How to calculate Vapor Molecular Weight

[GoBigRed7]

Hello, I am trying to figure out the vapor molecular weight of a substance in order to estimate its emissions. It is for a fatty acid, and I have not found readily available data for the material. It is an agricultural substance, so I am assuming the amount vary greatly. I can estimate the overall molecular weight, and I can run a simulation to get a vapor pressure. Other than that, I don't know much about the substance. Is there some way to correlate the vapor pressure with the molecular weight to get a vapor molecular weight? Or any other suggestions. Any help would be greatly appreciated, thank you very much.

 

[chicopee]

I would do a search of the Chemical Engineering periodicals.





e

 

[dcasto]

the vapor molecular weight is equal to the liquid molecular weight is equal to the solid molecular weight for a pure substance.

 

[25362]

The following link may be of some help:

http://www.stratusengr.com/Articles/MoleWt.pdf

 

[cheute79]

I suppose knowing the MW for the fatty acid might give you some peace of mind for estimating emissions, but if you don't know the formula, the name of the compound, or anything more than that it is a fatty acid, perhaps look up a few fatty acids, take an average, or assume a "highest case" (worst case for emissions), and use that? I did a quick check on adipic acid. it's very common. At 18.5 C, it has a vapor pressure of 10 Pa (quite low). Oleic acid vapor pressure is 1 mBar at 20 C (1 mBar is 100 Pa), so you have a variation there of a factor of 10. Oleic is a smaller molecule than adipic. Both values are quite small, as far as emissions go, so go with 100 Pa. I'd guess your emissions are more dependent on ambient temperature variation than much anythign else.