What gas has the lowest adiabatic index at room temperature?

[Sunfire]

Hello,

to the best of your knowledge, what is

1) the gas with the lowest adiabatic index $\gamma=\frac{c_p}{c_v}$ at room temperature
2) a very common, non-flammable, non-toxic gas with the lowest adiabatic index at room temperature

Many thanks

 

[Q_Goest]

In general, you're looking for molecules with the largest number of atoms. Octafluoropropane, also called Freon 218, has a ratio of specific heats of ~1.066 at room temperature and atmospheric pressure. Regarding toxicity, check the http://msdssearchengine.com/local_msds.php?id=118976.

 

[jlefevre76]

Just dug out the old thermo book...300 K and 1 ATM

Butane 1.091
Octane 1.044
Propane 1.126
Ethane 1.186

Those are the lowest ratios I found, not sure on toxicity though. However, I'm going to guess they're a heck of a lot less toxic than anything with Chlorine or Flourine in it...