetic Theory Of Temperature And Pressure

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Kinetic Theory Of Temperature And PressureQuestion 1 of 20

Two identical containers hold different ideal gases at the same temperature and pressure. Container A holds helium (molar mass 4 g/mol) and container B holds oxygen (molar mass 32 g/mol). Which statement correctly compares the molecular properties of these gases?

Helium molecules have higher average kinetic energy and higher average speed than oxygen molecules

Helium molecules have the same average kinetic energy but higher average speed than oxygen molecules

Helium molecules have higher average kinetic energy but the same average speed as oxygen molecules

Oxygen molecules have higher average kinetic energy and higher average speed than helium molecules

Helium molecules have the same average kinetic energy but lower average speed than oxygen molecules

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College Physics Quiz

College Physics Quiz: Kinetic Theory Of Temperature And Pressure

Practice Kinetic Theory Of Temperature And Pressure in College Physics with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

What this quiz covers

This quiz focuses on Kinetic Theory Of Temperature And Pressure, giving you a quick way to practice the rules, question types, and explanations that matter most for College Physics.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

All questions

Question 1

Helium molecules have higher average kinetic energy and higher average speed than oxygen molecules
Helium molecules have the same average kinetic energy but higher average speed than oxygen molecules (correct answer)
Helium molecules have higher average kinetic energy but the same average speed as oxygen molecules
Oxygen molecules have higher average kinetic energy and higher average speed than helium molecules
Helium molecules have the same average kinetic energy but lower average speed than oxygen molecules

Explanation: When you encounter questions comparing molecular properties of different gases at the same temperature and pressure, you need to distinguish between kinetic energy and speed using kinetic molecular theory. For ideal gases at the same temperature, the average kinetic energy depends only on temperature, following the relationship

KE_{avg} = \frac{3}{2}kT

. Since both gases are at identical temperatures, helium and oxygen molecules have exactly the same average kinetic energy. However, speed depends on both kinetic energy and molecular mass. Using

KE = \frac{1}{2}mv^2

, we can derive that average speed is inversely related to molecular mass:

v_{avg} \propto \sqrt{\frac{T}{M}}

. Since helium has a much smaller molar mass (4 g/mol) than oxygen (32 g/mol), helium molecules must move faster to have the same kinetic energy. Specifically, helium molecules move

\sqrt{\frac{32}{4}} = \sqrt{8} \approx 2.8

times faster than oxygen molecules. Choice A incorrectly suggests helium has higher kinetic energy, but kinetic energy depends only on temperature, not molecular mass. Choice C makes the opposite error, incorrectly claiming same speeds but different kinetic energies. Choice D completely reverses the relationship, suggesting the heavier oxygen molecules are both more energetic and faster, which contradicts both principles. Remember this key distinction: at the same temperature, all ideal gas molecules have identical average kinetic energy, but lighter molecules must move faster to achieve that energy. This concept appears frequently in thermodynamics problems involving gas mixtures or effusion rates.

Question 2

A cylindrical container with a movable piston contains an ideal gas. The piston is pushed down, reducing the volume by half while the temperature remains constant. According to kinetic theory, what happens to the pressure and the frequency of molecular collisions with the container walls?

Pressure doubles; collision frequency with each unit area of wall doubles (correct answer)
Pressure doubles; collision frequency with each unit area of wall remains the same
Pressure doubles; collision frequency with each unit area of wall quadruples
Pressure remains the same; collision frequency with each unit area of wall doubles
Pressure quadruples; collision frequency with each unit area of wall doubles

Explanation: When you encounter problems about gas behavior under changing conditions, you need to apply both the ideal gas law and kinetic molecular theory to understand what happens at the molecular level. Starting with the ideal gas law

PV = nRT

, if temperature T remains constant and volume V is halved, pressure P must double to maintain the equality. This is Boyle's Law in action - pressure and volume are inversely proportional at constant temperature. Now for the molecular perspective: pressure results from molecular collisions with container walls. When you compress the gas to half its volume, you're forcing the same number of molecules into half the space. This doubles the molecular density throughout the container. Since collision frequency depends on how many molecules are near each unit area of wall, and you've doubled that density, the collision frequency per unit area also doubles. The molecules aren't moving faster (temperature is constant), but there are twice as many in any given region. Looking at the choices: A correctly identifies both effects. B incorrectly suggests collision frequency stays the same - this ignores that molecular density has increased. C suggests collision frequency quadruples, which would only happen if both density doubled AND molecular speed doubled (but temperature is constant, so speed doesn't change). D incorrectly states pressure remains constant, violating Boyle's Law. Study tip: For isothermal processes (constant temperature), always remember that molecular speeds stay constant, but changing volume affects molecular density. Pressure changes come from density changes, not speed changes.

Question 3

An ideal gas undergoes a process where its pressure increases from $P_1$ to $4P_1$ while its volume decreases from $V_1$ to $V_1/2$ . What is the ratio of the final temperature to the initial temperature, and how does the average molecular speed change?

Temperature ratio is 2; average molecular speed increases by a factor of $\sqrt{2}$ (correct answer)
Temperature ratio is 4; average molecular speed increases by a factor of 2
Temperature ratio is 2; average molecular speed increases by a factor of 2
Temperature ratio is 1; average molecular speed remains unchanged
Temperature ratio is 8; average molecular speed increases by a factor of $2\sqrt{2}$

Explanation: When you encounter ideal gas problems involving changes in pressure, volume, and temperature, always start with the ideal gas law:

PV = nRT

. Since the amount of gas remains constant, you can use the relationship

\frac{P_1V_1}{T_1} = \frac{P_2V_2}{T_2}

. Given the initial state

(P_1, V_1, T_1)

and final state

(4P_1, V_1/2, T_2)

, you can solve for the temperature ratio:

\frac{T_2}{T_1} = \frac{P_2V_2}{P_1V_1} = \frac{(4P_1)(V_1/2)}{P_1V_1} = \frac{2P_1V_1}{P_1V_1} = 2

For molecular speed, use the relationship between kinetic energy and temperature. The average molecular speed is proportional to

\sqrt{T}

, so:

\frac{v_2}{v_1} = \sqrt{\frac{T_2}{T_1}} = \sqrt{2}

Now examining each choice: Answer A correctly identifies both the temperature ratio of 2 and the speed increase by

\sqrt{2}

. Answer B incorrectly calculates the temperature ratio as 4 (perhaps by multiplying the pressure and volume changes:

4 \times \frac{1}{2} = 2

, not 4) and wrongly states the speed increases by factor 2 (confusing temperature ratio with speed ratio). Answer C gets the temperature ratio right but incorrectly uses the temperature ratio instead of its square root for the speed change. Answer D completely misses that temperature changes when both pressure and volume change simultaneously. Study tip: Always check your ideal gas calculations by ensuring the units work out, and remember that molecular speed depends on

\sqrt{T}

, not

T

directly.

Question 4

According to kinetic theory, the pressure exerted by an ideal gas on the walls of its container is given by $P = \frac{1}{3}nm\langle v^2 \rangle$ , where $n$ is the number density of molecules, $m$ is the molecular mass, and $\langle v^2 \rangle$ is the mean-square speed. If the number of gas molecules is tripled while keeping the temperature constant, what happens to the pressure?

Pressure increases by a factor of 3 because molecular density increases while molecular speeds remain constant (correct answer)
Pressure increases by a factor of 9 because both molecular density and molecular speeds increase
Pressure increases by a factor of 3 because molecular speeds increase while molecular density remains constant
Pressure remains constant because the container volume must increase to accommodate more molecules
Pressure decreases by a factor of 3 because the molecular speeds decrease to maintain constant temperature

Explanation: When you encounter kinetic theory problems, focus on what each variable in the pressure equation represents and how changes in conditions affect each term independently. The pressure formula

P = \frac{1}{3}nm\langle v^2 \rangle

tells us pressure depends on three factors: number density (

n

), molecular mass (

m

), and mean-square speed (

\langle v^2 \rangle

). The key insight is understanding what "constant temperature" means for molecular motion. At constant temperature, the average kinetic energy of molecules remains fixed, which means

\langle v^2 \rangle

stays constant. When you triple the number of molecules in the same container, only the number density

n

changes—it triples. Since molecular mass

m

is obviously unchanged, pressure increases by exactly the factor that

n

increased: three times. Looking at the wrong answers: B incorrectly claims molecular speeds increase with more molecules, but temperature determines molecular speeds, not the number of molecules present. C has the causality backwards—it claims speeds increase while density stays constant, which contradicts the problem setup. D suggests the container volume increases, but the problem implies a fixed container where we're adding more gas molecules. The fundamental misconception in B and C is confusing density effects with temperature effects. Remember: at constant temperature, molecular speeds are determined solely by temperature, regardless of how many molecules are present. Study tip: In kinetic theory problems, always identify which variables change and which stay constant based on the given conditions. Temperature controls molecular speeds; changing the amount of gas at constant temperature only affects number density.

Question 5

An ideal gas at temperature $T_1 = 273\text{ K}$ has an rms molecular speed of $v_1$ . The gas is heated until its rms speed becomes $2v_1$ . What is the final temperature of the gas?

