Weight-average molar mass
= Weight-average molar mass
{title2=$M_w$}
The <weight-average molar mass> weights each population by its contribution to total <mass>:
$$
M_w=\frac{\sum_i n_iM_i^2}{\sum_i n_iM_i}.
$$
Here $n_i$ counts <molecules> with <molar mass> $M_i$. <SEC light-scattering detection> can infer this average for an eluting slice under its optical and <concentration> assumptions; overlapping species need not have the same individual <molar mass>.