Mole concentration means number of moles per unit volume, also called "molarity", and is used in the construction mole_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'. "Divalent mercury" means all compounds in which the mercury has two binding sites to other ion(s)in a salt or to other atom(s) in a molecule.