$546\text{ K}$
$819\text{ K}$
$1092\text{ K}$ (correct answer)
$1365\text{ K}$
$2184\text{ K}$

Explanation: When you encounter problems involving molecular speeds and temperature in ideal gases, you're working with kinetic theory. The key relationship connects the root-mean-square (rms) speed of gas molecules to absolute temperature. The rms speed formula is

v_{rms} = \sqrt{\frac{3RT}{M}}

, where R is the gas constant, T is absolute temperature, and M is molar mass. Since we're dealing with the same gas, R and M remain constant, so

v_{rms} \propto \sqrt{T}

. Setting up the proportion:

\frac{v_2}{v_1} = \sqrt{\frac{T_2}{T_1}}

Given that

v_2 = 2v_1

, we get:

\frac{2v_1}{v_1} = 2 = \sqrt{\frac{T_2}{273}}

Squaring both sides:

4 = \frac{T_2}{273}

Therefore:

T_2 = 4 \times 273 = 1092\text{ K}

Answer A (546 K) represents the common error of assuming rms speed is directly proportional to temperature rather than to the square root of temperature. This gives

T_2 = 2 \times 273 = 546\text{ K}

. Answer B (819 K) might result from incorrectly using

T_2 = 3 \times 273

, perhaps confusing the factor of 3 from the rms speed formula. Answer D (1365 K) could come from using

T_2 = 5 \times 273

, likely from misapplying the kinetic energy relationship. Remember: molecular speed depends on the square root of temperature, not temperature directly. When speed doubles, temperature quadruples. Always square the speed ratio to find the temperature ratio in kinetic theory problems.

Question 6

Two containers of equal volume contain the same ideal gas. Container A is at temperature $T_A = 300\text{ K}$ and pressure $P_A = 2\text{ atm}$ . Container B is at temperature $T_B = 600\text{ K}$ and pressure $P_B = 3\text{ atm}$ . What is the ratio of the number of molecules in container B to container A?

$\frac{1}{2}$
$\frac{3}{4}$ (correct answer)
$1$
$\frac{4}{3}$
$2$

Explanation: When you encounter gas problems comparing different containers, immediately think about the ideal gas law:

PV = nRT

, where

n

is the number of moles. Since the number of molecules is proportional to the number of moles, you can find the ratio by comparing

n

values. Since both containers have equal volume, you can write the ratio directly from the ideal gas law. For container A:

n_A = \frac{P_A V}{RT_A}

and for container B:

n_B = \frac{P_B V}{RT_B}

. Taking the ratio:

\frac{n_B}{n_A} = \frac{P_B V \cdot RT_A}{RT_B \cdot P_A V} = \frac{P_B T_A}{P_A T_B}

Substituting the values:

\frac{n_B}{n_A} = \frac{3 \text{ atm} \times 300 \text{ K}}{2 \text{ atm} \times 600 \text{ K}} = \frac{900}{1200} = \frac{3}{4}

This confirms answer (B) is correct. (A) $\frac{1}{2}$ would result if you incorrectly used only the temperature ratio

\frac{T_A}{T_B} = \frac{300}{600}

, ignoring pressure entirely. (C) $1$ suggests equal numbers of molecules, which would only be true if

\frac{P_B}{P_A} = \frac{T_B}{T_A}

. Here, the pressure ratio is

\frac{3}{2}

but the temperature ratio is

2

, so they're not equal. (D) $\frac{4}{3}$ results from incorrectly inverting the ratio or making an algebraic error in the calculation. Study tip: For ideal gas comparisons, always write the ratio

\frac{P_2 T_1}{P_1 T_2}

when volumes are equal. This avoids the common trap of considering only one variable while ignoring the other.

Question 7

According to kinetic theory, the average kinetic energy of molecules in an ideal gas is $\langle KE \rangle = \frac{3}{2}k_BT$ for a monatomic gas. If the temperature of a monatomic ideal gas increases from $27°C$ to $127°C$ , what is the ratio of the final average kinetic energy to the initial average kinetic energy?

$\frac{127}{27} = 4.7$
$\frac{400}{300} = 1.33$ (correct answer)
$\frac{154}{54} = 2.85$
$\frac{100}{27} = 3.7$
$\left(\frac{400}{300}\right)^2 = 1.78$

Explanation: When you encounter kinetic theory problems involving temperature changes, remember that average kinetic energy depends on absolute temperature, not Celsius. The key insight is that

\langle KE \rangle = \frac{3}{2}k_BT

shows kinetic energy is directly proportional to temperature in Kelvin. First, convert both temperatures to Kelvin:

T_1 = 27°C + 273 = 300K

and

T_2 = 127°C + 273 = 400K

. Since kinetic energy is proportional to absolute temperature, the ratio of final to initial kinetic energy equals the ratio of temperatures:

\frac{\langle KE_2 \rangle}{\langle KE_1 \rangle} = \frac{T_2}{T_1} = \frac{400K}{300K} = 1.33

. Choice A (

\frac{127}{27} = 4.7

) represents the common mistake of using Celsius temperatures directly without converting to Kelvin. This ignores the absolute nature of thermodynamic temperature. Choice C (

\frac{154}{54} = 2.85

) appears to add some arbitrary offset to both temperatures, perhaps confusing this with a different temperature conversion or formula. Choice D (

\frac{100}{27} = 3.7

) uses the temperature difference (100°C) in the numerator with the initial Celsius temperature in the denominator, which has no physical basis in kinetic theory. The correct answer is B:

\frac{400}{300} = 1.33

. Study tip: Always convert Celsius to Kelvin for thermodynamics problems. Temperature ratios in kinetic theory must use absolute temperature scales, never Celsius or Fahrenheit directly.

