## Perfect Gas or Ideal Gas

Experimental information about gases at low pressures i.e. Charles's law, Boyle's law and Avogadro's principle may be combined to one equation:
```	P V=n R T
```
known as perfect gas equation. Where,
P= absolute pressure,
T= absolute temperature,
V= volume of the gas,
n= number of moles,
and R is a constant, known as gas constant.
```	R=8314.51 J/(kmol.K)
```
The surface of possible states, (P,V,T), of a fixed amount of a perfect gas is shown in figure below.

Any gas that obeys the above mentioned equation under all conditions is called a perfect gas (or ideal gas). A real gas (or an actual gas), behaves like a perfect gas only at low pressures. Some properties of actual gases such as specific heat at constant pressure and specific enthalpy are dependent on temperature but the variation due to pressure is negligible. There are empirical relations that calculate gas properties. The following polynom is a good approximation for the specific enthalpy of gases:

where a1 to a6 are constants depending only on the type of the gas. It should be noted that this formulation will agree with Joule's law and we obtain a set of thermodynamically consistent equations. The above equation can be used directly for calculation of specific heat capacity of the gas:

By using the relationship:

The specific entropy of the gas, s, will be:

where a7 is a constant and P0 is a reference pressure. Related topics: