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 limonene is C10H16. The IUPAC name for limonene is 1-methyl-4-prop-1-en-2-yl-cyclohexene. Limonene is a member of the group of hydrocarbons known as terpenes. There are standard names for the terpene group as well as for some of the individual species.