Question 8

A sealed container holds an ideal gas. The walls of the container experience molecular collisions that create the gas pressure. If the volume of the container is suddenly doubled while keeping the temperature constant, what happens to the average force per collision and the collision frequency per unit area of the wall?

Average force per collision decreases; collision frequency per unit area decreases
Average force per collision remains the same; collision frequency per unit area decreases (correct answer)
Average force per collision increases; collision frequency per unit area decreases
Average force per collision remains the same; collision frequency per unit area remains the same
Average force per collision decreases; collision frequency per unit area remains the same

Explanation: This question tests your understanding of kinetic molecular theory and how gas molecules behave when conditions change. When analyzing gas pressure problems, remember that pressure results from both the force of individual molecular collisions and how frequently those collisions occur. Let's examine what happens when volume doubles at constant temperature. The average force per collision depends on the momentum change of molecules hitting the wall, which relates to molecular speed. Since temperature remains constant, the average kinetic energy (and thus average molecular speed) stays the same. Therefore, the average force per collision remains unchanged. For collision frequency per unit area, consider two effects: First, doubling the volume means molecules are spread over twice the space, reducing molecular density by half. Second, the walls now have twice the surface area. These effects combine so that any given unit area of wall experiences half as many collisions per unit time. Choice A incorrectly assumes force per collision decreases. The force depends on molecular speed, which is determined by temperature, not volume. Choice C makes the opposite error, suggesting force increases when it actually stays constant. Choice D fails to recognize that collision frequency must decrease when molecular density drops - this represents a common misconception that only molecular speed matters for collision rates. When tackling kinetic theory problems, always separate the effects on individual molecular behavior (which depends on temperature) from collective effects like density and collision rates (which depend on volume and number of molecules).

Question 9

In kinetic theory, the pressure of an ideal gas is related to molecular motion by $P = \frac{1}{3}\rho\langle v^2\rangle$ , where $\rho$ is the gas density and $\langle v^2\rangle$ is the mean-square molecular speed. A gas sample has density $\rho_1$ and pressure $P_1$ . If the density is reduced to $\rho_1/3$ while the pressure is maintained at $P_1$ , what happens to the mean-square speed?

Increases by a factor of 3 (correct answer)
Increases by a factor of 9
Decreases by a factor of 3
Remains the same
Decreases by a factor of 9

Explanation: This question tests your understanding of how molecular properties change when macroscopic conditions are altered in kinetic theory. When you see the kinetic theory pressure equation, think about how changing one variable affects the others. Start with the given relationship:

P = \frac{1}{3}\rho\langle v^2\rangle

. In the initial state, you have

P_1 = \frac{1}{3}\rho_1\langle v_1^2\rangle

. After the change, the density becomes

\rho_1/3

while pressure remains

P_1

, so:

P_1 = \frac{1}{3}\cdot\frac{\rho_1}{3}\cdot\langle v_2^2\rangle

. Since both expressions equal

P_1

, you can set them equal:

\frac{1}{3}\rho_1\langle v_1^2\rangle = \frac{1}{3}\cdot\frac{\rho_1}{3}\cdot\langle v_2^2\rangle

. Simplifying:

\rho_1\langle v_1^2\rangle = \frac{\rho_1}{3}\langle v_2^2\rangle

. Dividing both sides by

\rho_1

\langle v_1^2\rangle = \frac{1}{3}\langle v_2^2\rangle

. Therefore:

\langle v_2^2\rangle = 3\langle v_1^2\rangle

. The mean-square speed increases by a factor of 3, making A correct. Choice B (factor of 9) likely comes from incorrectly squaring the factor of 3. Choice C (decreases by factor of 3) represents confusion about the direction of change—since density decreased, students might think speed should also decrease. Choice D (remains same) ignores that when density changes while pressure stays constant, molecular motion must compensate. Strategy tip: In kinetic theory problems, when one variable in the equation is held constant while another changes, use algebra to find how the third variable must adjust. The math will guide you to the physical insight.

Question 10

An ideal gas undergoes an isothermal expansion where its volume triples. According to kinetic theory, which statement best describes what happens during this process?

