The unit of parts per million (ppm) and parts per billion (ppb) are commonly used by environmental chemistrs. In general, 1 ppm means 1 part of solute for every parts of solution. Mathematically, by mass: In the case of very dilute aqueous solutions, a concentration of 1.0 ppm is equal to 1.0 of solute per 1.0 mL, which equals 1.0g of solution (as aqueous solution). Parts per billion is defined in a similar fashion. Calculate the molarity of 5.0 ppb Hg in water.