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An ideal gas is kept in a container of constant volume. The pressure of the gas is also kept constant. (a) If the number of molecules in the gas is doubled, does the rms speed increase, decrease, or stay the same? Explain. (b) If the initial rms speed is \(1300 \mathrm{m} / \mathrm{s},\) what is the final rms speed?

Short Answer

Expert verified
(a) rms speed decreases, (b) final speed is approximately 919.24 m/s.

Step by step solution

01

Understand the Ideal Gas Law

To solve this problem, we need to consider the Ideal Gas Law, written as \( PV = nRT \), where \( P \) is the pressure, \( V \) is the volume, \( n \) is the number of moles of gas, \( R \) is the ideal gas constant, and \( T \) is the temperature. Given that \( P \) and \( V \) are constant, \( nT \) must remain constant for this scenario.
02

Relate to Root Mean Square Speed

The root mean square (rms) speed \( v_{rms} \) of gas molecules is given by \( v_{rms} = \sqrt{\frac{3kT}{m}} \), where \( k \) is the Boltzmann constant, \( T \) is temperature, and \( m \) is the mass of a molecule. This equation indicates that \( v_{rms} \) is dependent on \( T/m \).
03

Influence of Doubling Molecules

If the number of molecules is doubled and the pressure and volume are constant, then the temperature \( T \) must be halved to maintain constant \( nT \). In the formula for \( v_{rms} \), this implies \( v_{rms} \) will decrease because \( T \) decreases.
04

Calculation of Final rms Speed

Given that \( T \) is halved, the new \( v_{rms} = \sqrt{\frac{1300^2}{2}} = 1300 / \sqrt{2} \approx 919.24 \) m/s.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Root Mean Square Speed
Root mean square speed, often abbreviated as rms speed, is a way to determine the average speed of particles in a gas. It's particularly useful when considering how these particles move and react at various temperatures. This speed is calculated using the equation \( v_{rms} = \sqrt{\frac{3kT}{m}} \), where \( k \) stands for the Boltzmann constant, \( T \) is the temperature, and \( m \) is the mass of an individual molecule. This formula helps us understand that the rms speed is directly influenced by temperature and inversely related to the mass of the particles. By observing changes in rms speed, we can infer how particles in a gas respond to alterations in temperature, making it a key concept in thermodynamics and kinetic theory.
Temperature
Temperature is a crucial factor in the behavior of gases and their molecular activities. In the context of the Ideal Gas Law, it determines how gas molecules move and the energy they possess. A higher temperature means more energetic movements, translating to a higher rms speed. Conversely, a lower temperature would generally lead to a lower rms speed. Keeping the Ideal Gas Law in mind, where the equation \( PV = nRT \) includes temperature, it is clear that temperature significantly impacts pressure and volume when the number of moles and the gas constant remain unchanged. In relation to rms speed, if pressure and volume are constant and the number of molecules doubles, the temperature must decrease to maintain balance, as seen by the need to halve the temperature in such scenarios.
Number of Moles
The number of moles, symbolized by \( n \), represents the quantity of substance in chemistry, conveying the amount of gas in a system. In the Ideal Gas Law formula, \( n \) is key to determining the state of a gas in combination with pressure, volume, and temperature. If the number of molecules, and by extension moles, in a gas is altered while maintaining constant pressure and volume, adjustments must occur in temperature to preserve equality in the equation \( nT = \text{constant} \). For example, doubling the number of molecules requires halving the temperature to maintain the balance, illustrating the interconnectedness of these factors. Thus, analyzing changes in mole quantity gives insights into the behavior and properties of gaseous systems under set conditions.
Boltzmann Constant
The Boltzmann constant, denoted as \( k \), acts as a bridge between energy at the microscopic scale and macroscopic temperature. With a value of \( 1.38 \times 10^{-23} \text{JK}^{-1} \), the Boltzmann constant is fundamental in relating the kinetic energy of particles to temperature, particularly within the Ideal Gas Law framework. It appears in equations involving rms speed as \( v_{rms} = \sqrt{\frac{3kT}{m}} \), connecting the speed and energy of gas molecules to temperature. Understanding the Boltzmann constant helps in grasping how energy distributions among particles relate to observable properties like temperature and pressure. This constant is intrinsic to statistical mechanics and thermodynamics, simplifying the descriptions of behaviors at an atomic or molecular level.Overall, the Boltzmann constant symbolizes the link between the microscopic and macroscopic worlds, highlighting the role of individual particle dynamics in larger scale phenomena.

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Most popular questions from this chapter

Suppose the absolute temperature of an ideal gas is doubled from \(100 \mathrm{K}\) to \(200 \mathrm{K}\). (a) Does the average speed of the molecules in this gas increase by a factor that is greater than, less than, or equal to \(2 ?\) (b) Choose the best explanation from among the following: I. Doubling the Kelvin temperature doubles the average kinetic energy, but this implies an increase in the average speed by a factor of \(\sqrt{2}=1.414 \ldots\) which is less than 2 . II. The Kelvin temperature is the one we use in the ideal-gas law, and therefore doubling it also doubles the average speed of the molecules. III. The change in average speed depends on the mass of the molecules in the gas, and hence doubling the Kelvin temperature generally results in an increase in speed that is greater than a factor of 2

A hollow cylindrical rod (rod 1) and a a solid cylindrical rod (rod 2 ) are made of the same material. The two rods have the same length and the same outer radius. If the same compressional force is applied to each rod, (a) is the change in length of rod 1 greater than, less than, or equal to the change in length of rod \(2 ?\) (b) Choose the best explanation from among the following: I. The solid rod has the larger effective cross-sectional area, since the empty part of the hollow rod doesn't resist compression. Therefore, the solid rod has the smaller change in length. II. The rods have the same outer radius and hence the same cross-sectional area. As a result, their change in length is the same. III. The walls of the hollow rod are hard and resist compression more than the uniform material in the solid rod. Therefore the hollow rod has the smaller change in length.

A brick has faces with the following dimensions: face 1 is \(1 \mathrm{cm}\) by \(2 \mathrm{cm} ;\) face 2 is \(2 \mathrm{cm}\) by \(3 \mathrm{cm} ;\) face 3 is \(1 \mathrm{cm}\) by \(3 \mathrm{cm} .\) On which face should the brick be placed if it is to have the smallest change in dimensions due to its own weight? Explain.

The rms speed of a sample of gas is increased by \(1 \%\). (a) What is the percent change in the temperature of the gas? (b) What is the percent change in the pressure of the gas, assuming its volume is held constant?

A large punch bowl holds \(3.99 \mathrm{kg}\) of lemonade (which is essentially water) at \(20.5^{\circ} \mathrm{C} .\) A \(0.0550-\mathrm{kg}\) ice cube at \(-10.2^{\circ} \mathrm{C}\) is placed in the lemonade. What is the final temperature of the system, and the amount of ice (if any) remaining? Ignore any heat exchange with the bowl or the surroundings.

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