Dear, calculation of average molecular weight of heavy naphtha is difficult because there are ~ 50 (C7~C12) range components.
So, it is better to predict the molecular weight based upon specific gravity and boiling point or some other methods available in the literature. One of the methods is attached here.
Handbook of Petroleum Processing by Steven A. Treese, Peter R. Pujadó, David S. J. Jones
second edition