The "Mass Fraction" is the fraction of one substance with mass to the mass of the total mixture .
Applicable units are those of quantitykind:MassFraction
http://en.wikipedia.org/wiki/Mass_fraction_(chemistry)
The "Mass Fraction" is the fraction of one substance with mass to the mass of the total mixture .
$w_B = \frac{m_B}{m}$, where $m_B$ is the mass of substance $B$ and $m$ is the total.
http://www.iso.org/iso/catalogue_detail?csnumber=31894