Molecular speeds increase while collision frequency per unit area decreases, keeping pressure constant
Molecular speeds remain constant while collision frequency per unit area decreases, causing pressure to decrease (correct answer)
Molecular speeds decrease while collision frequency per unit area increases, keeping pressure constant
Molecular speeds remain constant while collision frequency per unit area remains constant, keeping pressure constant
Molecular speeds decrease proportionally to volume change while collision frequency per unit area increases

Explanation: When you encounter isothermal processes in kinetic theory, remember that "isothermal" means constant temperature, and temperature is directly related to the average kinetic energy of gas molecules. In an isothermal expansion, temperature stays constant, which means the average molecular speeds remain unchanged. However, as the volume triples, the same number of molecules now occupy three times more space. This increased volume means molecules have farther to travel between collisions with the container walls, reducing the collision frequency per unit area. Since pressure depends on both the force per collision (related to molecular speed) and the frequency of collisions, keeping molecular speeds constant while decreasing collision frequency results in lower pressure. This follows the ideal gas law:

PV = nRT

. With constant

T

n

, and

R

, if

V

increases,

P

must decrease proportionally. Choice A incorrectly suggests molecular speeds increase during isothermal expansion - this would require temperature to increase. Choice C wrongly claims molecular speeds decrease, which would mean temperature drops, contradicting the isothermal condition. Choice D incorrectly states that collision frequency stays constant despite the volume increase, which would violate the ideal gas law since pressure couldn't remain constant with tripled volume at constant temperature. Study tip: For isothermal processes, always remember that constant temperature means constant molecular speeds. The key variable that changes with volume is collision frequency, which directly affects pressure according to

PV = \text{constant}

Question 11

A gas thermometer works by measuring the pressure of a fixed volume of gas. If the gas pressure increases from $1.00\text{ atm}$ at $0°C$ to $1.25\text{ atm}$ , what is the new temperature according to kinetic theory principles?

$68°C$ (correct answer)
$25°C$
$341°C$
$273°C$
$-205°C$

Explanation: When you encounter gas thermometer problems, you're dealing with Gay-Lussac's Law, which states that for a fixed volume of gas, pressure is directly proportional to absolute temperature:

\frac{P_1}{T_1} = \frac{P_2}{T_2}

. The key insight is that you must work in absolute temperature (Kelvin), not Celsius. Convert the initial temperature:

T_1 = 0°C + 273 = 273K

. Now apply the law:

\frac{1.00\text{ atm}}{273K} = \frac{1.25\text{ atm}}{T_2}

Solving for

T_2

T_2 = \frac{1.25 \times 273}{1.00} = 341K

Converting back to Celsius:

341K - 273 = 68°C

This confirms answer A is correct. Let's examine why the other answers are wrong. Answer B (

25°C

) likely comes from incorrectly calculating the temperature difference as

\frac{1.25}{1.00} \times 25 = 31.25

, then rounding poorly. Answer C (

341°C

) is the trap of forgetting to convert from Kelvin back to Celsius—you calculated

341K

but reported it as Celsius. Answer D (

273°C

) suggests confusion about the relationship between Celsius and Kelvin scales. Remember this pattern: gas law problems almost always require absolute temperature. Always convert Celsius to Kelvin for calculations, then convert your final answer back to the requested units. The proportional relationship only works with absolute temperature because it represents the actual kinetic energy of gas molecules.

Question 12

According to kinetic theory, the pressure exerted by an ideal gas results from molecular collisions with container walls. Two containers have the same volume and contain the same type of gas at the same temperature. Container A has twice as many molecules as container B. What is the ratio of the average time between molecular collisions with the walls in container A compared to container B?

$2:1$
$1:2$ (correct answer)
$1:1$
$4:1$
$1:4$

Explanation: When analyzing gas behavior using kinetic theory, you need to connect molecular density to collision frequency. The key insight is that more molecules in the same space means more frequent collisions with the walls. Let's think about collision rate per unit wall area. This rate depends on how many molecules are available to hit the walls and how fast they're moving. Since both containers have the same temperature, the molecular speeds are identical. However, container A has twice the molecular density (twice as many molecules in the same volume). The collision rate is proportional to molecular density. With double the density, container A experiences twice as many collisions per unit time per unit wall area. Since the average time between collisions is the inverse of collision rate, container A has half the time between collisions compared to container B. This gives us a ratio of

1:2

for A compared to B. Looking at the wrong answers: Choice A (

2:1

) incorrectly suggests that more molecules somehow lead to longer times between collisions, which defies physical intuition. Choice C (

1:1

) would only be true if molecular density didn't affect collision rates, ignoring the fundamental principle that collision frequency increases with particle concentration. Choice D (

4:1

) appears to incorrectly square the density ratio, perhaps confusing this with pressure relationships or other kinetic theory formulas. Remember: collision frequency is directly proportional to molecular density, so time between collisions is inversely proportional. Double the molecules means half the waiting time between hits on the walls.

