Gas Properties
Specific gas constant R = Ru/M with Ru = 8314.46 J/(kmol·K). The heat capacities below are derived from M and k so that Mayer's relation cp − cv = R is satisfied exactly; published tables round cp independently and the small inconsistency shows up as spurious heat in an adiabatic calculation.
| Gas | M (kg/kmol) | R (J/kg·K) | c_p (J/kg·K) | c_v (J/kg·K) | k = c_p/c_v |
|---|---|---|---|---|---|
| Air | 28.970 | 287.0 | 1005 | 718 | 1.400 |
| Nitrogen (N₂) | 28.013 | 296.8 | 1039 | 742 | 1.400 |
| Oxygen (O₂) | 31.999 | 259.8 | 918 | 658 | 1.395 |
| Carbon dioxide | 44.010 | 188.9 | 843 | 654 | 1.289 |
| Methane (CH₄) | 16.043 | 518.3 | 2252 | 1733 | 1.299 |
| Helium | 4.003 | 2077.1 | 5191 | 3114 | 1.667 |
| Argon | 39.948 | 208.1 | 520 | 312 | 1.667 |
| Hydrogen (H₂) | 2.016 | 4124.2 | 14308 | 10183 | 1.405 |
| Steam (superheat) | 18.015 | 461.5 | 1873 | 1411 | 1.327 |
Values are for moderate temperatures near ambient; cp rises with temperature, so a calculation spanning many hundreds of degrees needs temperature-dependent properties. Note k = 1.67 for the monatomic gases (helium, argon) and about 1.4 for diatomic ones — that follows directly from the number of ways a molecule can store energy. Run a process on the Ideal Gas Process tool.
A reference aid, not a substitute for engineering judgment. Verify against the governing standard.