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 of CFC113a CCl3CF3. The IUPAC name for CFC113a is 1,1,1-trichloro-2,2,2-trifluoro-ethane.