Question 13

A container holds a mixture of two ideal gases: nitrogen ( $N_2$ , molar mass 28 g/mol) and carbon dioxide ( $CO_2$ , molar mass 44 g/mol) at thermal equilibrium at temperature $T$ . What is the ratio of the rms speeds of nitrogen molecules to carbon dioxide molecules?

$\sqrt{\frac{28}{44}} = 0.80$
$\sqrt{\frac{44}{28}} = 1.25$ (correct answer)
$\frac{28}{44} = 0.64$
$\frac{44}{28} = 1.57$
$1.00$ because both gases are at the same temperature

Explanation: When you encounter questions about molecular speeds in gas mixtures, you're dealing with kinetic theory and the relationship between molecular mass and motion. The key insight is that at thermal equilibrium, all gas molecules have the same average kinetic energy, regardless of their identity. The root-mean-square (rms) speed formula is

v_{rms} = \sqrt{\frac{3RT}{M}}

, where

R

is the gas constant,

T

is temperature, and

M

is molar mass. Since both gases are at the same temperature, when you take the ratio of their rms speeds, the

3RT

terms cancel out:

\frac{v_{rms}(N_2)}{v_{rms}(CO_2)} = \sqrt{\frac{M_{CO_2}}{M_{N_2}}} = \sqrt{\frac{44}{28}} = 1.25

Notice that the heavier gas's molar mass goes in the numerator - this is because lighter molecules move faster to maintain the same kinetic energy. Looking at the wrong answers: Choice A has the masses inverted, giving you the ratio of

CO_2

speed to

N_2

speed instead. Choices C and D eliminate the square root entirely, which ignores that kinetic energy (proportional to

mv^2

) is what remains constant, not momentum (proportional to

mv

). Choice D also has the masses in the wrong positions. Remember this pattern: for molecular speed ratios, lighter molecules are faster, and you always need the square root of the inverse mass ratio. The heavier molecule's mass goes on top under the square root sign.

Question 14

A container is divided by a removable partition into two equal sections. Section A contains oxygen gas at temperature $T_A = 300\text{ K}$ , and section B contains the same amount of oxygen gas at temperature $T_B = 400\text{ K}$ . When the partition is removed and the gases reach thermal equilibrium, what is the final equilibrium temperature?

$325\text{ K}$
$350\text{ K}$ (correct answer)
$375\text{ K}$
$346\text{ K}$
$335\text{ K}$

Explanation: When you encounter thermal equilibrium problems, you're dealing with energy conservation—specifically, the principle that heat lost by the hotter gas equals heat gained by the cooler gas. Since both sections contain the same amount of oxygen gas in equal volumes, this becomes a straightforward application of energy conservation. The heat lost by the hot gas (section B) must equal the heat gained by the cold gas (section A). Using the relationship

Q = nC_v\Delta T

for each section, where

n

is the number of moles and

C_v

is the heat capacity: Heat lost by B:

Q_{lost} = nC_v(400 - T_f)

Heat gained by A:

Q_{gained} = nC_v(T_f - 300)

Setting them equal:

nC_v(400 - T_f) = nC_v(T_f - 300)

The

nC_v

terms cancel out:

400 - T_f = T_f - 300

Solving:

700 = 2T_f

, so

T_f = 350\text{ K}

This confirms answer B is correct. Answer A (325 K) is too low—it would mean the cooler gas didn't gain enough energy. Answer C (375 K) is too high—it would violate energy conservation by having the final temperature closer to the initially hotter gas. Answer D (346 K) might result from calculation errors or incorrect weighting of the temperatures. Study tip: For thermal equilibrium problems with equal amounts of the same substance, the final temperature is always the arithmetic average of the initial temperatures. This shortcut can save you time:

(300 + 400)/2 = 350\text{ K}

Question 15

A sample of ideal gas has molecules with an average kinetic energy of $6.21 \times 10^{-21}\text{ J}$ per molecule. Using kinetic theory, what is the temperature of this gas? ( $k_B = 1.38 \times 10^{-23}\text{ J/K}$ )

$150\text{ K}$
$300\text{ K}$ (correct answer)
$450\text{ K}$
$600\text{ K}$
$900\text{ K}$

Explanation: When you encounter problems linking molecular kinetic energy to temperature, you're working with one of the fundamental relationships in kinetic theory. The key insight is that temperature is directly proportional to the average kinetic energy of gas molecules. The relationship you need is:

\langle KE \rangle = \frac{3}{2}k_B T

, where

\langle KE \rangle

is the average kinetic energy per molecule,

k_B

is Boltzmann's constant, and

T

is absolute temperature. To find the temperature, rearrange this equation:

