Mass concentration means mass per unit volume and is used in the construction mass_concentration_of_X_in_Y, where X is a material constituent of Y. 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 hexachlorobiphenyl is C12H4Cl6. This structure of this species consists of two linked benzene rings, each of which is additionally bonded to three chlorine atoms.