Most gauges read the excess of pressure over atmospheric pressure and this excess is called "gauge pressure". To be sure, it has zero useful pressure in it, and your tire gauge would read zero pounds per square inch. Standard volume of 1 mole of an ideal gas at STP: 22.4 litersĭoes the flat tire on your automobile have zero air pressure? If it is completely flat, it still has the atmospheric pressure air in it. Standard pressure = 1 atmosphere = 760 mmHg = 101.3 kPa The standard temperature is the freezing point of water and the standard pressure is one standard atmosphere. STP is used widely as a standard reference point for expression of the properties and processes of ideal gases. Avogadro's number Standard Temperature and Pressure One mole of an ideal gas will occupy a volume of 22.4 liters at STP (Standard Temperature and Pressure, 0☌ and one atmosphere pressure). The accurate masses of pure elements with their normal isotopic concentrations can be obtained from the periodic table. For an isotope of a pure element, the mass number A is approximately equal to the mass in amu. For example, carbon has an atomic mass of exactly 12.0 atomic mass units - a mole of carbon is therefore 12 grams. A mole of any material will contain Avogadro's number of molecules. Some texts just use the term "thermodynamic variable" instead of the description "state variable".Ī mole (abbreviated mol) of a pure substance is a mass of the material in grams that is numerically equal to the molecular mass in atomic mass units (amu). Enthalpy is one of the four " thermodynamic potentials", and the other three, internal energy U, Helmholtz free energy F and Gibbs free energy G are also state variables. If a property, e.g., enthalpy H, isdefined as a combination of other state variables, then it too is a state variable. In the ideal gas law, the state of n moles of gas is precisely determined by these three state variables. For example in the heat-work example, the final state is characterized by a specific temperature (a state variable) regardless of whether it was brought to that state by heating, or by having work done onit, or both.Ĭommon examples of state variables are the pressure P, volume V, and temperature T. It must be inherently single-valued to characterize a state. The average translational kinetic energy of any kind of molecule in an ideal gas is given byĪ state variable is a precisely measurable physical property which characterizes the state of a system, independently of how the system was brought to that state. Avogadro's number is the number of molecules in a mole of any molecular substance. A convenient standard quantity is the mole, the mass of gas in grams equal to the molecular mass in amu. The mass of the gas will then be proportional to the molecular mass. Gas properties are described in terms of state variables.Ī given volume V of any ideal gas will have the same number of molecules. Molecular Constants In the kinetic theory of gases, there are certain constants which constrain the ceaseless molecular activity. Calculationĭeparture from an ideal gas: van der Waals equation of state One mole of an ideal gas at STP occupies 22.4 liters. The temperature is taken to be proportional to this average kinetic energy this invokes the idea of kinetic temperature. But there is also a statistical element in the determination of the average kinetic energy of those molecules. The ideal gas law can be viewed as arising from the kinetic pressure of gas molecules colliding with the walls of a container in accordance with Newton's laws. N A = Avogadro's number = 6.0221 x 10 23 /mol.R = universal gas constant = 8.3145 J/mol K.The relationshipbetween them may be deduced from kinetic theory and is called the In such a gas, all the internal energy is in the form of kinetic energy and any change in internal energy is accompanied by a change in temperature.Īn ideal gas can be characterized by three state variables: absolute pressure (P), volume (V), and absolute temperature (T). One can visualize it as a collection of perfectly hard spheres which collide but which otherwise do not interact with each other. An ideal gas is defined as one in which all collisions between atoms or molecules are perfectly eleastic and in which there are no intermolecular attractive forces.
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