Mass fraction is used in the construction mass_fraction_of_X_in_Y, whereX is a material constituent of Y. It means the ratio of the mass of X to the mass of Y (including X). A chemical species denoted by X may be described by a single term such as 'nitrogen' or a phrase such as 'nox_expressed_as_nitrogen'. The chemical formula for ethane is C2H6. Ethane is a member of the group of hydrocarbons known as alkanes. There are standardnames for the alkane group as well as for some of the individual species.