
The ideal gas law describes the behaviour of real gases under most conditions, relating the pressure, volume, and temperature of a gas to the number of moles of the gas. Moles are used in the ideal gas law equation because the identity of the particular gas molecule doesn't matter; regardless of the compound, gas molecules have the same kinetic energy spread. The ideal gas law can be expressed as PV = NkT, where P is the absolute pressure of a gas, V is the volume it occupies, N is the number of atoms and molecules in the gas, and T is its absolute temperature. The constant k is the Boltzmann constant, with the value k = 1.38 × 10−23 J/K. The molar form of the ideal gas law can be expressed as PV = nRT, where n is the number of moles.
| Characteristics | Values |
|---|---|
| Molar form of the ideal gas law | PV = nRT |
| P | Pressure |
| V | Volume |
| n | Number of moles |
| R | Molar gas constant |
| T | Temperature |
| N | Total number of atoms and molecules |
| k | Boltzmann constant |
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What You'll Learn

The ideal gas law and the law of conservation of energy
The ideal gas law, also called the general gas equation, is an equation of state of a hypothetical ideal gas. It is a good approximation of the behaviour of many gases under various conditions. The ideal gas law can be considered a manifestation of the law of conservation of energy. The work done on a gas results in an increase in its energy, which increases pressure and temperature or decreases volume. This increased energy can be viewed as increased kinetic energy, given the gas's atoms and molecules.
The ideal gas law can be derived from the kinetic theory of gases, which assumes that gas molecules are point masses with mass but no significant volume. These molecules undergo elastic collisions with each other and the sides of their container, conserving linear momentum and kinetic energy. The ideal gas law relates the pressure, volume, and temperature of an ideal gas to the number of moles of the gas. The equation for this relationship is PV = nRT, where P is pressure, V is volume, n is the number of moles, T is temperature, and R is the ideal gas constant.
The ideal gas law can be expressed in molar form, relating the pressure, volume, temperature, and number of moles of an ideal gas to the molar gas constant. This form of the equation can be used to calculate the pressure, volume, temperature, or number of moles of a gas. For example, given a gas with a certain pressure, volume, and temperature, the number of moles can be determined by rearranging the equation and plugging in the known values.
The ideal gas law is closely related to energy, with both sides of the equation having units of joules. Pressure multiplied by volume is energy, and this energy can be changed when the gas is doing work as it expands, similar to what occurs in gasoline or steam engines and turbines. The ideal gas law can also be used to calculate pressure, temperature, volume, or the number of molecules or moles in a given volume.
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The ideal gas law equation
The ideal gas law, also known as the general gas equation, is a hypothetical equation of state for an ideal gas. It is a good approximation of the behaviour of many gases under various conditions. The ideal gas law was first stated by Benoît Paul Émile Clapeyron in 1834 as a combination of Boyle's law, Charles's law, Avogadro's law, and Gay-Lussac's law.
The ideal gas law assumes that gases behave ideally, with no intermolecular forces or volume, allowing for simpler calculations. However, it is important to note that no gas exhibits these ideal properties. The ideal gas law is most accurate for monatomic gases at high temperatures and low pressures, where molecular size and intermolecular forces have less impact.
The molar form of the ideal gas law is particularly useful when dealing with a given number of moles of an ideal gas. It can be expressed as PV = nRT, where P is pressure, V is volume, T is temperature, n is the number of moles, and R is the molar gas constant. This form of the equation allows for calculations involving the initial or final values of volume or temperature, provided that pressure and the number of moles remain constant.
The ideal gas law is a fundamental concept in chemistry and physics, providing valuable insights into the behaviour of gases and serving as a foundation for more complex gas equations and theories.
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The ideal gas law and the kinetic theory of gases
The kinetic theory of gases provides a microscopic perspective on gas behaviour. It assumes that gases consist of numerous small particles, typically atoms or molecules, that are in constant random motion. These particles are widely separated, with negligible volume compared to the volume of their container. The particles collide with each other and the walls of the container, and these collisions are assumed to be perfectly elastic. The theory explains that gas pressure results from these molecular collisions, with the number of collisions and the kinetic energy of the collisions determining the pressure of the gas.
The kinetic molecular theory (KMT) is a specific application of kinetic theory to ideal gases. The assumptions of the KMT align closely with the ideal gas law, and it effectively explains the behaviour of gases under typical conditions. According to KMT, the properties of a gas are determined solely by the number of moles of gas molecules, not their identity. This theory also introduces the concept of the Maxwell-Boltzmann distribution, which describes the speed of molecules in a gas.
The ideal gas law and KMT have practical applications in laboratory experiments, such as those involving moles of gas. For example, consider a container with a volume of 0.245 m3 that holds gas at a temperature of 350 K and a pressure of 120 kPa. By using the ideal gas law and KMT assumptions, we can calculate the number of moles of gas particles present. This illustrates how the combination of the ideal gas law and KMT provides a powerful framework for understanding and predicting gas behaviour in various scenarios.
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The ideal gas law and the Maxwell-Boltzmann distribution
The ideal gas law relates the pressure, volume, and temperature of a gas to the number of moles of the gas. The equation for the ideal gas law in its molar form is:
PV = nRT
Where P is pressure, V is volume, T is temperature, n is the number of moles, and R is the molar gas constant. This equation is useful for solving problems where any three of the variables are known, and the fourth can be calculated.
The Maxwell-Boltzmann distribution is a result of the kinetic theory of gases, which provides a simplified explanation of fundamental gaseous properties, including pressure and diffusion. It defines the distribution of speeds for a gas at a certain temperature. The distribution was first derived by James Clerk Maxwell in 1860 based on the molecular collisions of the kinetic theory of gases. Later, in the 1870s, Ludwig Boltzmann carried out significant investigations into the physical origins of this distribution.
The Maxwell-Boltzmann distribution applies fundamentally to particle velocities in three dimensions but depends only on the speed (magnitude of the velocity) of the particles. It assumes that the velocities of individual particles are much less than the speed of light. On average, heavier molecules move more slowly than lighter molecules, so heavier molecules will have a smaller speed distribution, while lighter molecules will have a more spread-out speed distribution.
The Maxwell-Boltzmann distribution is used to determine the number of molecules moving between velocities v and v + dv, as it is impossible to measure the velocity of each molecule at every instant of time when looking at a mole of ideal gas. Three speed expressions can be derived from the distribution: the most probable speed, the average speed, and the root-mean-square speed.
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The ideal gas law and the Boltzmann constant
The ideal gas law states that the pressure (P) and volume (V) of a given amount of an ideal gas are directly proportional to each other when temperature (T) is held constant. This relationship can be expressed as PV = k, where k is a constant. The ideal gas law can also be written in terms of the number of moles (n) of the gas and the molar gas constant (R), as PV = nRT. The value of R is approximately 8.31 J/K·mol.
The Boltzmann constant, denoted as k or kB, is a proportionality constant that relates the average relative thermal energy of particles in a gas to the thermodynamic temperature of the gas. It is named after Austrian scientist Ludwig Boltzmann, who first linked entropy and probability in 1877. The Boltzmann constant is defined as the gas constant per molecule, given by the equation k = R/NA, where NA is the Avogadro constant (Avogadro's number). The value of the Boltzmann constant is approximately 1.38 x 10^-23 J/K.
The Boltzmann constant is one of seven defining constants of the International System of Units (SI). It is used in the definitions of the kelvin (K) and the molar gas constant, as well as in Planck's law of black-body radiation and Boltzmann's entropy formula. The Boltzmann constant is also used in calculating thermal noise in resistors.
The molar form of the ideal gas law can be expressed as PV = nRT, where P is pressure, V is volume, T is temperature, n is the number of moles, and R is the molar gas constant. This equation allows for the calculation of various properties of an ideal gas, such as pressure, volume, temperature, or the number of moles, given the other variables.
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Frequently asked questions
The ideal gas law describes the behaviour of real gases under most conditions. It states that PV = NkT, where P is the absolute pressure of a gas, V is the volume it occupies, N is the number of atoms and molecules in the gas, and T is its absolute temperature.
The ideal gas law works reasonably well, but empirical equations can fit the relationship between pressure, volume, and temperature better. Moles are used in these equations because they allow for the relationship to be defined by the molecular weight of the gas.
The number of moles can be calculated using the ideal gas law equation, PV = NkT, where P is the absolute pressure of a gas, V is the volume, T is the absolute temperature, and N is the number of moles.
The molar gas constant is approximately equal to 8.31 J/K⋅mol, which can also be written as m2⋅kg/s2⋅K⋅mol in SI base units.






































