ZFactor is an Excel add-in for calculating compressibility, molecular weight, density, critical pressure/  temperature and other gas mixture properties using the Peng-Robinson or Soave-Redlich-Kwong equations of state (EOS).  Data is provided for most common components however the database also supports addition of new or psuedo components as well as customisation of interaction parameters if required.

Reference:

http://firstprincipleseng.net/index.htm