Table: Gas Constant for
Common Gases*

Gas
Constant, R (kJ/kgK) 
Air 
0.2870 
Ammonia 
0.4882 
Carbon Dioxide 
0.1889 
Helium 
2.0771 
Hydrogen 
4.1243 
Nitrogen 
0.2968 
Oxygen 
0.2598 
R12 
0.06876 
R134a 
0.08149 
Universal Gas Constant Versus
Gas Constant


The relationship between pressure and temperature for most gases
can be approximated by the ideal gas law,
where p is the absolute
pressure, V is the
volume, m is the mass, T is the absolute temperature (units in Kelvin
or Rankine) and R is the gas constant. Kelvin is related to Celsius by
T_{K} = T_{C} + 273.15 and Rankine is related to Fahrenheit
byT_{R} = T_{F} + 459.67.
This
equation is also referred to as the perfect gas law or the equation of
state for
an ideal gas. The gas constant R for some common gases is given in the
table. Note that the density ρ is given by m/V,
hence the ideal gas law can be written in terms of the density as
p = ρRT
The ideal gas law can also be written in per mole basis as follows:
where n is the number of moles and is
the universal gas constant. The number of moles is given by n = m/M where M is
the molecular weight of the
gas. The universal gas constant is
8.314 kJ/kmolK for all gases, and it is related to the gas constant by:

Compressibility Chart for
Various Gases**
Table: Critical Pressure and
Temperature for Various Gases*

Critical Temp. T_{c} (K) 
Critical Pressure p_{c} (MPa) 
Air 
133.0 
3.77 
Ammonia 
405.5 
11.3 
Carbon Dioxide 
304.1 
7.38 
Helium 
5.19 
0.227 
Hydrogen 
33.2 
1.30 
Nitrogen 
126.2 
3.39 
Oxygen 
154.6 
5.04 
R12 
385.0 
4.14 
R134a 
374.2 
4.06 
Validity of the Ideal Gas Law** 

Having introduced the ideal gas law, the
next important question is: Under what conditions will the ideal gas
law be a good approximation
for real gases? To address this issue, the compressibility factor
Z is introduced, and it is defined as:
Z = pV/mRT
Obviously, when Z approaches unity, then the ideal gas law holds.
The compressibility
factor for various gases is plotted as a function of the reduced pressure
(p_{R}) and reduced temperature (T_{R}) in the compressibility
chart as shown in the figure. The reduced pressure and temperature are
defined as follows:
p_{R} = p/p_{c}
T_{R} = T/T_{c}
where p_{c} and T_{c} are the critical pressure and
critical temperature, respectively. The critical temperature is the maximum
temperature that liquid and vapor phases can coexist in equilibrium.
The corresponding
pressure is the critical pressure. Values of critical pressure and temperature
for some common gases are summarized in the table.
From the compressibility chart, it is observed that Z approaches unity
when p_{R} < 0.1 or T_{R} > 2. Hence, these are
the conditions when the behavior of real gases can best be approximated
using the ideal gas law.
* Source: Sonntag, R. E. and Borgnakke, C., "Introduction
to Engineering Thermodynamics," John Wiley, Inc., New York,
2001.
** Reference: Su, G. J., "Modified Law
of Corresponding States," Ind. Eng. Chem. (international ed.), Vol.
38, pp. 803, 1946. 