The relationship between three state variables, absolute pressure (P), volume (V), and absolute temperature (T), that is deduced from kinetic theory and characterizes an ideal gas is called the ideal gas law, PV = nRT, where P is absolute pressure, V is volume, n is the number of moles of a gas, R is the universal gas constant, and T is temperature. Emil Clapeyron first wrote down the ideal gas law in 1834 although it is a combination of the three empirical gas laws by Robert Boyle, Jacques Charles and Amedeo Avogadro that were known some time before that.
An ideal gas is one in which all collisions between molecules or atoms are perfectly elastic and in which there are no intermolecular attractive or repulsive forces. An ideal gas also obeys Boyle's law, Charles's law, and Avogadro's (or Gay-Lussac's) law. Although no real gas is actually ideal and so can only approach the behavior implied by the ideal gas equation, they do act as if they were ideal gases at and below atmospheric pressure and so the concept is very useful in reality.
Robert Boyle made the first quantitative measurements of gases in a systematic manner. Using a manometer, which measures differences in pressure, and a barometer, which measures the total pressure of the atmosphere, he developed what is now known as Boyle's law. This law states that at any constant temperature, the product of the pressure and the volume of any size sample of any gas is a constant (V1/P). Several years later, after developing a quantitative temperature scale, French chemist Jacques Charles formulated a general law known as Charles's law. This law states that at any constant pressure, the volume of any sample of any gas is directly proportional to the temperature (VT). Amedeo Avogadro formulated the last of the empirical laws required to derive the ideal gas law in 1811. This law is an interpretation of Gay-Lussac's law, which has to do with combining volumes of gases. Avogadro's law states that equal volumes of gases under the same conditions of temperature and pressure contain equal numbers of molecules (Vn). This law is equivalent to the statement that volume is directly proportional to the number of atoms or molecules.
These three concepts in conjunction produce the ideal gas law, PV = nRT where R is the universal gas constant. The gas constant arises from a combination of the proportionality constants in the three empirical gas laws and has a value that depends only upon the units in which the pressure and volume are measured. Two of the more common values employed are 0.08206 L atm/mol K and 8.3145 J/mol K.
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