# 18.1: Molecular Weight


The molecular weight (MW) of each of the phases in a VLE system is calculated as a function of the molecular weight of the individual components (MWi), provided that both the composition of the gas (yi) and the liquid (xi) are known:

$M W_{g}=\sum_{i=1}^{n} y_{i} M W_{i} \label{18.1a}$

$M W_{l}=\sum_{i=1}^{n} x_{i} M W_{i} \label{18.1b}$