T = \frac{2\langle KE \rangle}{3k_B}

Substituting the given values:

T = \frac{2 \times 6.21 \times 10^{-21}}{3 \times 1.38 \times 10^{-23}} = \frac{1.242 \times 10^{-20}}{4.14 \times 10^{-23}} = 300 \text{ K}

This confirms answer B is correct. Looking at the wrong answers: A (150 K) would result from using the incorrect relationship

\langle KE \rangle = k_B T

instead of the proper factor of

\frac{3}{2}

. C (450 K) might come from computational errors or using an incorrect proportionality constant. D (600 K) could result from forgetting the factor of 2 in the numerator when rearranging the equation. Remember this key relationship:

\langle KE \rangle = \frac{3}{2}k_B T

for ideal gases. The factor of

\frac{3}{2}

comes from equipartition theorem for three translational degrees of freedom. Many students forget this factor, so always double-check that you're using the complete relationship when connecting kinetic energy to temperature.

Question 16

Two identical balloons are filled with different gases at the same temperature and pressure. Balloon A contains hydrogen ( $H_2$ , molar mass 2 g/mol) and balloon B contains xenon ( $Xe$ , molar mass 131 g/mol). Compare the number of molecules and the average molecular speeds in the two balloons.

Same number of molecules; hydrogen molecules move faster than xenon molecules (correct answer)
More hydrogen molecules; hydrogen molecules move faster than xenon molecules
Same number of molecules; xenon molecules move faster than hydrogen molecules
Fewer hydrogen molecules; hydrogen molecules move faster than xenon molecules
More hydrogen molecules; xenon molecules move faster than hydrogen molecules

Explanation: When you encounter gas problems involving different molecular masses at the same conditions, you need to apply two key principles: Avogadro's Law and kinetic molecular theory. Since both balloons have identical volumes and contain gases at the same temperature and pressure, Avogadro's Law tells us they contain the same number of moles, and therefore the same number of molecules. The molecular mass doesn't affect how many molecules fit in a given volume under specified conditions. For molecular speeds, kinetic molecular theory shows that average kinetic energy depends only on temperature:

\frac{1}{2}mv^2 = \frac{3}{2}kT

. Since both gases are at the same temperature, they have the same average kinetic energy. However, since kinetic energy equals

\frac{1}{2}mv^2

, lighter molecules must move faster to achieve the same kinetic energy. Hydrogen molecules (2 g/mol) are much lighter than xenon molecules (131 g/mol), so they move significantly faster. Looking at the wrong answers: Choice B incorrectly assumes more hydrogen molecules exist, violating Avogadro's Law. Choice C makes the critical error of thinking heavier molecules move faster, which contradicts kinetic theory. Choice D combines two errors: incorrectly predicting fewer hydrogen molecules while correctly noting their higher speed. Choice A correctly identifies that identical conditions mean identical molecular quantities, while lighter hydrogen molecules move faster than heavier xenon molecules. Study tip: For gas mixture problems, remember that at fixed T and P, molecular count depends only on volume (Avogadro's Law), while molecular speed depends on both temperature and molecular mass.

Question 17

A container holds an ideal gas at temperature $T_1 = 300\text{ K}$ . If the temperature is increased to $T_2 = 450\text{ K}$ while keeping the volume constant, what happens to the average kinetic energy per molecule and the root-mean-square (rms) speed of the gas molecules?

Average kinetic energy increases by a factor of 1.5; rms speed increases by a factor of 1.5
Average kinetic energy increases by a factor of 1.5; rms speed increases by a factor of 1.22 (correct answer)
Average kinetic energy increases by a factor of 1.22; rms speed increases by a factor of 1.5
Average kinetic energy increases by a factor of 2.25; rms speed increases by a factor of 1.5
Average kinetic energy increases by a factor of 1.5; rms speed increases by a factor of 2.25

Explanation: When you encounter temperature changes in an ideal gas, you need to understand how molecular motion relates to temperature through kinetic theory. Two key relationships govern this: average kinetic energy is directly proportional to temperature, while rms speed depends on the square root of temperature. For average kinetic energy per molecule, the relationship is

\langle KE \rangle = \frac{3}{2}k_BT

, where

k_B

is Boltzmann's constant. Since kinetic energy is directly proportional to temperature, when temperature increases from 300 K to 450 K, the kinetic energy increases by the same factor:

\frac{450}{300} = 1.5

. For rms speed, the relationship is

v_{rms} = \sqrt{\frac{3k_BT}{m}}

, where

m

is molecular mass. Since rms speed is proportional to

\sqrt{T}

, the speed increases by

\sqrt{\frac{450}{300}} = \sqrt{1.5} = 1.22

. Looking at the wrong answers: Choice A incorrectly applies the direct temperature ratio (1.5) to both quantities, missing that rms speed depends on the square root. Choice C reverses the factors, mistakenly applying the square root relationship to kinetic energy instead of speed. Choice D correctly identifies the kinetic energy factor but uses 2.25 (which is

1.5^2

) instead of 1.5, then incorrectly applies 1.5 to rms speed. Remember this pattern: for ideal gases, average kinetic energy scales directly with temperature, but speeds (including rms speed) scale with the square root of temperature. This square root relationship appears frequently in kinetic theory problems.

Question 18

According to kinetic theory, the root-mean-square speed of molecules in an ideal gas is $v_{rms} = \sqrt{\frac{3RT}{M}}$ , where $R$ is the gas constant, $T$ is temperature, and $M$ is molar mass. At what temperature would nitrogen molecules ( $M = 28\text{ g/mol}$ ) have the same rms speed as hydrogen molecules ( $M = 2\text{ g/mol}$ ) at $300\text{ K}$ ?

$21.4\text{ K}$
$4200\text{ K}$ (correct answer)
$600\text{ K}$
$150\text{ K}$
$42.9\text{ K}$

Explanation: When you encounter kinetic theory problems involving different gases at different conditions, focus on setting up equivalent expressions and solving for the unknown variable. Since both gases need the same rms speed, you can set their velocity expressions equal:

\sqrt{\frac{3RT_{\text{N}_2}}{M_{\text{N}_2}}} = \sqrt{\frac{3RT_{\text{H}_2}}{M_{\text{H}_2}}}

The

\sqrt{3R}

terms cancel out, giving you:

\sqrt{\frac{T_{\text{N}_2}}{M_{\text{N}_2}}} = \sqrt{\frac{T_{\text{H}_2}}{M_{\text{H}_2}}}

Squaring both sides eliminates the square roots:

\frac{T_{\text{N}_2}}{M_{\text{N}_2}} = \frac{T_{\text{H}_2}}{M_{\text{H}_2}}

Solving for the nitrogen temperature:

T_{\text{N}_2} = T_{\text{H}_2} \times \frac{M_{\text{N}_2}}{M_{\text{H}_2}} = 300\text{ K} \times \frac{28}{2} = 4200\text{ K}

This confirms answer (B) 4200 K. (A) 21.4 K results from incorrectly dividing instead of multiplying the mass ratio, giving

300 \times \frac{2}{28}

. (C) 600 K comes from only considering that nitrogen is heavier and arbitrarily doubling the temperature. (D) 150 K represents halving the original temperature, perhaps from confusing the relationship direction. Study tip: In kinetic theory problems, remember that lighter molecules move faster at the same temperature. To make a heavier molecule match a lighter one's speed, you must increase its temperature proportionally to the mass ratio. Always set up the equality first, then solve algebraically rather than guessing the relationship direction.

Question 19

A sealed container holds an ideal gas at temperature $T_1 = 300$ K. If the container is heated until the average kinetic energy per molecule doubles, what happens to the root-mean-square (rms) speed of the gas molecules?

The rms speed increases by a factor of $\sqrt{2}$ (correct answer)
The rms speed increases by a factor of 2
The rms speed increases by a factor of 4
The rms speed remains unchanged since it depends only on molecular mass

Explanation: The average kinetic energy per molecule is

\frac{3}{2}k_BT

, and the rms speed is

v_{rms} = \sqrt{\frac{3k_BT}{m}}

. If the kinetic energy doubles, then

\frac{3}{2}k_BT_2 = 2 \cdot \frac{3}{2}k_BT_1

, so

T_2 = 2T_1

. Since

v_{rms} \propto \sqrt{T}

, the rms speed increases by

\sqrt{2}

. Choice B incorrectly assumes direct proportionality to temperature. Choice C confuses the factor for kinetic energy with speed. Choice D ignores the temperature dependence.

Question 20

Two identical containers each hold the same ideal gas at the same temperature. Container A has twice the volume of container B. According to kinetic theory, which statement correctly compares the pressure and molecular motion in the two containers?

Container A has half the pressure of container B, and molecules in A move faster on average
Container A has half the pressure of container B, and molecules have the same average speed in both containers (correct answer)
Container A has the same pressure as container B, but molecules in A move slower on average
Container A has twice the pressure of container B, and molecules have the same average kinetic energy in both containers

Explanation: Since both containers have the same temperature, the average molecular kinetic energy and speed are identical (

\frac{1}{2}mv^2 = \frac{3}{2}k_BT

). However, using

PV = nRT

with the same amount of gas and temperature but twice the volume, container A has half the pressure. Choice A incorrectly links pressure to molecular speed. Choice C wrongly suggests speed depends on pressure at constant temperature. Choice D reverses the pressure relationship.